{"schemaVersion":1,"id":"en-mechanism-dietary-fat","type":"mechanism","pillar":"energy","locale":"en","translationGroupId":"mechanism-dietary-fat","slugPath":"energy/what-happens-to-dietary-fat","title":"What happens to dietary fat","shortTitle":"Dietary fat","summary":"Fat does not dissolve in water: its route out of the intestine resembles that of no other nutrient. It joins an enormous store, and its share during effort follows a curve that rises and then falls.","level":"intermediate","estimatedMinutes":9,"synonyms":["dietary fat","fats","fatty acids","triglycerides","body fat","fat oxidation"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"A nutrient that does not mix with water","anchor":"a-nutrient-that-does-not-mix-with-water"},{"id":"b2","type":"paragraph","content":[{"kind":"link","value":"Protein","href":"/en/adaptations/what-happens-to-dietary-protein","external":false},{"kind":"text","value":" and "},{"kind":"link","value":"carbohydrate","href":"/en/energy/what-happens-to-dietary-carbohydrate","external":false},{"kind":"text","value":" share one thing: once cut up, their fragments dissolve in the blood and travel as they are. Fat cannot. Its whole story follows from that single property."}]},{"id":"b3","type":"callout","tone":"insight","title":"The answer in three sentences","content":[[{"kind":"text","value":"A dietary fat must first be dispersed into fine droplets before it can be cut up, and it is then rebuilt and packaged to travel through a fluid that will not accept it."}],[{"kind":"text","value":"That package does not take the route of the other nutrients: it joins the lymphatic circulation, and enters the blood without passing through the liver first."}]]},{"id":"b4","type":"heading","level":2,"text":"From the plate to the circulation","anchor":"from-the-plate-to-the-circulation"},{"id":"b5","type":"definition","term":"Triglyceride","definition":"A molecule made of three fatty acids fixed to a common backbone. It is the form in which fat is eaten, transported and stored.","alsoKnownAs":["storage fat"]},{"id":"b6","type":"paragraph","content":[{"kind":"text","value":"Fat digestion really begins in the small intestine. Bile, produced by the liver, disperses it into minute droplets — without which enzymes would only reach the surface of a compact mass. Lipases then detach the fatty acids from their backbone."}]},{"id":"b7","type":"paragraph","content":[{"kind":"text","value":"Those fragments cross the intestinal wall, where they are "},{"kind":"strong","value":"reassembled"},{"kind":"text","value":" into triglycerides, then wrapped in transport particles. Those particles are too large to cross the wall of the blood capillaries: they take the lymphatic circulation and join the blood only after a long detour, upstream of the heart."}]},{"id":"b8","type":"paragraph","content":[{"kind":"text","value":"The consequence is clear. A carbohydrate must pass through the liver before any other organ; a fat need not. It circulates in the blood first, where muscle and adipose tissue take their share, and the liver sees only what is left."}]},{"id":"b9","type":"heading","level":2,"text":"A store on another scale","anchor":"a-store-on-another-scale"},{"id":"b10","type":"paragraph","content":[{"kind":"text","value":"The body stores fat in two places, and both matter."}]},{"id":"b11","type":"list","ordered":false,"items":[[{"kind":"strong","value":"Adipose tissue"},{"kind":"text","value":" is the main store. It is spread throughout the body and has no fixed limit: it extends according to what it receives."}],[{"kind":"strong","value":"The muscle fibre itself"},{"kind":"text","value":" holds small lipid droplets, right next to the mitochondria that consume them."}]]},{"id":"b12","type":"paragraph","content":[{"kind":"text","value":"That second store deserves a note, because the "},{"kind":"link","value":"how muscle grows","href":"/en/adaptations/how-muscle-grows","external":false},{"kind":"text","value":" page dismisses the idea that fibres \"fill with fat\". That is correct, and it is not a contradiction: intramuscular droplets are a distinct compartment, small in size, whose presence is a normal feature of trained muscle rather than a sign of fattening."}]},{"id":"b13","type":"paragraph","content":[{"kind":"text","value":"Compared with the "},{"kind":"link","value":"glycogen","href":"/en/energy/muscle-glycogen-and-water","external":false},{"kind":"text","value":" store, the fat store is of a different order. Glycogen empties within a few hours of sustained effort; the fat store is not exhausted within a session, however long it runs."}]},{"id":"b14","type":"heading","level":2,"text":"What decides the share of fat","anchor":"what-decides-the-share-of-fat"},{"id":"b15","type":"paragraph","content":[{"kind":"text","value":"This is the least intuitive point on the subject, and it completes what the "},{"kind":"link","value":"aerobic system","href":"/en/energy/the-aerobic-system","external":false},{"kind":"text","value":" page describes."}]},{"id":"b16","type":"evidence","level":"established","statement":"As exercise intensity rises, the amount of fat oxidised per minute increases at first, then falls beyond an intermediate intensity, while the relative share of fat decreases continuously.","detail":"Both protocols combine isotopic tracers and muscle biopsies, on a cycle ergometer, in small samples of trained subjects put through increasing intensity stages. They measure oxidation rates during effort, not a change in body fat. The turning point varies with training, with recent diet and with the protocol.","referenceIds":["romijn1993","vanloon2001"]},{"id":"b17","type":"paragraph","content":[{"kind":"text","value":"Two different curves therefore hide behind the same question. In "},{"kind":"strong","value":"proportion"},{"kind":"text","value":", the share of fat falls as soon as intensity rises. In "},{"kind":"strong","value":"absolute amount"},{"kind":"text","value":", it rises at first, because total expenditure grows faster than the proportion falls — and then it comes back down as intensity keeps increasing."}]},{"id":"b18","type":"evidence","level":"established","statement":"The regulation of fat use plays out not at a single point but along the whole chain: entry into the cell, internal transport, storage and release, then entry into the mitochondrion.","detail":"The review gathers a decade of work identifying the transport proteins involved and their movement towards the membranes during exercise. That multiplicity of control points explains why no single factor — a food, an intensity — determines the share of fat on its own.","referenceIds":["spriet2014"]},{"id":"b19","type":"heading","level":2,"text":"Three received ideas","anchor":"three-received-ideas"},{"id":"b20","type":"callout","tone":"caution","title":"What the measurements contradict","content":[[{"kind":"strong","value":"That there would be a zone where you burn fat."},{"kind":"text","value":" The "},{"kind":"link","value":"aerobic system","href":"/en/energy/the-aerobic-system","external":false},{"kind":"text","value":" page already covers this: the share of fat is indeed much higher at low intensity, but total expenditure is lower there, and none of it transfers directly to body fat."}],[{"kind":"strong","value":"That fat would come into play only once glycogen is gone."},{"kind":"text","value":" Both fuels are oxidised in parallel at all times. What changes is their proportion."}],[{"kind":"strong","value":"That the fat you eat becomes the fat you carry."},{"kind":"text","value":" A dietary fat is cut up, reassembled, packaged, distributed, taken up and reworked. The store is fed and drawn on continuously; its size depends on a balance over time, not on the journey of one food."}]]},{"id":"b21","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b22","type":"callout","tone":"limit","title":"A tissue absent from the models","content":[[{"kind":"text","value":"Body Lab's scenes derive from anatomical sources providing the skeleton, the muscles, the joints and the regions of the body. Adipose tissue does not appear there as a distinct part, and the lipid droplets of the fibre are far below what an anatomical mesh represents."}],[{"kind":"text","value":"This page therefore joins "},{"kind":"link","value":"fascia","href":"/en/body/fascia","external":false},{"kind":"text","value":" and "},{"kind":"link","value":"cartilage","href":"/en/body/cartilage","external":false},{"kind":"text","value":": the corpus describes it without being able to show it, and says so rather than attaching it to a scene that is about something else."}]]},{"id":"b23","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b24","type":"callout","tone":"limit","title":"No quantity, no pace, no ranking","content":[[{"kind":"text","value":"Body Lab describes mechanisms. It gives no exercise intensity, no duration, no amount of fat and ranks no type of dietary fat: that would be personalised advice."}],[{"kind":"text","value":"Questions of body composition, diet, blood lipids or cardiovascular health belong to a health professional, who has the context a page does not."}]]},{"id":"b25","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b26","type":"sourceList","title":"Main sources","referenceIds":["spriet2014","romijn1993","vanloon2001"]},{"id":"b27","type":"shapierAction","actionId":"nutrition-macros","label":"Set your macronutrient split","description":"Setting the share of fat in your daily intake is done in Shapier; Body Lab only explains what happens to that fat once eaten.","webTarget":"https://shapier.app/en/conseils/calculer-macros","appTarget":"shapier://conseils/calculer-macros"},{"id":"b28","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"Why do absorbed fats reach the blood through the lymphatic circulation?","choices":[{"id":"q1c1","label":"Because the particles carrying the reassembled triglycerides are too large to cross the wall of the blood capillaries","correct":true,"explanation":"As the page describes, unable to enter the capillaries, these particles take the lymph and only join the blood upstream of the heart."},{"id":"q1c2","label":"To pass through the liver first, like all nutrients","correct":false,"explanation":"That is the carbohydrate's route, not the fat's: a fat circulates in the blood first and the liver sees only what is left."},{"id":"q1c3","label":"Because fatty acids dissolve poorly in the blood and have to be diluted in lymph","correct":false,"explanation":"Fat is not diluted anywhere: it travels packaged in transport particles precisely because, unlike carbohydrate fragments, it is not soluble in water."}]},{"id":"q2","prompt":"As exercise intensity rises, what happens to the absolute amount of fat oxidised per minute?","choices":[{"id":"q2c1","label":"It increases at first, then decreases beyond an intermediate intensity","correct":true,"explanation":"Total expenditure initially grows faster than the share of fat falls, then the amount comes back down as intensity keeps increasing."},{"id":"q2c2","label":"It decreases continuously as soon as intensity rises","correct":false,"explanation":"That describes the relative share of fat, not the absolute amount: the two curves part ways from the lowest intensities."},{"id":"q2c3","label":"It increases continuously with intensity","correct":false,"explanation":"The curve comes back down beyond an intermediate intensity, whose position varies with training, recent diet and protocol."}]},{"id":"q3","prompt":"Does fat serve as fuel only once glycogen is depleted?","choices":[{"id":"q3c1","label":"No: both fuels are oxidised in parallel at all times, and only their proportion changes","correct":true,"explanation":"The page lists this idea among those that the measurements contradict."},{"id":"q3c2","label":"Yes: glycogen empties within a few hours of effort, and fat then takes over","correct":false,"explanation":"Glycogen does empty within hours, but fat does not wait for that moment: both fuels are oxidised in parallel at all times."},{"id":"q3c3","label":"Yes: as long as glycogen remains available, fat is not oxidised","correct":false,"explanation":"That assumes a queue between fuels; the measurements show simultaneous oxidation instead, with only the proportion varying."}]}]},{"id":"b29","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-aerobic-system","en-mechanism-dietary-carbohydrate","en-mechanism-dietary-protein"]}],"relations":[{"type":"prerequisite","targetId":"en-mechanism-aerobic-system"},{"type":"explains","targetId":"en-mechanism-dietary-carbohydrate"},{"type":"next-step","targetId":"en-mechanism-dietary-protein"}],"authors":["equipe-editoriale-shapier"],"reviewers":["Thanh Chau"],"references":[{"id":"spriet2014","authors":"Spriet LL","year":2014,"title":"New insights into the interaction of carbohydrate and fat metabolism during exercise","source":"Sports Medicine","kind":"review","doi":"10.1007/s40279-014-0154-1"},{"id":"romijn1993","authors":"Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E, Wolfe RR","year":1993,"title":"Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration","source":"American Journal of Physiology","kind":"journal-article","doi":"10.1152/ajpendo.1993.265.3.e380"},{"id":"vanloon2001","authors":"van Loon LJC, Greenhaff PL, Constantin-Teodosiu D, Saris WHM, Wagenmakers AJM","year":2001,"title":"The effects of increasing exercise intensity on muscle fuel utilisation in humans","source":"The Journal of Physiology","kind":"journal-article","doi":"10.1111/j.1469-7793.2001.00295.x"}],"review":{"publishedAt":"2026-08-02","reviewedAt":"2026-08-02","reviewDueAt":"2027-08-02","evidenceLevel":"established"},"limitations":["No scene accompanies this page: neither adipose tissue nor the lipid droplets of the muscle fibre exists as a separate mesh in the anatomical sources used.","Measurements of the share of fat during effort come from cycle ergometer protocols, in small samples of trained subjects, using isotopic tracers and biopsies: they describe rates, not a loss of body fat.","No intensity, no duration and no pace is given here: that would be personalised advice.","This page ranks no type of dietary fat and addresses no question of cardiovascular health, which belongs to a health professional."],"seo":{"title":"What happens to dietary fat — absorption, storage and use","description":"Why fat travels through the lymph, where it is stored, and how its share during effort rises and then falls as intensity increases.","canonicalPath":"/en/energy/what-happens-to-dietary-fat","image":"/og/en/mechanism-dietary-fat.png"},"app":{"offlineEligible":true},"version":1}