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RACHEL CARMODY (ASSISTANT PROFESSOR OF HUMAN EVOLUTIONARY BIOLOGY, HARVARD UNIVERSITY): I wanted to start off tonight with a little straw poll. So, put your hand up if you’ve eaten anything today. Awesome, awesome. We’re not a ravenous, this is good news.

So, now put your hand up if everything you ate today was raw or minimally processed. By which I mean no blending, no grinding, no pounding, no pressing, no—it’s New York, so like so spherification, no foam. How many? Show of hands. Right.

So, on the whole what we can say is, as a group, we eat a highly processed diet on average. And we’re really not alone in this at all. Cultures all around the world—in fact, every human civilization known actually processes its diet extensively, both by cooking as well as by a variety of non-thermal processing methods. And this includes cultures like the Inuit who famously do consume a portion of their hunted and fished materials raw. But even among the Inuit, the standard evening meal is both cooked and highly processed.

And so tonight I wanted to ask two basic questions about this unique and universal human behavior. And the first is: Why is food processing so universal? And the second is how might it threaten our health today?

So, first, let’s think about why is food processing so universal? And anytime we think about diet and why an organism consumes the diet it does, it’s really important to start thinking about the dietary adaptations of that particular species, because that really sets the playing field for what’s possible.

So if we think about what some of the human dietary adaptations are in comparison to our closest relative, the chimpanzee, we can see that humans have evolved a suite of features that necessitate a high daily energy budget. By which I mean we have to take in a lot of calories. And one bit of evidence for this is our increased body mass.

So, on the average, a human male, even in traditional populations that do not tend to be obese, human males tend to be 34% larger than male chimpanzees. Human females tend to be 56% larger than female chimpanzees. And all else being equal, a larger body does require more calories to sustain itself.

We also have a dramatically expanded brain size compared to chimpanzees. So, on average, our brains are about 1,350 cubic centimeters whereas chimpanzees are about 400 cubic centimeters. And the reason that this is relevant to a high energy budget is that the brain is an extraordinarily expensive tissue. It weighs about 2% of our body mass, but in a body at rest it consumes somewhere between 15 and 25 percent of our energy.

We also, especially in traditional societies, tend to be much more active than chimpanzees are. So, in wild chimpanzees, the average is they travel distance per day about 3 to 5 kilometers, whereas in traditional foraging populations, 10 to 20 kilometers per day of travel is the norm. And, obviously, travel does cost quite a bit.

So, when you do the math, this works out to humans having much higher rates of energy expenditure, even after you control for the differences in body size. So based on equations that were suggested by Leonard and Robertson several years ago, we calculate that human males expend about 44% more energy than do chimpanzee males, adjusted for body mass. And human females expend 17% more than do female chimpanzees, again adjusted for body mass. And that number is probably a dramatic underestimate because these equations didn’t account for the cost of gestation and lactation which are expected to be higher in humans due to our shorter interbirth intervals.

And so, in order to fuel our large bodies and our large brains and our pretty active lifestyles, humans require a lot of calories compared to chimpanzees. But we don’t just eat more of the same food. We eat fundamentally different food and we know this because humans also show adaptations that signal a loss of digestive capacity and I’ll explain what I mean.

So, what you’re looking at here are chimpanzee and human skulls shown from the underside. You can see the holes at the bottom, that’s the foramen magnum, that’s where the spinal cord passes through and inserts. So you’re looking at these skulls from the underside.

And what you can visualize here is that we have smaller mouths, shown here on the right, in blue, compared to chimpanzees when the two crania are scaled to the same size. We also have smaller muscles for chewing, including the temporalis, which attaches through to the side of the cranium and passes through the zygomatic arch, which is here shown kind of in cross-section in red. And then it attaches down to the mandible and allows us to chew. We’ve got these sort of small, little muscles that we can deduce by the size of the zygomatic arch.

We also have a mutation in the myosin component of the jaw muscle that effectively limits bite force. So for humans, chewing and chewing power is reduced compared to chimpanzees.

So, jointly, we’ve got this high daily energy budget and we’ve got low digestive capacity and this actually suggests two things about the human dietary niche that are rooted in our ancestry. The first is that we require foods that are rich in calories, a lot of calories in a limited amount of space. And we require foods that are easy to digest. And I’m going to argue here that by externalizing part of the digestive process, food processing allows foods to meet these needs.

So let’s consider for a second how the average American meets their caloric needs. We get on average about 50% of our calories from carbohydrates, primarily in the form of starch. We get about 12% of our calories from protein and the remaining 38% of our calories from fat. Of course, if you drink alcohol, there’s additional calories that are added into this mix, but on average, that’s what it looks like.

Food processing has differential effects on each of these macronutrients that I thought it’d be useful to just briefly review to give you an understanding of how food processing is shaping these different macronutrients.

So, first, as I mentioned, the majority of our carbohydrates come in in the form of starch. And heat has a very well-known effect on starch. Essentially, starch consists of long chains of glucose all strung together. And we’ve got enzymes called amylases that are specific to starch that come in and cleave off di- and trisaccharides. So little two glucose units and little three glucose units that our bodies can then break down with other enzymes and absorb.

But the problem is that amylases can’t always get to the starch string in the places they need to cleave. And that’s because, when foods are served raw, starch exists in these tightly packed granules. And in this form, amylases cannot penetrate the starch granule in order to get at the cleavage sites that can start cleaving off those sugars that we can absorb.

But when starch has been cooked, that granule essentially wiggles around and it swells and it takes up water and what you can see is this exploded mass. And this is what’s called, this is a process called gelatinization. And in this form it’s very easy for amylases to come in and cleave off those di- and trisaccharides. And that’s one of the reasons that, if you’ve ever tasted a raw potato next to a cooked potato, the cooked potato tastes relatively sweet in your mouth and that’s literally because salivary amylases are starting to create sugar while the potato is in your mouth, but that doesn’t happen when the potato is raw because it’s still in granule form.

So, when we think about protein, a lot of us will get the majority of our protein through animal products, particularly meat. If we think about the structure of meat, meat essentially is muscle-fiber bundles that are surrounded by a collagen matrix. And what heat basically does is it causes the muscle fibers to toughen and shrink along the grain. But at the same time, it gelatinizes the collagen matrix. And this sort of toughening of the muscle-fiber bundles but the gelatinization of the collagen means that it’s easier for cracks to propagate through the tissue. That’s why it’s easier to chew through cooked meat than it is to chew through raw meat. It also makes mechanical digestion in the stomach more efficient when you’ve got easy cracks propagating through the tissue.

The other thing that goes on is that protein in nature exists in these tightly wound bundles. It’s almost like a massive of yarn, tangled yarn. And what heat does is it causes that ball of yarn that is the protein to unwind. And that allows protein-degrading enzymes called proteases to come in and get access to the cleavage points that again create the peptides that our bodies can absorb.

Now, for a really long time, it was thought that heat didn’t have any impact on fact and this is because by and large, very little fat appears in feces, so the assumption was always that, well, we must be absorbing 100% of it so it doesn’t really matter it’s form, it’s all going to use by the body.

But we now know that that’s just not the case. And a key reason is that fat, when it comes in in a food, is often encased in other structures that depend on cooking. And so, for example, what you can see here—this is an example of peanuts with raw peanuts and cooked peanuts in the lower panel. And the fat globules have been stained in red. And, in the raw form, what you can see are these little fat globules that are encased within intact cell walls. Whereas in the cooked you can see the cell walls have broken open because the polysaccharides that keep those cells nice and intact have broken open. And the other thing that’s happened is that the fat globules themselves, the reason they’re round, is you’ve got fat that’s surrounded by this layer of protein called the oleosin layer. And, as we just talked about, heat degrades protein, causing them to unwind. And, for the same reason, when you heat this oleosin layer, it unwinds and that allows fat-degrading enzymes like lipases to come in and to be able to metabolize that fat.

And so jointly, these effects of heat on starch and protein and fat can be expected to lead to important increases in nutrition digestibility. But what about non-thermal processing, right? What I’ve shown you so far is all about heat.

Now, non-thermal processing can only modify the physical structure. It can’t change these compounds chemically, so it can’t, for example, gelatinize starch or it can’t denature protein. It can essentially deal, though, with particle size and we’ll consider this for just a second.

So, for example, this is an example of starch digestibility given cooking versus milling. And what you can see in the white bars is that the fraction of starch that is digested quickly rises to close to 100% when that starch is cooked. So, the white bars are cooked, the green are raw. And that’s true whether that starch has been coarse-milled or fine-milled.

But when the starch is raw, it lags at every point and, in the coarse-milled case, even after 24 hours, the digestibility of that raw starch never approximates that of cooking. And in the fine-milled case, it really only approximates the digestibility of cooked starch at 24 hours and, if you think about passive rate of starch through the gut, it’s really only in the small intestine for 4 to 6 hours. So this is actually never achieved. These levels are never achieved in real life.

So, the effect of cooking and non-thermal processing on starch and protein and lipid allow us to make two predictions. And the first is that the consumption of foods processed by these methods should lead to increased energy gain. And the second is that the energy gain conferred by cooking should exceed the energy gain conferred by non-thermal processing.

So, those are pretty basic questions and you would think, gosh, we’ve all been eating forever. We’ve been cooking, we’ve been processing our food, everybody does it. Surely somebody would have researched this and this would be known. And this is like my nightmare that one day I’m going to just find the text that overrides all of my research. But we haven’t found it yet. Surprisingly, no one has actually studied the energy gain due to non-thermal and thermal food processing. And so we decided to set up a really simple experiment.

And in our experiment, we selected two foods for testing. We selected sweet potato and we selected lean beef. And the reason we selected these foods is partly because they have very different macronutrient profiles so that we could see how food processing was affecting all of these nutrients. But we also selected it because I’m a human evolutionary biologist and these are examples of two food classes that were though to provide the bulk of calories for ancestral humans, so it gave us perspective into how the adoption of food processing techniques may have enhanced energy gains for ancestral humans.

And we took these sweet potato and lean beef and we processed them in four ways. Either what we call NP—Not Processed. We pounded it up. We cooked it. Or we both cooked and pounded it. And we fed these treatments to mice for a period of just four days to see what would happen with the energy metabolism of those mice. And I can attest that these mice actually were really happy about the experiment and they enjoyed eating both the sweet potato and the beef compared to their normal chow. Which was nice.

And what you’re looking at here are the changes in body mass that these mice experienced over just the period of four days, and this is controlled for differences in activity as well as slight differences in food intake across these treatments. And what you can see is that, on the sweet potato diet, when mice were eating the raw, not processed sweet potato, they lost about 4 grams. When they ate the pounded sweet potato, they lost about 3 grams. But when they ate either of the cooked sweet potato, whether it was whole or whether it was pounded, they were able to maintain their body masses just fine, no problem at all. And they actually did so despite eating less overall sweet potato.

Now, the story was similar in meat. In the case of meat, when mice were placed on the meat diets, they actually all lost weight and this was to be expected because mammalian omnivores actually don’t deal with lean meat very efficiently in terms of our metabolism. But what’s important is that, on the raw treatments, mice lost more weight than they did on the cooked and there was really no difference between whether those were served whole or pounded.

So we see across and diverse foods—right, sweet potato and lean beef and peanuts—processing increases energy gain and that cooking does to a greater extent than non-thermal processing.

And so we can say that processed foods meet our requirement for a diet rich in calories—right? But what about a diet that’s easy to digest? If you eat meat and if you’ve ever eaten a large steak or a large burger and after you’ve eaten you feel really tired and you feel really warm and maybe you sweat, you are not making it up. Your body’s working really, really hard to break down that food, to produce all the acids and enzymes that create the digestive juices that break down the food. You’re conducing peristalsis to squish that food down the gastrointestinal tract. You’re absorbing, you’re assimilating all of these nutrients. And that is costly.

The cost of diet-induced thermogenesis actually differs by different macronutrient. So, for carbohydrates, it’s about 5-to-10% of the caloric value of that carbohydrate is essentially used in its own digestion. If you eat a protein-rich food, that’s a whopping 20-to-30% of the caloric value essentially is spent in its own digestion. For fat, the costs are very low, zero to 3%. And the good news is, for those of you who are drinking tonight, the cost of digesting alcohol are also quite high, so actually you’re saving on calories purely by spending some energy digesting your drinks. So, you go.

And, on the whole, this phenomenon of diet-induced thermogenesis accounts for about 10 to 15 percent of your daily energy expenditure. It’s an amount similar to locomotion or physical activity. All the moving about that you do every day is matched in the calorie expenditure by just you sitting there digesting your food.

And you would think that because food processing externalizes part of the digestive process, it should actually make your foods easier to digest. Right? It’s not rocket science. But nobody had actually shown this before. So we decided to do it. And we chose to do it in pythons—admittedly, a weird choice, but they’re actually perfect animal models for this system. They don’t move, so you don’t have to worry about energy expenditure due to physical activity. You can control their body temperature because they’re cold-blooded. They basically only eat once a month so you can be sure that you have a baseline where they’re not still digesting their prior meal. And, when they do eat, they eat an enormous meal, which gives you a nice huge peek that you can measure. That’s why we chose pythons.

And we fed them lean beef that was either not processed, ground, cooked, or both ground and cooked. And we compared the diet-induced thermogenesis. And what we found is that ground meat led to 12% less cost of digestion, cooked meat led to 13% less cost of digestion, and both ground and cooked meat, there was almost an additive effect, where it was 23% less costly to digest compared to the unprocessed meat treatment.

Other researchers have subsequently shown something similar, even for degrees of highly processed food. So, eating a white bread and processed cheese sandwich resulted in about a 40% less cost of digestion compared to eating a whole wheat bread and I think it was a maybe cheddar cheese…?  It was some sort of artisanal cheese. And they nevertheless were able to measure a difference in diet-induced thermogenesis.

So, the evidence does suggest that processing makes foods both richer in calories and easier to digest. And so one simple reason why food processing is so universal among human cultures is that it renders food suitable for human consumption. And, for most of human evolution, maximizing energy gain from food and minimizing the cost that went into digesting that food would have been advantageous, since suitable foods were so hard to come by. And so processing food would have actually given ancestral humans a competitive advantage, both in terms of survival but then also in terms of reproductive success.

But of course today we live in a really different environment and we’ve got, you know, I don’t know if you have it here in New York, but we’ve got Prime Now where you can get all of your Whole Foods groceries delivered to your door, there’s no such thing as foraging, really, anymore. We don’t really spend a lot of energy to go collect these foods. We don’t really have meaningful seasonality in the food supply. And by and large, a lot of our foods are processed to such an extent that they almost rarely resemble the ingredients from which they’re actually prepared.

And this means that some of the energetic advantages that ancestral humans gained by processing are today likely to be disadvantages.

And so how might it threaten our health today? I don’t have to be depressing and sobering, but it is worth reminding that we have an obesity epidemic in this country. I’ve just put up data for the last 30 years, comparing 1990, 2000, 2010. There’ll be a new survey coming out next year, which I’m sure will be even worse. But if we just look at this kind of 20-year time span, what we can see is that, in 1990, there was no state in the country where the rate of obesity reached 15%. So, every state had a rate of obesity less than 15%. And within a 20-year span, we reached the state where there was no state where the rate of obesity was less than 20% over a 20-year span. And today we’ve got 2 of 3 U.S. adults being overweight. We’ve got about 35% of adults and 15% of children being clinically obese. And this is a diagnosis that contributes to at least 400,000 deaths per year and something like $100 billion drain on the healthcare system. So, obviously, this is a problem.

And, ultimately, obesity is a problem of too many calories in and not enough calories out. We all know that. But rather than me standing here and tell you what you should eat or how you should exercise, I wanted to highlight something that is less well-publicized, and that’s the main tool that we all have for managing caloric intake. The food label. It’s actually not a very good tool. It’s woefully inadequate in reporting the number of calories that our bodies are actually gaining from our diet. And I’ll illustrate what I mean here.

So, if we take the three foods for which I’ve already presented data showing that there is a net energy gain associated with processing—we’ve got lean beef, sweet potato and peanuts. And if we look up the food labels for these items served raw and cooked, we get this.

So you’re saying to yourself, what is she talking about? We’re reporting that the cooked item has more calories, isn’t this what the whole point was about? She’s crazy.

But actually, no. Well, yes, maybe, but not at least for this point. And the reason is that 100 grams, so much of food is water and 100 grams of items that have been cooked actually have lost a lot of that water so there’s more food in 100 grams of cooked food than there is in 100 grams of raw food. And so if we actually scale these values on a dry matter basis where we just look at how are these labels actually capturing the caloric gains for the stuff that’s not water? Here’s what we see instead.

So, somehow these food labels are not capturing the caloric benefits that we know come with cooking.

And why is this? Helps to understand a little bit about digestion. I promise not to quiz you on this. But essentially if we just consider what goes on—let’s say, imagine you eat this cashew. So you eat a cashew. You chew it in your mouth. You’ve got all this saliva being produced and salivary amylase that begins that process of starch digestion. At some point this all comes together and forms what we call a bolus. That bolus is swallowed. It passes down your esophagus aided by the muscular contractions that we call peristalsis. Goes into the stomach. In the stomach you’ve got squeezing going on that’s mechanically breaking down your food. You’ve got gastric acids and enzymes that are basically starting the process of protein digestion. It basically turns your food into a slurry called [kimes]—really gross, really mushy. That passes out of the stomach and into the small intestine where secretions from the pancreas and liver start to break down carbohydrates and emulsify fats and continue to process the protein digestion. And all the stuff that you can possibly absorb gets absorbed.

But some of that cannot be absorbed by the end of the small intestine. And what can’t passes into the colon where you can’t really do much with it anymore, but your microbial community takes over. It ferments that food. It can produce short-chain fatty acids, some of which we use as energetic substrates. And then you are sucking out the water, you’re sucking out additional minerals and, finally, you’re getting rid of what’s left over. That’s a lot of stuff that happens between Point A and Point B.

But the food labels that we’ve got today only look at what goes in and what comes out. It actually ignores everything that happens in-between. And that’s kind of a problem. And I’ll explain why.

So, what it measures is what we call total tract digestibility. What goes in minus what comes out. We assume that everything that has disappeared between Point A and Point B has gone to us. What we know it doesn’t include are diet-induced thermogenesis, which we just talked about. And, as I’ve already shown you, diet-induced thermogenesis can be really different based on different macronutrients. So it’s not just that, oh, we haven’t accounted for diet-induced thermogenesis, we know that all the calories are wrong by 10%. No, it’s variably wrong between different foods depending on that macronutrient composition, but also depending on whether that food has been processed.

We also know that food labels don’t take into account a property we called ileal digestibility and this just means we need to know where in the gastrointestinal tract things were absorbed. And I’ll give you an example.

So, this is my poor drawing of the gut. It’s a simplified version of the small intestine followed by the colon. And these little red, blue and green dots—those are nutrients coming in and some of them get absorbed and there’s lingering amounts that then pass into the colon.

Now, if you absorb nutrients in the small intestine, you get the kilo-calorie per gram values that probably all of you are familiar with. Nine kilo-calories per gram for fat, 4 kilo-calories per gram for either carbohydrate or protein.

But, here’s what. So, the nutrients that are not absorbed in the small intestine enter the colon where, as I’ve mentioned before, microbes ferment these nutrients and they produce short-chain fatty acids, some of which we can absorb and utilize for energy gain. But these short-chain fatty acids, if we can use them for energy gain, are only worth 2 kilo-calories per gram to us. And so a gram of carbohydrate absorbed in the small intestine is worth 4, but if that gram of carbohydrate is actually metabolized by microbes, it’s worth 2. Which is a pretty big difference. And this happens all the time. Our microbial communities are very active digesting our food and food labels are not accounting for this differential.

And so, on the whole, by failing to capture diet-induced thermogenesis, the distinction between digestion in the small intestine and the colon, and host-microbial interactions and energy gain, all of which we’ve now seen are likely to be influenced by food processing, our standard energy assays fall pretty short.

And so, without tools that capture the energetic effect of food processing in a world full of processed foods, it’s going to be difficult indeed for consumers to manage their caloric intake, even if they really try.

And I hope that our journey tonight just gave you a little bit of a new perspective on why we eat the way we do. And, to sum up, I think it’s quite simple—we eat processed foods because we can. And because for most of our history, the increased energy from these processed foods gave us an advantage in terms of survival and in terms of reproduction. And, although we may not appreciate or benefit from these energetic advantages today in the modern world, I think we can take the reins of our energetic legacy just by keeping in mind that it’s not just the food that matters, but it’s also the form of the food that matters.