Differential effects of fats on gut–host health

Dietary fats are ubiquitous and essential, while their quantity and quality modulate health. Recently, effects on the gut microbiome are being revealed. This post explores their differential effects on the gut–host dialog and underlying mechanisms relevant to many diseases.

Dietary fats appear to differentially affect human physiology; and perhaps most notoriously in the case of cardiovascular disease (CVD), the leading cause of death globally. For instance, in large observational studies, substitution analyses suggest opposing effects of saturated vs. monounsaturated and polyunsaturated fatty acids (i.e. SFAs vs. MUFAs and PUFAs, respectively) on CVD ^1–3; a relationship tested and supported by meta-analyses of randomised controlled trials (RCTs) ⁴, and referenced in many dietary guidelines. Further, in 3–4 week RCTs on healthy adults, adjusting the habitual palmitate/oleate ratio (i.e. the most abundant SFA/MUFA) affects blood/tissue lipids, alongside energy metabolism, immune activity and brain function ^5–11. And even single meals with different fats can have markedly different effects on postprandial cardiometabolic biomarkers ¹².

From a cellular perspective, lipids serve various fundamental roles (e.g. as membrane components, signalling molecules and calorie-dense fuel) requiring the properties of specific fatty acids and their spatiotemporal regulation. Consequently, while dietary fats vary widely in quantity/quality, cell membrane composition is highly regulated across organs and different diets ¹³, and perturbations can induce rapid lipidome remodelling to maintain biophysical properties and fitness ¹⁴. Nonetheless, such adaptability depends on differential modulation of lipid metabolism ^14–16 and genetic/kinetic constraints therein (e.g. PUFA synthesis) ^13,17, underlying dietary sensitivity. From a whole-body perspective, dietary fats may interact with many tissues to affect physiology; but notably, foods initially encounter the gut, a host–environment interface directly exposed to dietary extremes and potential perturbations, and home to a rich microbiome and much of the immune system. Accordingly, in human trials ^18–22 and rodent models ^23–31, high fat diets (i.e. 30–70% kcal; varied total) can induce heterogeneous changes to gut microbiome in association with host physiology (e.g. inflammation ^{19,21,24,26,28–31} and metabolic dysfunction ^{18–20,22,23,25,27}; reviews ^32,33). Crucially, such diets are typically high in SFAs (e.g. animal ²⁰, lard ^23,25,30 or dairy ^18,26,27,29), MUFAs (e.g. lard or olive oil ²⁷) or omega-6 PUFAs (e.g. soybean ¹⁹, corn ^24,31 or sunflower oil ²⁵), which can have differential effects (at same % kcal) ^{18,25–27,29,30}; as can moderate omega-3 PUFAs (e.g. fish oil ^{24,25,31,34–36} or EPA/DHA ³⁷; vs. lard ²⁵, dairy ^36,37 or corn oil ^24,31). Hence this post explores some of the differential effects of (natural) fats on the gut–host dialog and underlying mechanisms relevant to many diseases and systemic health.

Gut microbiome

The gut microbiome contains trillions of microbes from hundreds of species, which populate specific metabolic and mucosal niches, thereby supplementing host metabolism and colonisation resistance, and underlying a mutual symbiosis shaped by host physiology and diet. In particular, in observational studies, dietary fats have (differentially) been associated with bacterial diversity ^34,38,39, enterotypes ⁴⁰ and species/strains ^41–43; and as above, high fat diet interventions can modulate the gut microbiome. For instance, in perhaps the largest and longest RCT so far, conducted in China, with 217 healthy young adults on isocaloric 20, 30 or 40% fat diets for 6 months, higher fat consumption (mainly via soybean oil) lowered microbiota α-diversity, while modulating specific genera and metabolites ^19,44. Notably, diet can induce significant gut microbiome changes within 1–5 days in young adults ^20,21,40, although long-term diet may have stronger associations ⁴⁰. Also, in animal models, high fat diets may further modulate susceptibility to exogenous pathogen infection/colonisation (e.g. Salmonella ⁴⁵, C. difficile ⁴⁶ and C. albicans ⁴⁷).

How does fat shape the gut microbiome? In typical diets, most energy comes from carbs (4 kcal/g) and fats (9 kcal/g), which exist in balance; so generally higher fat = lower carb. This may underlie indirect effects, since gut microbes typically ferment non-digested carbs/fibre as a major energy source, resulting in production of short-chain fatty acids (SCFAs), which in turn fuel colonocytes, while lowering lumen pH and oxygen to inhibit pathogens ^48–51. Accordingly, many studies use refined sources of fats/oils, with unmatched fibre ⁵², and high fat/low carb diets can decrease SCFAs in humans ^21,22. Whereas whole nuts (i.e. fat with fibre, polyphenols, etc.) can increase SCFA-producing bacteria (review ⁵³); as also occurred with overfeeding unsaturated fat ¹⁸, in part via nuts ⁵⁴. However, other high fat diet studies affect the microbiome despite matched fibre ^{19,25,28,29,45} (and even carbs in hypercaloric studies ^18,27), with different fats having different effects (refs above). Moreover, dietary and blood levels of omega-3/DHA positively correlated (independent of fibre) with bacterial diversity and Lachnospiraceae spp. (i.e. major SCFA-producers) in a large cohort of female twins (n=876) ⁴². Accordingly, a 6-week RCT with walnut (i.e. 65% fat; 9% α-linolenic acid—C18:3, n-3) or equivalent α-linolenic acid similarly increased Roseburia (family Lachnospiraceae) ⁵⁵; and another with EPA/DHA increased Coprococcus (also family Lachnospiraceae), which correlated butyrate ⁵⁶. Therefore, omega-3s may actually support SCFA/butyrate production.

As with carbs, a portion of fat intake also escapes digestion and can be utilised by gut microbes for growth, as demonstrated in vitro with a human colon simulator ⁵⁷. Specifically, fat-only medium (resembling that in a typical western colon, Table 1) promoted growth of Alistipes, Bilophila and Gammaproteobacteria, while reducing glycan and protein degraders (e.g. Bacteroides, Clostridium and Roseburia spp.), SCFAs and antioxidants; i.e. similar to high fat diets (e.g. Table 3) ⁵⁷. Also, culturing mouse microbiome with high fat diet media (i.e. lard-based) promoted growth of Enterobacteriaceae (class Gammaproteobacteria) ²³, while EPA/DHA directly inhibited E. coli (family Enterobacteriaceae) and promoted C. glomerans (phylum Actinobacteria) ⁵⁸; i.e. both replicating changes seen in mice ^23,58. In the relatively anoxic environment of the gut, “lipophilic” bacteria may perform β-oxidation of fats via anaerobic respiration with various alternate terminal electron acceptors (e.g. sulfate, nitrate, fumarate, etc.) ⁵⁷. Exogenous fatty acids may also modulate microbial membranes and signalling/metabolism ^59–63. Notably, fungi can also metabolise fats ⁴⁷, while PUFAs ⁶⁴, MCFAs (i.e. coconut oil) ⁴⁷ and SCFAs ⁶⁵ can have antifungal activity.

Dietary fats also alter the intestinal environment, foremost by stimulating bile release, which aids digestion and also modulates the microbiome ⁶⁶. Accordingly, faecal bile acids were >3x higher in omnivores than vegans (n=32/32), and associated with higher fat and lower fibre intake ⁶⁷; while in young adults (n=297), habitual animal fat intake correlated lower bacterial diversity and SCFAs, alongside increased bile acids (ns) ³⁹. Moreover, in the large Chinese RCT above, higher fat consumption (i.e. mainly soybean oil) induced several microbiota changes (i.e. ↓ α-diversity, Faecalibacterium, SCFAs; ↑ Alistipes, Bacteriodes) ¹⁹ in association with increased bile acids ⁴⁴. Bile acids are detergents with antimicrobial properties, although some microbes are more resistant than others (e.g. Gammaproteobacteria; Fig. 2e) ^45,66; so their administration can lower microbial diversity ^66,68 and favour growth of bile-resistant microbes ^26,45. Similarly, high fat diets can increase intestinal bile acids ^{21,26,44,45,69}, while their manipulation replicates microbial changes ^26,45,66,68. For instance, high fat diets promoted Salmonella infection via bile-mediated (i.e. CA or TCA) suppression of core microbiota, but only in mice lacking competitive E. coli (another Enterobacteriaceae) ⁴⁵. A particularly susceptible bacterium was Prevotella copri ⁴⁵, which may generally decline on high fat diets ^21,27 and with Westernisation ⁷⁰. Notably, animal fats also come with cholesterol (i.e. bile acid precursor), but a high cholesterol diet did not significantly affect gut bile or microbiome, suggesting dietary fat content is more important ⁷¹.

Regarding differential effects, the abundance of sulfide-producing bacteria in human colon biopsies (from 9 healthy subjects) over several months was positively associated with intake of dairy/SFAs and negatively with MUFAs/PUFAs ⁴¹. Accordingly, high SFA/lard diets seem to particularly increase Desulfovibrionaceae (i.e. Bilophila ^{18,20,21,26,30,33,68} and Desulfovibrio ^25,27,72,73), which reduce sulfur compounds to hydrogen sulfide (H₂S). With milk fat, expansion of Bilophila wadsworthia was linked to taurine-conjugated bile acids (i.e. Fig. 3a) ²⁶, which provide organic sulfonate for sulfite respiration; and may be stimulated by long-chain SFAs (e.g. C16:0/18:0) in relation to their greater hydrophobicity ^36,74,75 and slower absorption ^76–78. Conversely, sulfide-producing bacteria did not increase with oils rich in omega-6 ^25,26, and were even suppressed by addition of moderate omega-3 ^25,36,37 (i.e. at 1–7%; not 21% ³⁰). However, both butter and refined olive oil (i.e. high MUFA) similarly increased Desulfovibrio (a sulfate-reducing genus), but not extra virgin olive oil, thus implicating other minor constituents (e.g. additives or polyphenols) ²⁷. Notably, low fibre diets can also increase Desulfovibrio ⁵², and specifically D. piger ⁷⁹, along with degradation of the mucus layer (releases sulfate) and enhanced pathogen susceptibly ⁷⁹. Conversely, prior infection may actually enhance colonisation resistance via taurine-induced sulfide production ⁸⁰.

Gut barrier

The gut microbiome is separated from the body by the gut barrier, which is demarcated by a single outer layer of epithelial cells and the mucus they exude. Diet greatly influences barrier permeability to microbes and their antigens, such as lipopolysaccharides (LPS, aka. endotoxin) from Gram-negative bacteria. In humans and animals, even single high energy/fat meals can induce a postprandial (0–5hr) increase in blood LPS ^81–83 (with inflammation and oxidative stress ^84–87), which is blocked by adding orange juice, polyphenols or fibre ^88–90.

Moreover, many studies specifically implicate saturated fat (e.g. postprandial ^85,87,91,92 and longer ^{18,23,25,30,54,93}; reviews ^81,83). For instance, in healthy adults, blood LPS was increased by 30g of cream (70% sat fat; low LPS content), but not orange juice or glucose (i.e. Fig. 3) ⁸⁵. In pigs and humans, blood LPS was increased by a porridge-based meal made with 16g coconut oil (86% sat fat), but decreased with fish oil (500mg DHA), while there was no significant effect with other oils (i.e. olive, vegetable and grapeseed oil) ^91,92; and no meal affected blood inflammatory markers ⁹². Further, in mice, 1 week of a high fat diet induced translocation of live commensal intestinal bacteria into blood and mesenteric adipose ⁹⁴; while in a subsequent study, 8 weeks of a high saturated fat diet increased colonic permeability and translocation of bacterial DNA, whereas omega-6 did not and omega-3 was ameliorative ²⁵. However, other mouse models show an elevated omega-6:3 ratio (e.g. 20:1) can increase blood LPS ^24,31,95; and in people at risk of colon cancer, the dietary omega-6:3 ratio correlated blood LPS-binding protein (LBP) ⁹⁶.

Modulation of intestinal permeability may occur via transcellular or paracellular routes (i.e. through or between cells, respectively) and various mechanisms (reviews ^81,97). Firstly, fat absorption and incorporation into chylomicrons can promote auxiliary transport of LPS (a fat-containing molecule) into peripheral blood ^81,98,99. However, the acute differential effects of saturated vs. omega-3 fats on LPS above were linked to modulation of intestinal membrane lipid rafts, which may represent an initial gateway ⁹¹. And recently, fat was shown to promote rapid transcellular transport of LPS from small intestine into portal vein via a lipid raft and CD36-dependent pathway, with slower and smaller transport into lymph via the chylomicron pathway ¹⁰⁰.

Fat digestion also liberates free fatty acids (FFAs), which increase on a high fat diet and can induce lipotoxicity ^101,102. In particular, saturated FFAs (e.g. palmitate) can induce ER/oxidative stress in intestinal cells, which was associated with inflammation and an impaired mucosal barrier ¹⁰². In addition, fat-induced changes to faecal bile acid profile (i.e. ↑ DCA:UDCA ratio) were linked to barrier dysfunction ¹⁰³; and suggested to involve DCAs surfactant-related ability to disrupt lipid bilayers (e.g. cell membranes) ¹⁰³. In an ex vivo study, fat-induced permeability mainly occurred in intestinal crypts, possibly due to the presence of immature cells lacking detergent-resistant lipid rafts with increased susceptibility to bile and FFAs ¹⁰⁴. Notably, diets high in soybean oil or lard (40% fat) similarly decreased colon tight junction expression and increased paracellular permeability in relation to secondary bile acids ¹⁰⁵; whereas saturated fat-specific barrier dysfunction has been linked to H₂S-producing bacteria ^25,37,69. Accordingly, B. wadsworthia can induce host bile and lower faecal butyrate, a barrier-supportive SCFA ⁶⁹; while H₂S can further impair SCFA metabolism by colon cells ^33,106, and reduce disulfide bonds in the mucus network, thus increasing exposure of bacteria to the epithelium ^51,107.

Host immunity

The gut also contains the largest collection of immune cells in the body (i.e. GALT), which support barrier function and interact reciprocally with microbes. As such, dietary fats can modulate immune activity via the gut microbiome. For instance, LPS binds to toll-like receptor 4 (TLR4) expressed on the surface of many cells to modulate inflammatory signalling. In the human gut, LPS largely originates from the Gram-negative Bacteroidetes phylum and has anti-inflammatory effects, as compared to that from Proteobacteria ¹⁰⁸. However, high fat diets can alter the Firmicutes to Bacteroidetes ratio ^19,21,23,26, and especially induce growth of Proteobacteria (i.e. Desulfovibrionaceae ^18,25,27 or Enterobacteriaceae ^23,31,95); as well as LPS biosynthesis ^19,23,28, translocation (see above) and TLR-related inflammation in blood ^{84,85,88–90} and organs ^23,28,30.

In addition, fats may have more direct effects. In 3-week RCTs on healthy adults, lowering the dietary palmitate/oleate ratio lowered (fasting) blood and tissue fatty acid ratios in relation to cytokine release (to LPS in vitro) ^9,10 and muscle NLRP3 inflammasome (not ER stress) ¹⁰. In healthy humans, fat-induced postprandial inflammation has been suggested to involve saturated FFAs, rather than LPS ¹⁰⁹. And in mouse models, high fat diets may impair intestinal immunity ¹⁰¹ and mucosal barrier via FFAs ¹⁰², and induce loss of gut neurons and dysmotility via both LPS and saturated FFA-mediated TLR4 signalling ²⁸. Accordingly, fats make up cell membranes, including bacterial LPS—so named because it contains fat (lipo-) and carbohydrate (polysaccharide). In particular, saturated fats acylated in the lipid A portion of LPS are essential for signalling via TLR4, as well as lipopeptide signalling via TLR2 ¹¹⁰. Further, cell membranes are heterogeneous and contain microdomains known as lipid rafts, which result from associations between cholesterol, sphingolipids and other saturated lipids, while proteins can be recruited via palmitoylations ¹¹¹. Stimulation of cells with LPS triggers formation of a TLR4–lipid raft complex, which requires activation of sphingomyelinase to produce ceramide ¹¹² (also produced de novo from palmitate). Moreover, saturated FFAs (e.g. lauric and palmitic) can also stimulate TLR4/2 signalling, via (redox-dependent ¹¹³) receptor dimerization and translocation into lipid rafts, which can be inhibited by DHA/omega-3 ^{110,113–116}. Note, intestinal enterocytes also mediate uptake of LPS via TLR4 ^117,118, which may mediate the differential effects of fats (and antioxidants) on intestinal permeability above ⁹¹ (e.g. figure below); while omega-3s were further shown to enrich the microdomains of tight junctions with unsaturated fats and prevent intestinal permeability induced by inflammatory cytokines ^119,120. Inside cells, palmitate can induce synthesis of saturated glycerolipids/ceramide, lower ER membrane fluidity ¹²¹, and activate ER stress and NF-kB/NLRP3 inflammasome pathways ^122–124, all of which is inhibited by unsaturated fats (i.e. oleate and DHA). Note however, some potential methodological issues with saturated fats in cell studies ^125,126.

Omega-6 and 3 PUFAs broadly serve as precursors to pro- and anti-inflammatory mediators (i.e. oxylipins/eicosanoids), respectively, which may be paralleled by corresponding regulation of pro- and anti-inflammatory bacteria ^{24,25,31,35,37,95,127}. For instance, in a 6-month RCT, higher dietary fat (i.e. 20 vs. 40% kcals; mostly via soybean oil—rich in C18:2, n-6) induced proinflammatory changes, including bacterial LPS biosynthesis and arachidonic acid (AA; i.e. C20:4, n-6) metabolism, while changes in faecal AA correlated its derivatives (i.e. PGE₂ and TXB₂) and CRP in plasma ¹⁹. An omega-3 trial also revealed a modest post-treatment inverse relation between bacterial diversity and colonic PGE₂ ³⁴. In mice, systemic administration of resolvin D1 (derived from omega-3/DHA) ameliorated saturated fat-induced gut dysbiosis, permeability and inflammation ²⁵. Further, in transgenic mouse models, a high tissue omega-6:3 ratio promoted systemic inflammation and gut dysbiosis characterised by overgrowth of Enterobacteriaceae and suppression of Bifidobacteria ^31,95. This was linked to modulation of intestinal alkaline phosphatase (IAP) activity; where omega-3 induction of IAP reversed gut dysbiosis and decreased LPS production, translocation and inflammation ³¹. However, while a high omega-6 diet exacerbated infectious colitis, addition of omega-3 promoted sepsis, alongside suppression of immune/IAP responses ²⁴; thus illustrating the potential for harm from excess (reviews ^128,129). Notably, both saturated ^23,25 and omega-6 fats ^24,95 can promote intestinal inflammation, which itself can induce luminal release of respiratory acceptors (e.g. nitrate) for Enterobacteriaceae allowing them to bloom ^50,130.

All these pathways further regulate T and B cells, and thus adaptive immunity. For instance, LPS–TLR4 signalling promotes Th17 activity and worsens experimental autoimmune encephalomyelitis (EAE) ¹³¹, an animal model of brain inflammation and multiple sclerosis (MS). Similarly, another study showed that dietary long-chain saturated fats (i.e. palmitic and lauric acid) promoted Th1/17 activity and worsened EAE, via suppression of intestinal SCFAs which induce Tregs ¹³². Interestingly, in humans, Dr. Swank published repeatedly on a link between animal/saturated fat and MS, including the long-term effect of a low fat diet on disease activity (YouTube). More recently, a low cholesterol diet (i.e. low animal fat?) was also shown to lower autoimmune potential (i.e. Th17/Treg balance) in people with HCV ¹³³. Notably, compared to typical high fat diets, ketogenic diets (e.g. 80% fat, 5% carb) have distinct effects on the microbiome, mediated by the antimicrobial activity of ketones, resulting in lower intestinal Th17 cells ¹³⁴.

Host energy

A primary function of the gut is to digest and absorb nutrients, where a symbiotic relationship with microbes extends and modulates systemic metabolism ^135–138. As a macronutrient, dietary fat also fundamentally shapes metabolism, both directly and via microbes ^33,139. For instance, putting healthy young, but slightly overweight, men on eucaloric diets high in SFAs or PUFAs (64% kcal) for 6 weeks induced extensive metabolic adaptation, with maintenance of insulin sensitivity ¹⁴⁰. However, excessive fat intake (i.e. SFAs, MUFAs or PUFAs) can rapidly induce metabolic dysfunction ^141–146, with saturated fat seeming most detrimental ^54,141,147. For instance, in humans and mice, a single high dose of palm oil (50% sat fat) initiated whole-body, liver and adipose insulin resistance, alongside liver LPS signalling (not blood inflammation) ¹⁴³.

Importantly, the small intestine also becomes insulin resistant early in human obesity ¹⁴⁸, while itself regulating systemic glucose control (e.g. whole-body ¹⁴⁹ and brain ¹⁵⁰), as revealed by gastric bypass. This may involve intestinal gluconeogenesis and portal glucose, a signal sensed by nerves regulating gut–brain signalling and systemic energy homeostasis ¹⁵¹. In the post-absorptive period, this signal may be sustained by protein or fibre, which serve as reservoirs of gluconeogenic substrates, promoting satiety ¹⁵¹. Conversely, a high fat diet (36% safflower oil—rich in C18:2, n-6) impaired small intestinal glucose uptake and gluconeogenesis ^151,152. In human obesity, enterocyte GLUT2 location is altered (i.e. apical and endosomal), in relation to insulin resistance, intestinal inflammation (i.e. CD8αβ T cells) and dietary calories/macros ^153,154, while high fat diet models support causality ¹⁵³. In fasting hyperglycaemic mice, apical GLUT2 increased blood–lumen glucose flux, with potential consequence for gut microbiome ¹⁵³; note, small intestinal bacterial overgrowth (SIBO) is increased in metabolic disease ⁸¹ and obesity in relation to refined carb intake ¹⁵⁵. Insulin also normally suppresses enterocyte lipid secretion (via chylomicrons), which was inhibited in mice fed palm vs. olive oil, which may promote postprandial hyperlipidaemia (a risk factor for CVD) ¹⁵⁶.

In the colon, bacterial fermentation of carbs/fibre generates SCFAs and succinate which fuel intestinal metabolism and mediate gut–host signalling to regulate systemic energy metabolism (reviews ^137,138). For instance, in overweight men, acute colonic infusion of SCFAs induced fasting fat oxidation ¹⁵⁷; and in mice on a high fat diet, butyrate prevented insulin resistance and obesity, while increasing energy expenditure and mitochondrial function ¹⁵⁸. Accordingly, high fat diets may lower SCFAs (as above) and suppress Prevotella copri ^{21,27,40,45,70}, a succinate-producer with metabolic benefits ¹⁵⁹ and human health association (n=1098) ¹⁶⁰, while promoting overgrowth of pathobionts with detrimental effects. For instance, transplantation of specific Enterobacteriaceae from obese humans to mice fed a high fat diet induced endotoxin-dependant obesity, insulin resistance ¹⁶¹ and fatty liver ¹⁶². Also, in overweight adults, overfeeding saturated fat increased Proteobacteria and liver fat ⁵⁴, which was associated with baseline Bilophila ¹⁸. In mice, B. wadsworthia aggravated metabolic dysfunctions, via inflammation-dependent and independent pathways ⁶⁹. Similarly, in other studies, high fat diets induced gut microbiome changes which promoted inflammation and metabolic dysfunction, but largely independently ^23,25,30. For instance, both high saturated and omega-6 fat diets induced weight gain; while only saturated fat induced H₂S-producing bacteria (i.e. Desulfovibrio and Bilophila), leaky gut, inflammation and insulin resistance; and omega-3 ameliorated gut changes, but not insulin resistance ²⁵. Omega-6 fats are also precursors to endocannabinoids which regulate gut–host physiology ¹⁶³; while low–high fat diets with a high omega-6:3 ratio induced endocannabinoids and adiposity (not insulin) ^164,165.

In humans and mice, acute high fat diet-induced muscle insulin resistance was causally linked to increased mtROS and oxidation of GSH redox ¹⁴⁵. Accordingly, compared to carbs, fat oxidation generates more ROS, although this was lower with medium vs. long-chain fats (i.e. C10:0/12:0 vs. C16:0/18:1/18:2) ¹⁶⁶ and may involve LCAD ¹⁶⁷. In human tracer studies, dietary fat oxidation also decreases with chain length (e.g. C12:0>16:0>18:0 ¹⁶⁸ and C18:3>22:6 ¹⁶⁹) and saturation (e.g. C16:0/18:0 vs. C18:1/18:2/18:3; Fig. 4) ^168,170,171. Further, in a series of 3–4 week RCTs on healthy adults, lowering the habitual dietary palmitate/oleate ratio (i.e. C16:0/18:1) lowered cytokines ^9,10 and raised systemic energy/lipid metabolism ^5–8,11. And in humans spanning the range of insulin sensitivity (i.e. athletes–lean–obese–T2D), there were inverse associations with intramuscular saturated fats and sphingolipids ^172,173 (and SFA intake was increased in T2D ¹⁷²); while in myotubes, palmitate (not oleate) oxidation was impaired in T2D ¹⁷⁴. Blood FFAs are also elevated in metabolic diseases and ageing, with their saturated fat content (e.g. palmitate) linked to systemic lipotoxic effects (e.g. inflammation, insulin resistance, endothelial dysfunction, etc.) ^114,175,176 and unsaturated fat (e.g. oleate and omega-3s) to protection ^177–180. For instance, plasma palmitate fuels intramuscular ceramide synthesis in vivo ^181,182, and induces ceramide-dependent insulin resistance in vitro (e.g. gut ¹⁵⁶, vascular ^175,183 and muscle cells ^184–186), via TLR4-dependent ceramide biosynthesis ¹⁸⁵ and mtROS ¹⁸⁴, which can be ameliorated by oleate ^184,186. Moreover, cerebrospinal palmitate is also increased in cardiometabolic disease; while palmitate (not oleate) infusion impaired memory in mice and neuronal insulin signalling in vitro via TNFα ¹⁸⁷.

Host lipids

Importantly, fatty acids are also synthesised endogenously via de novo lipogenesis (from sugars), while FFAs are released by triglyceride lipolysis (inhibited by insulin). Consequently, eucaloric low carb/high fat diets can decrease lipogenesis and triglycerides ^140,188, whereas overfeeding simple carbs increased lipogenesis and triglyceride saturated fat % ⁵⁴. On the other hand, an Atkins-style low carb/high fat weight loss diet decreased postprandial insulin and increased FFAs ¹⁸⁹, while overfeeding saturated fat (not unsaturated fat or simple carbs) increased lipolysis and blocked insulin-dependent suppression of FFAs ⁵⁴; alongside increased endotoxemia, ceramides and insulin resistance ⁵⁴. Notably, in people injected with LPS, insulin infusion also completely suppressed LPS-induced nitro-oxidative stress and FFAs ¹⁹⁰. Low carb diets high in saturated fat also increase cholesterol (esp. LDL-c ^140,188,191, which correlated FFAs ¹⁸⁹), while those favouring unsaturated fat can induce lower blood lipids (e.g. lipogenesis/desaturation ¹⁴⁰, triglycerides ¹⁹¹, SFA/PUFA ratios ^140,188 and LDL-c ^140,188,191) with higher ketones (e.g. after 5 days ¹⁹¹ or 6 weeks ^140,188). Notably, fats and ketogenic diets can also differentially affect insulin sensitivity ¹⁹¹, while hepatic insulin signalling regulates lipid/lipoprotein metabolism ^192–194; although differential lipid responses are still present even with preservation of insulin sensitivity ^140,188. Saturated fats may favour more lipogenesis–desaturation to produce MUFAs (e.g. C16:1/18:1) ^15,195, which support storage (as triglycerides) and maintain cell physiology (e.g. membrane fluidity and insulin sensitivity), buffering toxicity ¹⁵; whereas unsaturated fats lower desaturation/SCD activity ^140,196,197. Further, the greater oxidation rate of long-chain unsaturated vs. saturated fats may favour more ketogenesis ^169,198; and ketones can modulate gut microbiome and immunity ¹³⁴. Long-chain omega-3s in particular can also lower triglycerides ¹⁹⁹, by lowering VLDL production and increasing fatty acid uptake/oxidation ^200,201. Moreover, in a 6-week RCT, omega-3 supplementation increased Coprococcus, which correlated butyrate (+) and VLDL-TGs (–), suggesting a role for the microbiome ⁵⁶.

The blood cholesterol-raising effect of saturated fats is mainly attributable to C12–16:0 (not C18:0/stearic) ¹⁷⁰ and related to lower LDL receptor expression (i.e. hepatic LDL uptake) ^16,202,203; and potentially differential effects on cellular lipid homeostasis (e.g. via SREBPs) ^{16,195,204,205} and systemic turnover (e.g. intestinal absorption ^206,207 and bile/excretion ^16,202). Further, diets high in saturated fat can also promote lipoprotein oxidative/inflammatory modifications (e.g. vs. low fat ^208–210, MUFAs ^210–212 or PUFAs ^213,214). In particular, in mice, a high saturated fat diet (vs. MUFA diet) increased body weight and HDL-c but impaired reverse cholesterol transport (RCT) from liver–faeces, alongside increased liver/adipose cholesterol and inflammation, and enrichment of HDL with acute-phase proteins ²¹². A similar diet also enriched liver and LDL with sphingomyelin (SM) and ceramide ²⁰⁸, which can mediate LDL aggregation, oxidation ²⁰⁸, inflammatory and apoptotic activity ²¹⁵; and deliver ceramide to macrophages and muscle inducing insulin resistance ²¹⁶. In humans, LDL aggregation was independently associated with CVD mortality and reduced by a ‘healthy Nordic diet’ in relation to increased PUFA intake and LDL-PC/SM ratios (i.e. Fig. S5B) ²¹⁷. Note, both aggregated and oxidised LDL can induce foam cell formation (an early stage of atherosclerosis) ²¹⁸, via TLR4 ²¹⁹ and in synergy with LPS ²²⁰.

As discussed earlier, dietary fat can modulate LPS translocation via portal and lymph-chylomicron pathways ^98,100. The former would deliver LPS to the liver, where it may induce inflammation ¹⁰⁰ (even in absence of blood inflammation ¹⁴³); albeit limited by enteric release of HDL₃, which is complexed with LBP and can bind and deactivate LPS ²²¹. Similarly, the affinity of chylomicrons (aka. ULDL) for LPS is mediated by LBP and prevents endotoxin activity ²²²; although LPS-containing chylomicrons still activated lymph nodes ⁹⁸, and may be delivered to other tissues such as adipose ^223,224 and endothelium ²²⁵. In blood, LPS and Gram-positive LTA have affinity for all lipoproteins ²²⁶, especially HDL ²²⁷, and are subsequently transferred to LDL ^228,229, while the hepatic LDL receptor plays a major role in their clearance ^230,231 (e.g. figure below). Therefore lipoproteins (forward–reverse) transport both host and microbial lipids, subserving lipid homeostasis and innate immunity ²³². Further, systemic infections/LPS induce many changes to lipid pathways, including lipid biosynthesis (e.g. fatty acids and sphingolipids), lipoprotein modifications and suppression of RCT, which may acutely support immunity, but if prolonged become harmful by promoting atherosclerosis ^226,232,233; while diet can also chronically modulate these pathways. Accordingly, in a meta-analysis of 3 Finnish cohorts (n=7178), blood LPS was associated with an adverse metabolic profile (incl. lipoprotein sizes, fatty acids, glycation and amino acids), and more so with increased BMI and MetS ²³⁴. Further, compared to healthy weight people, metabolic disorders (e.g. obesity ⁹⁹ and T2D ²³⁵) may be even more sensitive to saturated fat, with greater increases in postprandial LPS (and chylomicron enrichment ⁹⁹), alongside decreasing LDL-c and HDL-c (not total cholesterol) ²³⁵; while in people with coronary disease (CORDIOPREV cohort), a high postprandial LPS predicted later development of T2D ²³⁶. This sensitivity might involve differences in gut LPS ^{18,108,161,162} and blood transport/clearance ^99,235. Also, in human obesity, a subtly altered small intestinal barrier was exacerbated by lipid challenge (in vitro) and correlated intestinal/systemic inflammation and T2D ²³⁷; while postprandial LPS in chylomicrons correlated triglyceride responses ^99,238, which may relate to intestinal insulin resistance, as above.

Dietary fats may also differentially affect metabolism of amyloid-beta (Aβ)—the major protein in brain plaques characterising Alzheimer’s disease (AD). In humans, brain amyloid burden is highly correlated with blood Aβ peptides ^239,240, which largely associate with triglyceride-rich lipoproteins originating from gut and liver (i.e. chylomicrons and VLDL, respectively), also detectable in human CSF ^241,242. In mice, a diet high in saturated fat (not MUFAs or PUFAs) increased Aβ production in enterocytes (concomitant with apoB-48 chylomicrons ²⁴³) and accumulation in blood ²⁴⁴; and induced disruption of blood–brain barrier allowing delivery of plasma proteins, including apoB-lipoproteins enriched with Aβ, into the brain ²⁴⁵. High (saturated) fat diet-induced fatty liver was also linked to AD ²⁴⁶; and Aβ production restricted to liver was shown to induce neurovascular dysfunction and inflammation with parenchymal extravasation ²⁴⁷. Moreover, both apoB-lipoproteins ²⁴⁸ and Aβ ^249,250 are cleared by hepatic LDLR and LRP-1, while their induction lowers brain amyloid ^249,251,252; further supporting a lipoprotein-Aβ vascular–brain axis ^243,247,253. Interestingly, people with Alzheimer’s had increased (3-fold) postprandial chylomicrons, which may increase exposure to peripheral Aβ ²⁴¹. In addition, the gut microbiome also likely plays an important role in AD ^254,255; and in particular, gut Proteobacteria/SCFAs and blood LPS/SCFAs/cytokines have correlated CSF/brain amyloid ^255–257, and can be modulated by diet as above. Further, LPS may be elevated in the brain and induce AD-like pathology ^254,258, while Aβ may serve as an antimicrobial peptide in innate immunity ²⁵⁹.

Fats may also modulate systemic health via the meta-organismal TMAO pathway; where gut bacteria convert dietary choline/carnitine to TMA, which host FMO3 converts to TMAO. In humans, a 5-day high fat diet (50% sat fat) increased postprandial TMAO ²⁶⁰, while 4 weeks of a high (animal) fat Atkins diet increased fasting TMAO ²⁶¹. Also, in a transgenic mouse model, an increased tissue omega-6:3 ratio elevated both faecal and blood TMAO ⁹⁵. Fats may influence TMAO at various levels. Firstly, saturated fats can come with TMAO precursors in animal foods ²⁶⁰. Also, TMAO increases with age ²⁶², which in mice was accompanied by increased Proteobacteria and Desulfovibrio (capable of producing TMA) ²⁶³, which can also be induced by fats (as above). Further, FMO3 is also under complex genetic and dietary regulation, including by bile–FXR signalling ²⁶⁴; and itself controls cholesterol balance ²⁶⁵ and obesity ²⁶⁶.

Perspective

In summary, dietary fats may differentially affect gut–host health; with higher levels of saturated (vs. unsaturated) and omega-6 (vs. omega-3) fats having unfavourable effects at multiple levels (i.e. gut microbiome and barrier; host immunity and metabolism). Mechanistically, the different physiochemical properties of saturated and unsaturated fats may require different bile acid responses for digestion and metabolic responses for homeostasis, while specific fatty acids serve as mediators/precursors regulating specific metabolic-immune pathways, with imbalances in any of these factors promoting dysfunction. In particular, as the first organ exposed to diet, the gut may represent ‘ground zero’ for dietary perturbations, while gut–host reciprocity may modulate systemic pathways (e.g. LPS/TLR signalling) involved in many chronic diseases (e.g. NAFLD, MetS, T2D, CVD, AD, etc.) and ageing, contributing to the differential effects of fats on systemic health.

In accord, dietary guidelines typically advise lowering saturated fats (e.g. <10% kcals) ²¹² and favouring foods richer in unsaturated fats and omega-3s (FAO). Some further considerations also seem noteworthy. Dietary fat content varies widely around the world, and generally increases with latitude, wealth and Westernisation ^19,70,267. In 2013, in the UK and US respectively, 36% and 39% of calories came from fats, and much from refined sources such as oils (OWID). Compared to natural wholefoods, these sources isolate the fat/calories and fat-soluble nutrients, at the expense of everything else. For instance, compared to extra virgin olive oil (100% fat), whole olives are ~10–15% fat, which comes with gut-supporting fibre and >5x more polyphenols ²⁶⁸ (e.g. oleuropein and hydroxytyrosol); although curing does add salt. Moreover, the contemporary increase in oil consumption may have particularly increased omega-6 intake, modulating endogenous omega-3 levels and 6:3 ratios ^{17,165,269,270}, and chronic inflammation/disease ^{13,19,95,165,271}. On the other hand, with the popularisation of fish oil, its excessive consumption also has potential to be harmful, especially during infections ^128,129. PUFAs are also prone to oxidation, which may be promoted by oil isolation (i.e. removal of protective food matrix/antiox), heating and even digestion; the latter being catalysed by co-ingestion of heme-iron (in red meat) and inhibited by polyphenols (in plants) ²⁷². This may impact bioactivity, since oxidised fish oil had less favourable effects on lipoproteins in humans ²⁷³, and worsened gut dysbiosis in an animal model ²⁷⁴. Notably, both animal and plant fat sources (esp. dry heated) can also be high in advanced glycation end-products (AGEs) ²⁷⁵, which may also affect gut–host health ²⁷⁶ and blood lipids (incl. omega-3) ²⁷⁷.

Finally, PUFA status is not only related dietary content, but also fatty acid metabolism and genetic variation therein, supporting personalised nutrition ¹⁷. Strong relative correlations have been reported between changes to the blood omega-3 index (i.e. RBC EPA + DHA) and gastrointestinal tissues, suggesting it could serve as a surrogate measure ²⁷⁸.

Resources

• Sources of saturated fats
• Omega-6/3 content of foods

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