Shortcut to Slim

Season 1: Episode 4

The Energy Theory of Cooking, Comparative Anatomy, & What Causes Obesity

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After the last episode, a few listeners emailed asking if there was other, non-physiological evidence that supports Wrangham’s conclusion that humans have ADAPTED to cooked foods.

I’m glad you asked because I love talking about comparative anatomy. (Sidebar: Dr. Milton Mills has done incredible work in this field comparing humans to herbivores. I’ve included a few resources in the show notes if you want a deeper study. Visit for the links)

Why can’t humans eat an all raw diet from an anatomical perspective?

In Catching Fire: How Cooking Made Us Human, Richard Wrangham explains that compared to our ape and chimpanzee cousins, as well as other mammals, we have small mouths, weak jaws, small teeth, small stomachs, small colons, and small guts overall.

In the past, the small size of these body parts has been attributed to eating meat but Wrangham says the design is better explained as an ADAPTION to eating cooked food rather than raw meat.

Wrangham notes, “given that the mouth is the entry to the gut, humans have an astonishingly tiny opening for such a large species.”

If you’re anything like me, you’re probably stretching your jaw as wide as possible right now.

...And if you’re not, go find a mirror and do that, everybody else is.

Now compare your face to a picture of a chimpanzee with an open mouth. (I’ve included a picture with the show notes on for easy access).

Chimps can open their mouths twice as far as humans, as they regularly do when eating.

Our mouths also hold very little, as I rediscover pitifully every time I try to eat popcorn.

Interestingly, we actually hold the same volume as chimps in our mouths, though we weigh 50% more.

We also have weaker jaw muscles by comparison and I know I’ve personally experienced muscle fatigue chewing hard foods before like big sourdough pretzels or Cliff bars.

Our teeth and molars are also the smallest of ANY primate species in relation to body size and our stomach is LESS than ⅓ the size expected for a typical mammal of our body weight.

The large intestine, meaning the colon, where plant fibers are broken down to be used as energy, is LESS than 60% of the mass that would be expected for a primate of our body weight. This is why we can’t “digest” fiber and basically send it back out whereas apes actually break it down and use it for energy.

For me, this fact alone provides a convincing argument that we are anatomically adapted to cooked foods because if we weren’t eating the high caloric density of cooked foods we would have to eat all day and probably eat twice our body weight as apes do. Even then we would still probably starve because the sheer amount of food needed is insane AND we’d basically have to eat all day while simultaneously foraging. I’m exhausted just thinking about that…

To summarize (in case I lost you back there), we have small teeth which are adapted finely mashing softer, cooked foods NOT chewing tough raw material. The high caloric density of cooked food also allows our stomachs to be small and our intestines to be short.

This next part is for the vegans. The “Man-the-Hunter” hypothesis assumes our ancestors were originally plant eaters, but that we evolved because we ate meat, and it was this meat eating that caused us to have small everythings and become the humans we are today.

But small mouths, teeth, and jaws are clearly not well adapted to eating raw game meat which is tough. Cooked meat is easier to consume, true, but it doesn’t move through the human body the same way it does in the bodies of true carnivores and omnivores. Humans are extremely inefficient at processing chunks of meat. It passes out of our stomachs quickly and then languishes in the intestines, where for other meat-eaters such as true carnivore and omnivores, it stays in the stomach for a really long time, with a short ride in the intestines.

Plus, as Wrangham pointed out, our ancestors were still eating at least half their diets of plants. They were hunters AND gatherers. So, if the meat-eating hypothesis explains the small everythings, it faces difficulty with the plant component of the diet. It cannot explain how humans with diminished capacity for digestion could have digested raw plant foods efficiently.

I’ll stop myself before I fall deeper down this rabbit hole of comparative anatomy. As I mentioned earlier in the podcast, if you’re as supremely fascinated as I am about this, I’ve left a few reading links in the shownotes. You can also read chapter 2 of Wrangham’s book. For those who wanted non-physiological evidence, I hope I have satisfied your appetite (see what I did there?)

The reduced size of our digestive system limits our effectiveness at digesting raw food, but it enables us to process cooked foods with exceptional proficiency.

This GREAT news from an evolutionary standpoint but not-so-good news when we consider what that means for processed foods and the obesity epidemic, which I’ll talk more about later in this episode.

The energy theory of cooking is basically this: By cooking our food, we are better able to absorb the nutrients (and also the calories) in that food.

This is overwhelmingly true for all animals, even fish and insects.

But this wouldn’t be a science-backed research podcast if there wasn’t a giant, steamy caveat. (Science is so humbling!)

COOKING can also reduce calories. For example, cooking can lead to the loss of nutrient-rich juices or reduce the amount of sugars or amino acids which would consequently decrease the bioavailable energy. Cooking can also generate indigestible molecules or change the texture in such a way that the food becomes less digestible, though this is definitely the more rare exception. Though if you’ve ever burned something in the kitchen you’ve experienced this.

The effects of cooking on energy gain, however, are consistently positive.

Wrangham talks at length about gelatinization, denaturalization, and the reduced cost of digestion in exceptional detail.

I’ll share a few points or examples to give you a basic understanding, but if you’re even a little bit fascinated, or you want to know why bodybuilders started drinking raw eggs, or why we’re so attracted to marinades, pickles, lemon juice and beef jerky, see Chapter 3 in Catching Fire. Here’s a hint: it’s not just about the salt, denaturalization helps account for our enjoyment.

Here are the Cliff Notes:

We utilize cooked starches such as oats, wheat, potatoes, bananas, white bread very efficiently. About 95% gets digested. Raw starch, however, doesn’t fare so well. Digestibility drops to about 50%. It varies slightly for each starch food, potatoes being the worst, which I think explains why we don’t like to eat raw potatoes and also why raw bananas and uncooked oats don’t settle well for some people.

The principal way cooking increases digestibility is by gelatinization.Basically, the starch inside plant cells comes in these little dense packages that are hard to digest. BUT when you warm them up, they swell, weaken, fragment, and get goopy.

The more starch is cooked, the more it is gelatinized, and the more gelatinized it is, the easier it is for our digestive enzymes to reach it, and therefore, the more completely it is digested. Thus, we can assimilate more energy out of cooked starch than raw starch. (All of this is easily detected in blood measurements).

Interestingly, Wrangham notes that starch does not STAY gelatinized after it’s cooked. He says this might explain why we like to toast bread after it’s lost it’s initial freshness.

Gelatinization (and denaturalization) are chemical changes brought about by cooking, but as I mentioned earlier with the caveat, cooking can also have a physical effect. Usually that effect is that enhances digestion.

For example, the more tender, soft, or finely divided a food is, the more easily and completely it is digested.

Which brings us back to the very first episode, and each episode since, when I said a calorie is not always ‘a calorie’ and that the more processed or cooked a food is, the better we can absorb the calories (and also the nutrients) in that food.

Wrangham’s pet example is fantastic. Domestic pets become fat because the calories in processed pellets are so much more easily absorbed.

I parlayed this into my orange versus oreo example, that you’re probably not going to digest and absorb every calorie of bioavailability in an orange, but you can bet your bottom dollar you’re probably going to absorb every last calorie in an Oreo.

Why is that?

Softer food is digested faster and easier digestion demands less metabolic effort. Less metabolic effort means you’ve saved energy. You become more efficient. You become more like a Prius and less like a Hummer.

Here’s another way to think about it: Blending or pureeing is a form of predigestion. Your blender is doing the job your teeth and jaw would do. It’s literally chewing for you, saving you the energy and work of doing it yourself.

From an evolutionary perspective, this is like winning the Powerball, especially when food was so scarce. To get more calories out of my food all I have to do is cook it? No wonder we evolved into the badass humans we are today.

BUT when you consider processed foods and the wacky manipulations food giants have done to sugar crystals, the structure of salt, and a million other lab-created twists… YEOWZA!

Wrangham shared a study out of Japan that proved this point beautifully and if you like breakfast cereal, I apologize in advance.

In the study, 10 rats ate ordinary laboratory pellets which were hard enough to require substantial chewing. The other 10 ate a version of the standard food that was modified to be softer by increasing the air content. The pellets were basically puffed up like breakfast cereal and only took half the effort to chew.

In every other way, the conditions were IDENTICAL. The calorie intake and expenditure was identical, the pellets did not differ in nutritional composition, how they were cooked, or in water content.

Conventional theory based on the calculation of calorie intake, meaning the old calories in vs. calories out math formula, predicted that the rats should all grow at the same rate. They should be the same size and have the same body weight and body fat.

But they did not. Both groups were on the same diet the first four weeks of their life, and then they separated. After one week on the puffed pellets, there was a visible difference in the rats. By 22 weeks, the difference was significant.

The rats eating soft food slowly became heavier. On average they were 6% heavier and had more abdominal fat, enough to be classified as obese.

Soft, well-processed foods made the rats fat. The difference was in the cost of digestion.

The researchers concluded that the reason the softer diet led to obesity was simply that it was a little less costly to digest.

Wrangham wraps it up nicely: “If cooking softens food and softer food leads to greater energy gains, then humans should get more energy from cooked food than raw food not only because of processes such as gelatinization and denaturalization, but also because it reduces the cost of digestion.”

Completely and utterly anecdotal, but I had a similar experience in my own journey. At different times I eliminated juices and smoothies, breakfast cereal, and highly processed grains from my diet. It was always in an effort to reduce my calorie intake to lose weight. While I’m sure my weight-loss was due to my decreased caloric intake, this research does make me wonder if my perceived caloric deficit was even greater than what I thought.

I also stopped eating puffed food, not because of the calories, but because I noticed I always overate on them. I couldn’t seem to keep to portion size with popcorn, puffed kamut, rice cakes, and so on. After doing so I lost weight, which I still attribute to the decrease in calories but I do wonder if the puffed nature made a difference.)

I also wonder what this means for slow cooked foods and the pressure cooker...

Anyway, Wrangham also also references a python study (yes, pythons) and same results: Grinding reduced the snake’s cost of digestion and cooking had nearly identical results.

To me, this makes sense why we like foods that have been softened by cooking, blending, juicing, grinding, pounding, processing, and pulverization.

And it also supports eating the whole foods, plant-based diet we promote with the meal plans.

So what can we take from this? Cooking gives calories, but we are adapted to cooked foods. Processing also gives calories, but there’s a wide spectrum in terms of what processing means. Dieters please don’t start fearing chopped tomatoes, chunky stews, hummus, or applesauce. Their calories are not the same as the super absorbent calories in pretzels, Twinkies, and Big Macs.

Intuitively you know this, and now you know why 100 calories of doughnuts might lead to obesity while 100 calories of applesauce, tomato soup, or strawberries probably won’t.

Finally, I’d like to end this episode with my favorite Wrangham quote,“delicious” means high energy because what people like are foods with low levels of indigestible fiber and high levels of soluble carbohydrates.

Download your free research-based 7-day meal plan at and leave the guesswork and science to me. I’ll be back next week talking about gut bugs and sugar cravings.

Show notes:

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