Humans ferment fiber into fat

A short post on how fermentation of resistant starches in the gut into short chain fatty acids (SCFA) works and the role it plays in human and other animals’ diets.

I direct you all to a really nice review paper in the journal “Physiology Reviews” for way more expert and in-depth analysis than I can do. You better have some good biochemistry if you want to understand that paper though. Frankly, I was struggling by the end.

I am writing this because I have had a problem reconciling why mammals can have such varied diets yet end up running pretty similar organs and homeostatic biological systems. Even within humans, at least in the non-industrial food environment, we can flourish on such a wide range of macronutrients.

At one end of the animal kingdom we have herbivores, such as cows and gorillas, eating plant only diets. At the other end we have carnivorous cats of various sorts, like tigers and lions, eating meat. One end appears to be a low fat, high carb, moderate protein diet. The other is a high fat, high protein diet.

Humans are the same. We have Inuits at one end on a high fat, virtually no carb, moderate to high protein diet. At the other end we have the Kitavans in Papua New Guinea who eat high carbohydrate diets (have a look at Dr Staffan Lindberg’s site on this – it’s quite interesting). Yet both groups and all of the ones in between in their natural food environment are metabolically healthy and free of chronic disease. The same is true for the animals as well. All have no trouble maintaining a healthy weight even in the presence of plentiful food.

So what goes? And why do I talk about a low carbohydrate diet in the midst of all this evidence?

My short answer is that a whole food diet is fine in any macro-nutrient composition as long as you are metabolically healthy. But as soon as you become dysregulated (aka insulin resistant) – probably because of sugar and other lifestyle factors, like excess stress – then carbohydrate restriction is the way back out of this. This is probably half the western population, maybe more.

Why? Well the longer answer is that there is something else to consider in the animal and human digestive systems which further helps us (or me at least, and hopefully you too) reconcile the variation in carbs/fat/protein across animals and humans.

The emerging evidence it seems is that plant fiber is fermented in the gut of herbivores and omnivores, including humans. This provides energy in the form of short chain fatty acids, especially butyrate.

This is really cool because it means that the range of macro-nutrients which makes it through the gut into the actual bloodstream of all these very different, but similar, animals is similar. It’s much higher in fat than we previously thought. For example, studies with Western lowland gorillas shows that the majority of their energy (57%) comes from the SCFA fermented in the gut from vegetable fiber (SCFA are saturated fats!). Most of the animals we are considering, including humans, are on high fat, moderate protein, lower carb diets – give or take – once we take into account fiber fermentation in the (healthy) gut.

“The macronutrient profile of this diet would be as follows: 2.5% energy as fat, 24.3% protein, 15.8% available carbohydrate, with potentially 57.3% of metabolizable energy from short-chain fatty acids (SCFA) derived from colonic fermentation of fiber. Gorillas would therefore obtain considerable energy through fiber fermentation.”

Where things go astray is when any of these animals, including humans, eats processed carbohydrates. These aren’t high in fiber of course and go straight into the bloodstream. Insulin is constantly high in an attempt to move the carbs and it’s downhill from there.

Other researchers might have known about this for a long time, but I was sticking to the old “roughage hypothesis” of fiber digestion. That is, the idea that the fibre wasn’t digestible and helps clean out and stimulate the colon.

So what of humans then? I think we can take a few things away:

  1. Fermentation of fibre, like in herbivores, occurs in the human colon
  2. Much of this turns into usable SCFAs. Some feeds the actual bacteria, some the gut wall and some goes into the bloodstream and is processed from there.
  3. The calorie count on products which contain fiber is flawed, and another reason why a calorie is not a calorie. Celery is a good example of this; people claim celery contains less energy than it takes to digest – true enough, immediately available carbs are low, but the fiber fermented into fat contains significant amounts of calories in SCFAs.
  4. High carb diets which are high in fiber can turn into higher fat diets, and that is likely what has been the case historically for humans.
  5. Microbiome health is likely to depend on establishing quantities of bacteria which can digest fiber. This will depend on the history of feeding that sort of food. Processed carbs probably undermine the development of the gut bacteria needed to digest fiber.
  6. And processed carbs bypass the entire mechanism and dump insulin raising carbs into the system further upstream from the stomach and small intestine.

10 Comments on “Humans ferment fiber into fat

  1. I really like the idea and would be surprised if we didn’t get an answer about why healthy diets can be so different from on another from this type of inclusive theory.

    How were we able to determine the form and amount of energy (SCFA) from initial fiber consumption? (for humans or gorillas)

    I was under the impression that our abilities to ferment any significant amount of fibres to fat was lost due to an ‘evolved digestive system’ much better suited to absorbing nutrient dense foods rather than fermenting fibres. Can you point to something arguing to the contrary?


    PS: your blog made my health/nutrition bookmarks list 🙂

  2. The fact that ALL mammals are on a high fat diet when it comes down to actual energy usage is nothing new, but there’s still the constant turf wars of where to obtain them exogenously.

    Sure, it’s great that fiber *can* be fermented to SCFA, but in humans it’s ridiculously inefficient to the point of unusable – whereas chimps/gorillas/etc can do it quite well, not to mention outright herbivores.

    • Ash, I’ve seen studies of PNG highlanders on protein-deficient high-carb, high fibre diets where the gut bacteria are actually synthesizing enough amino acids to keep them healthy.
      But I don’t think New Zealanders could rely on that service.
      I envision a future where there is a trade in fecal transplants from third-world hunter-gather tribes to help those in the developed world cope with the impossible low-fat, high-fibre or vegan diets they’ve committed themselves to due to their various crazy ideological motivations.

  3. Soluble fibre is generally given an Atwater rating of 2 calories per gram. Considering that carbohydrate gives 4 calories and fat 9 (Atwater ratios are averaged and rounded off, but generally accurate overall), this shows quite a low availability of SCFA from fibre, at least in humans.
    However from such research as I’ve read into butyrate it seems that relatively small amounts have measurable benefits in animals that aren’t high fermenters. In this regard butyrate seems a bit like ketones – it doesn’t have to be the dominant fuel source to be beneficial.
    And humans are fortunate to be able to get SCFAs from butter (3-5% butyrate) and dairy fat, especially if FODMAP intolerance or chronic inflammatory diseases limit our tolerance of soluble fibre or resistant starch.

    • Thanks Chilisalsa, that actually answers a question I posed myself recently. The hummingbird, when hovering, is hoovering up fructose and glucose.
      Bees have designated “heater” drones, who consume much more honey than other bees, and who vibrate constantly to maintain the hive at an even temperature for incubation.
      Sperm are also fueled with fructose, the only use for it I know in mammalian metabolism.
      Live fast, die young.

  4. good to be reminded of something I already knew but didn’t really think to much about, that explains why a high fruit and veggie (fresh) diet can be so healthy (while still having some protein and fats from coconut oil or small amunts of meat and dairy if that is one’s preference and satisfying for many (if they are not metabolically dysfuntional like me). I was wondering if one really works on healing their digestive system with kefir, and probiotics in protein powder and green superfoods supplement powder (it says it has a bunch of wonderful stuff in there and is supposed to be good for digestion and supporting bacteria in the gut) I have been drinking alot of kefir taking probiotics and that green stuff and my stomach is doing so much better (now taking high potency cinnomon for my insulin troubles aka glucose intolerance that seems persistant and taking longer then I figured to reverse) I noticed my sinus and asthma are not acting up like theyusually do too. I attributed these to my stomach I read that gases from your stomach cause irritation to the lungs esophagus and sinus drainage and mild asthma being the result. I haven’t had to usemy inhaler for a long while even tho I only used it occassionally anyway. post nasal drip has reduce quite a bit too.

    • Hi Gaucho,

      Atherosclerosis in ancient peoples is probably caused by long exposure to sooty fires, not by dietary factors. It is a normal healing process as people age through exposures to damage by polluted air and infections. If you look at old photos of inuit you can see that soot was a constant companion.

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