Chopping and sautéing aren’t just steps in a recipe, they can fundamentally alter the chemical properties of the foods we eat and the ways our bodies respond to nutrients. Join Harvard University’s Rachel Carmody on a journey behind the chemistry of cooking, the effect it has had on human evolution, and why nutritional information on food labels only tell part of the story.
This SciCafe took place at the Museum on March 6, 2019.
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#Raw #Diet #Calories #Nutrition #Cooking #FoodScience #Health
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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 mob 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 you know it's New York so like no spherification no foam how many show of hands right so on the whole what we can say is that 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 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 the even among the Inuit comes a 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 sort of sets the playing field for what's possible so if we think about what's some of the human dietary adaptations are in comparison to our closest living 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 average human males even in traditional populations that are not though tend to be obese human males tend to be 34 percent larger than male pansies human females tend to be 56 percent larger than female chimpanzees and all else 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 thirteen hundred and fifty cubic centimeters whereas chimpanzees are about four hundred 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 two percent of our body mass but in a body at rest it consumes somewhere between 15 and 25 percent of our energy we also in especially in traditional societies tend to be much more active than chimpanzees are so in in wild chimpanzees the average is the travel distance per day of about 3 to 5 kilometers whereas in traditional foraging populations 10 to 20 kilometers per day of travel as the norm and obviously travel does those 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 letter Ned Robin Robertson several years ago we calculate that human males expend about 44 percent more energy than do chimpanzee males adjusted for body mass and human females expend 17 percent more energy 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 inner birth intervals it's so in order to fuel our large bodies and our large brains and are 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 note this because humans also show adaptations that signal kind of 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 compared to chimpanzees when the two crania are sort of scaled to the same size we also have smaller muscles for chewing including the temporalis which attaches sort of 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 choose 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 sort of 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 percent 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 would 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 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 dye and trisaccharides a little to glucose units and little 3 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 amylase is 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 this is what's called this a process called gelatinization and in this form it's very easy for amylase is to come in and cleave off those dye and trisaccharides and that's one of the reasons that if you've ever you know tasted a raw potato next to a cooked potato the cooked potato tastes relatively sweet in your mouth and that's literally because salivary amylase is 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 now 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 cos muscle fiber bundles that are surrounded by a collagen matrix and what he 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 like toughening of the muscle fiber bundles but the the gelatinize ation 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 as these tightly wound bundles almost like a massive yarn tangled yarn and what he does is it causes that ball 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 he didn't have any impact on fat and this is because by and large very little fat appears in feces so the assumption was always that we must be absorbing a hundred percent of it so it doesn't really matter its 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 there 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 Olli Osen layer and as we just talked about heat degrades protein causing them to unwind and for the same reason when you heat this oleoresin 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 nutrient 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 can denature protein it 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 a hundred percent when that starch is cooked so the white bars are cooked the green are raw and that that's true whether those starch has been coarse milled or fine Milt 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 only really approximates the digestibility of cook starch at 24 hours and if you think about passage rate of starch through the gut it's really only in the small intestine for four to six hours so this is actually never achieved these levels are never achieved in real life so the effects 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 think gosh we've all been eating forever we've all 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 find the text that like overrides all of my research but we haven't found it yet surprisingly no one has actually studied the energy gained 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 macro nutrient profiles so that we could sort of see how food processing was affecting all of these nutrients but we also selected it because I'm a human evolutionary biologist and are examples of two food classes that were thought 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 process 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 said these treatments to mice for a period of just four days to see what 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 they enjoyed eating both the sweet potato and the beef compared to their normal chow 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 kind of not processed sweet potato they lost about four grams when they eat the pounded sweet potato they lost about three 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 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 diet they actually all lost weight and this was to be expected because mammalian omnivores actually don't don't deal with lean meat very efficiently in terms of our metabolism but what's important is that on the 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 different diverse foods rice sweet potato and lean beef and peanuts processing increases energy gain and that cooking does so to a greater extent than on thermal processing and so we can say that processed foods meet our requirement for a diet rich in calories right but what about a guy 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 fit really warm and maybe you sweat you are not making it up your body is 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 conducting peristalsis to kind of squish that food down the gastrointestinal tract you're absorbing and 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 five to ten percent of the caloric value that carbohydrate is essentially used in its own digestion if you eat a protein rich food that's a whopping 20 to 30 percent of the caloric value essentially is spent in its own digestion for fat the costs are very low zero to three percent 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 some calories purely by spending some energy digesting your drink 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 although 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 this 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 the 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 there's still not they're not still digesting their prior meal and when they do eat they need an enormous meal which gives you a nice huge peak 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 met and we compared the diet-induced thermogenesis and what we found is that ground meat led to twelve percent less cost of digestion cooked meat led to 13 percent less cost of digestion and both ground and cooked meat there was almost an additive effect where it was 23 percent less costly to digest compared to the unprocessed to meat treatment other researchers have subsequently shown something similar even four degrees of highly processed food so eating a white bread and processed cheese sandwich resulted in about forty percent 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 like 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 and calories and easier to digest and so one simple reason why food processing is so universal among human cultures is that it renders foods suitable for human consumption and for most of human evolution maximizing energy gained from food at minimizing the cost that went into digesting that food would have been advantageous and suitable foods were so hard to come by and so you know 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 you know you think 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 any more we don't really expend 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 our 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 and 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 is now there was no state where the rate of obesity was less than 20% over a 20 year span and today we've got two of three u.s. adults being overweight we've got about thirty five percent of adults and fifteen percent of children being clinically obese and this is a diagnosis that contributes to at least 400,000 deaths per year and something like a hundred billion dollar drain on the 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 right that's we all know that but I'm rather than me kind of stand 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 is 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 wealth yes maybe but not at least for this point and the reason is that a hundred grams you know so much of food is water and a hundred grams of items that have been cooked actually have lost a lot of that water so there's more food in a hundred grams of cooked food then there isn't a hundred grams of raw food and so if we actually scaled 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 that for example with me hundred grams of dry matter gives you five hundred and fifty calories if raw 549 of cooked 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 like let's imagine you eat this cashew right so you eat a cashew you chew it in your mouth you've got all this saliva being produced in salivary amylase that prot that begins that process of starch digestion at some point this all comes together it forms what we call a bolus that bolus is swallowed and 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 pass is out of the stomach and into the small intestine where secretions from the pancreas and liver starts to break down carbohydrates and emulsify fats and continue the process of 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 it passes into the colon where you can't really do much with it anymore but your microbial community takes over ferments that food it can produce short chain fatty-acids some of which we can 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 – what comes out and we assume that everything that has disappeared between point A and point B has gone to us what we know it doesn't include our diet induced thermogenesis which we just talked about and as I've already shown you died 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 call 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 green and blue 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 kilocalorie per gram values that probably all of you are familiar with 9 kilocalories per gram for fat 4 kilocalories per gram for either carbohydrate or protein but here is 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 acid if we can use them for energy gain or only worth two kilocalories per gram to us and so a gram of carbohydrate absorbed in the small intestines worth four but if that gram of carbohydrate is actually metabolized by microbes it's worth two 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 died induced thermogenesis the distinction between digestion the small intestine in 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 effects 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 kind of 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 [Applause]