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Dietary fat contains lots of calories, just like carbohydrates.

But the metabolic response differs.

For example, for ingested fat, the body has a much lower insulin response.

Is all the fat you eat metabolised or stored, or does the body have a (protection?) mechanism to respond to large amounts of dietary fat by simply letting it pass straight through?

52d6c6af
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  • Comments are not for extended discussion; this conversation has been moved to chat. – Carey Gregory Aug 02 '19 at 13:30
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    The more fat you eat, the better you become at absorbing it: https://www.sciencedaily.com/releases/2018/04/180411131639.htm – Count Iblis Aug 02 '19 at 22:46
  • @CountIblis - Are you able to use this in an answer? – Chris Rogers Aug 04 '19 at 11:23
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    Welcome to MedicalSciences.SE! Please take the [tour] and read the [help]. For reasons mentioned in this post and in [ask], we require prior research information when asking questions. Please help us to help you and [edit] your question to provide more information on what you have read on this subject, what made you ask this question, and any problems you are having understanding your research. This helps to provide an answer which will be more helpful. If you found nothing, what did you Google? – Chris Rogers Aug 04 '19 at 11:24
  • @careygregory By being so quick to move discussion to chat, useful context has been lost. – 52d6c6af Aug 05 '19 at 10:18
  • @Ben It's not lost. It's preserved in the chat room. Comments aren't for extended discussion so if there's important context it needs to be edited into the question. – Carey Gregory Aug 05 '19 at 14:17
  • I'm aware it is not lost. But the number of click-throughs to the chat will be minimal. The comment discussion could simply have been left here, and the answer below might then have referred to it (because it also discussed the absorbtion of nutrients). Now, relevant discussion has been hidden in the chat, and this Q&A is the worse for it. Put another way: how has this Q&A been improved by hiding that discussion on another page? – 52d6c6af Aug 05 '19 at 14:32
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    Ben, I used most of that conversation in my answer below. – Jan Aug 07 '19 at 14:57

2 Answers2

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Not all the fat we eat is absorbed, but if we eat more fat then the body will become more efficient at absorbing fats. This is because certain intestinal microbes in the small bowel that help us to absorb fats, will increase in numbers in response to an increase in the amount of fat available there. There must, of course, be a good reason why the feedback mechanism is positive instead of negative, as evolution can be assumed to have led to a well designed body. In Nature animals don't have an obesity problem, they need to survive on the sources of calories they can actually find. So, if there is a shift from carbs to fats in the diet, it makes sense that the animal will become better at absorbing fats rather than becoming worse at absorbing fats.

Count Iblis
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  • The "in nature" argument needs more detail and backup, after reading about 'obese wild animals' and studies like https://www.livescience.com/10277-obesity-rise-animals.html – LаngLаngС Aug 05 '19 at 09:06
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After ingestion of even large amount of fat (like 200 g/day), most of it will be absorbed and then either used as an energy source or stored as body fat. In the context of weight gain, a healthy human body has no effective mechanism to "ignore" fat once it is ingested. Ingestion of abnormally large amounts of fat at once could trigger diarrhea, though.

How do we know that most of fat is absorbed?

There is a fecal fat test in which you are instructed to consume 100-150 g fat per day for few days and then the amount of fat in your stool is measured for few days. The normal amount of fat in the stool in healthy adults is up to 7 g/day (Rochester.edu). So, from 100-150 grams ingested only up to 7 g can normally escape absorption. Larger amounts of fat in stool (steatorrhea) occur in malabsorption conditions.

How do we know that fat, once absorbed, is not excreted as fat?

When dietary fat is absorbed, it is either burned or stored as body fat. Where else could it go? A healthy human body can excrete unmetabolized nutrients via the stool, urine and skin. A minimal amount of fat is secreted via bile into the intestine, but most of it is reabsorbed. Fat is not excreted via urine in any significant amounts, and when it is (lipiduria), it can be a sign of kidney disease. Sebaceous glands excrete only few grams of fat per day.

Jan
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  • I was with you right up until the last phrase. Based on my own research the response of the endocrine system to different macronutrients can deliver significantly different weight-loss outcomes, implying that counting calories is not enough: whether those calories come from carbs or fats (for example), appears to matter. – 52d6c6af Aug 07 '19 at 15:01
  • @Ben, they are always some studies that show that. Would you mind to link to one or two systematic reviews that support your belief. I also suggest you to intentionally search for another review that concludes the opposite from what you believe to see that the current evidence is not completely convincing. I've added one link to my last paragraph. – Jan Aug 08 '19 at 07:47
  • I am not saying that nutrient absorption is affected by macronutrient ratio. I am saying that endocrine response is affected. This is basic medical knowledge. For example, the insulin response to different kinds of macronutrient is very different. This is not controversial. – 52d6c6af Aug 08 '19 at 08:05
  • @Ben, can you link to the study that says what you said, so we will both know what we are talking about. So, which diet affects the endocrine system (insulin, leptin..?) in the way that encourages weight gain. – Jan Aug 08 '19 at 08:09
  • I get to define what I said and what I meant, so this is what I said, and what I meant: different macronutrients in the diet deliver a different response from the endocrine system. This is basic biology. I am merely pointing out that what you eat matters, so your comment "it's mainly about how many calories you consume" is too simplistic. – 52d6c6af Aug 08 '19 at 11:20
  • Here's a study: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0165378 describing the different endocrine responses to high-carb and low-carb diets. Specifically about insulin resistance. Elevated insulin is associated with decreased use of body fat for energy, because insulin is normally associated with the presence of blood glucose. Insulin resistance, linked to high-carb diets in the study, delivers chronically elevated insulin levels, meaning fat stores are less likely to be used for energy, making fat loss harder. – 52d6c6af Aug 08 '19 at 11:34
  • ...the implication is that eating 500 calories of fat will not contribute to insulin resistance and and that eating 500 calories of carbohydrate will. Fat loss could therefore be easier under a low-carb, higher fat diet. And this is borne out by the anecdotal evidence of countless near-miraculous weight loss stories of low carb diets, and decades of near-total failure on low-fat, higher carb diets. – 52d6c6af Aug 08 '19 at 11:36
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    I deleted the last paragraph to keep my answer focused on what was asked in the original question. – Jan Aug 08 '19 at 14:17