As you know if you’ve been reading this blog for a while, I think there’s too much focus on macronutrient intake in the nutritional community. I’ve repeatedly highlighted the fact that humans can be lean and healthy on both high-carbohydrate and high-fat diets and that the most important thing is that we eat the types of foods we are best adapted to eat, not that we restrict one of the three macronutrients. However, this clearly doesn’t mean that macronutrient intake is irrelevant, and as I’ve discussed in my posts on the topic and especially in my three lengthy articles on protein, fat, and carbohydrate, there are several considerations to keep in mind when determining how much you should eat of the different macronutrients. One of the things that is especially important is to get enough protein into your diet, as people tend to consume more total calories when they eat a diet that is very low in protein. But as you know, even if you consume a relatively high percentage of these essential building blocks compared to western standards, protein only constitutes a small part of your total caloric intake. Where should you get the rest of your energy from? Clearly, a combination of fat and carbohydrate is the way to go, but how much of each?
The question above has evoked one of the most long-lasting debates in the nutritional community, and while some proponents of very low-carbohydrate diets say that a low percentage of carbohydrate (<20%) is the way to go for pretty much everyone, official health authorities tend to make the case that carbohydrate should make up 45-65% of your daily caloric intake and thereby be the primary source of energy. I’ve previously discussed carbohydrate intake in a comprehensive article on the topic – and in several other posts, such as “Evolution: The Basis for Understanding Human Nutrition” and “What is a Healthy Diet?“. In this series of posts I’m going to examine things further and take a look at how much of this macronutrient you should actually be eating…
In discussions of carbohydrate intake, people tend to use different arguments. Some put emphasis on genetic adaptations (e.g., the number of AMY1 gene copies), some focus on the nutrient-density of different foods, and others are more concerned with what our ancestors ate. In this series of posts I’ll try to combine “all” of these different angles by looking into the following questions:
- What does our biology tell us about adaptation to carbohydrate consumption?
- What does our physiology tell us about adaptation to diet?
- What types of foods fueled the evolution of our large brains?
- What can the diets of the healthiest populations that have ever been studied tell us about optimal carbohydrate intake?
- What can a critical look at nutritional science and dietary advice tell us about optimal carbohydrate intake?
- Does carbohydrate intake impact longevity?
In part 1, we’ll focus on the three first items on the list, in part 2 we’ll get into question 4, in part 3 we’ll get into question 5, and in part 4 we’ll finish with question 6, a summary, and some practical takeaways.
What does our biology tell us about adaptation to carbohydrate consumption?
To be able to digest and metabolize the carbohydrates we eat, we depend on both human and microbial genes. Let’s do a quick run through of the various types of carbohydrates in our diet…
Starch is the major carbohydrate in most people’s diet and also the polysaccharide that tends to receive the most attention in the discussions of carbohydrate intake. Although there isn’t a lot of research in this area yet, there are studies which suggest that humans often differ markedly in our ability to digest starch. Here’s a quote from RobbWolf.com:
Our gene pool began to differentiate between one another when we began to settle in various locations around the globe. Some hunter-gatherer groups settled in cold climates, some in warm climates, and everything in between. Each location offered its own challenges and evolutionary pressures, one of them being diet.
For example, colder climates may have relied more heavily on animal meats for food and warmer, wetter climates may have relied more heavily on plant food (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2377015/#R8). This led to diversity in one specific gene responsible for the breakdown of carbohydrates, alpha-amylase (AMY1). AMY1 is a salivary enzyme that begins the breakdown of starch in the mouth and makes it taste sweet.
AMY1 variation exists between different members of the human species. This may be a major reason why there is so much variation from person to person when it comes to carbohydrate intake. Some people thrive on a higher carbohydrate diet and others thrive when carbohydrates are kept in check. This is also a reason why there will never be just one perfect human diet.
The USDA recommends that the entire population consumes 45% to 65% of their daily calories in the form of starch. Is this a correct recommendation to the part of the population that contains fewer copies of the AMY1 gene? It is not only unfair, but may be setting them up for a future filled with weight issues and all the diseases that accompany increased weight.
Abigail Manell and Paul Breslin have done some amazing research at the Monell Chemical Senses Center in Philadelphia. One study in particular looked at starch digestion between differing AMY1 groups. The experimental group was healthy, non-obese individuals and they were divided into a high amylase group and a low amylase group. They came into the lab twice, once to ingest starch (experiment) and glucose (control). The low amylase group had higher blood glucose levels then the high amylase group during starch consumption. This increase in blood glucose levels lasted for the two hours that the participants remained at the lab! Interestingly, when the low amylase group consumed the glucose blood sugar levels remained relatively consistent with the high amylase group and the blood sugar did not stay elevated as long as when they ingested the starch (http://jn.nutrition.org/content/142/5/853.abstract) (1).
How important is this difference in AMY1 copies? We don’t have studies to say for sure. What we do know is that individuals from populations with high-starch diets have on average more AMY1 copies than those with traditionally low-starch diets (2); an observation that can help explain why Europeans and Asians tend to do better on high-carbohydrate diets than those who descend from people who’ve been living in parts of the world where starch has not been consumed in large quantities. In other words, your ability to digest starch largely depends on what your ancestors ate.
Also, as mentioned in the quote; the difference in amylase gene (AMY1) copies can help explain why some people do better on the high-carbohydrate, grain-based modern diet than others. As Kevin Cann notes; recommending a starch-based diet to someone with few AMY1 copies is probably not a good idea.
How relevant is this varying ability to digest starch to those of us eating an ancestral, hunter-gatherer type diet? It does play a role, but not a major one. Unless you stack up on potatoes and yams, you’re not going to be consuming so much starch that it’s going to be problematic. After all, very few of us descend from Inuits and other indigenous people who’ve been eating diets extremely low in starch.
In terms of monosaccharides and disaccharides, the obvious one that stands out as problematic is lactose. Most of the world’s population loses the ability to completely digest a physiological dose of lactose after infancy, and the food sensitivity that then occurs – lactose intolerance – is one of those conditions that really highlights how long it can take for the human genome to adapt to dietary changes. Luckily, the human microbiome can adapt rapidly; meaning that although a person doesn’t carry lactase persistent alleles, he/she can still adapt to break down lactose by eating lactose-digesting bacteria – which can transfer genes to bacteria living in the gut through horizontal gene transfer.
What about other complex carbohydrates, such as oligosaccharides and non-starch polysaccharides? When discussing carbohydrate intake, it’s important to clarify whether dietary fiber is included or not. Dietary fibers are technically carbohydrates, but they aren’t digested and absorbed like for example starch, maltose, fructose, and glucose. Some of the dietary fibers we eat are passed through undigested, while others are fermented by gut bacteria. If we don’t carry the genes that code for the enzymes that are needed to digest these otherwise indigestible compounds, food intolerance (e.g., FODMAP intolerance) occurs. Solution: Manipulating the gut biome.
These fermentable fibers – such as resistant starch and inulin-type fructans – do provide calories in the sense that various nutrients, such as short-chain fatty acids, are produced by gut bacteria during fermentation. These end-products are sometimes used by other cross-feeders in the gut – or by the human host.
So, although fermentable fibers are considered as part of the carbohydrate group, it’s actually fats we derive from them. A good example of this transformation from carbohydrate to fat is seen in the digestive tract of ruminants (e.g., pasture-fed cattle), where fibers, especially cellulose and hemi-cellulose, are broken down by bacteria into the three volatile fatty acids (VFAs): acetic acid, propionic acid, and butyric acid. In other words, although the cattle you see grazing around seem to be eating a carbohydrate-based diet, they actually get most of their energy from fat.
But how much energy do we really get from these fermentable carbohydrates? There’s no clear consensus. Here’s what Wikipedia has to say:
Soluble fiber is partially fermented, with the degree of fermentability varying with the type of fiber, and contributes some energy when broken down and absorbed by the body. Dietitians have not reached a consensus on how much energy is actually absorbed, but some approximate 8 kJ/g (1.9 kcal/g). Regardless of the type of fiber, the body absorbs less than 17 kJ/g (4.1 kcal/g), which can create inconsistencies for actual product nutrition labels. In some countries fiber is not listed on nutrition labels and is considered to provide no energy. In other countries all fiber must be listed and is simplistically considered to provide 17 kJ/g (4.1 kcal/g) (because chemically fiber is a type of carbohydrate and other carbohydrates provide that amount of energy). In the US, soluble fiber must be counted as 4 kcal/g (17 kJ/g), but insoluble fiber may be (and usually is) treated as not providing energy and not mentioned on the label (3).
All in all, whether you include the energy from fermentable fiber in the fat category, carbohydrate category, or not at all really depends on how you look at things. If you include them in the percentage of energy you get from carbohydrate, it’s especially important not to go too low on the carbohydrate curve, as a very low-carb diet, rid of fermentable substrates, is not something most people can thrive on.
What does our physiology tell us about adaptation to diet?
What is special about the human body? Important features include…
- An unusually large brain.
- A smaller colon and relatively enlarged small intestine compared to other large-bodied primates (4).
- An ability to walk on two legs (bipedalism).
- An ability to run long distances and cool ourselves down through perspiration.
Brain development and gut size are especially important features to keep in mind when discussing diet. Why did humans evolve larger brains and smaller colons? According to the expensive tissue hypothesis, smaller gut size was simply a way of compensating for brain growth. When one energy expensive organ grew, the decrease in colon size worked as a compensatory mechanism – and our total energy expenditure remained somewhat the same. In other words, the expensive tissue hypothesis suggests that reduction in gut size was an adaptation that helped balance the increased metabolic costs associated with the expansion of brain size during human evolution.
However, while there are studies that support this theory, there are also those that don’t (5). Humans definitely have relatively small colons compared to most other large primates, but this reduction in gut size could simply have been a consequence of the transition over to a higher quality diet, which contained less plant foods that were subject to colonic fermentation. Anyways, it’s interesting to note that although humans have a much larger and more energy expensive brain than other primates, our basal metabolic rate (BMR) is fairly similar (in relation to our body size) (4,5). So, a reduction in gut size – regardless of the cause – was definitely important in terms of keeping our BMR “down”. In combination with this changing gut size, brain growth was probably also supported by changes in body composition.
Besides the change in gut size, there was also a reduction in size of molar dentition, indicating that hominins living 1.5–2 million years ago started consuming foods that required less mechanical breakdown, i.e., chewing (7).
But what triggered and/or supported the brain growth of Homo erectus? More complex social systems (the social brain hypothesis) and factors related to culture (e.g., language, art) can help explain why we developed such large brains, but what we’re more interested in here is diet. Which dietary conditions lay the foundation for the growth of the large hominin brain? Many theories have been proposed, but a general consensus is that increased dietary quality, more food sharing, perhaps division of foraging tasks, and a more flexible and versatile subsistence strategy probably played key roles.
As mentioned, while other large plant-eating primates have large colons that make them able to extract energy from their fibrous, low-quality diets, Homo evolved a relatively large small intestine and a small colon. Basically, our gut morphology is more similar to a carnivore than that of long-bodied primates; a change that reflects adaptation to an easily digested, relatively calorie-dense diet (4).
Overall, the staple foods for all human societies are much more nutritionally dense than those of other large bodied primates. This higher-quality diet for humans relative to other large-bodied primates means that we need to eat a smaller volume of food to get the energy and nutrients we require (4).
This change in gut size makes complete sense, as it would have been extremely difficult to fuel our large brains on a diet that is predominantly composed of low-calorie plant foods.
What types of foods fueled the evolution of our large brains?
The reason I think it’s important to go back and look at what early hominins ate is because it gives us a good indication of what types of foods we’re best adapted to eat, and it shows us the role carbohydrates have played in the human diet throughout our evolution.
In an ancestral environment, there are primarily three major foods that could drive increased dietary quality: Honey, meat (and perhaps some marine foods), and underground storage organs (USOs). Why? Because these foods are relatively calorie-dense (especially meat and honey) and they are primarily absorbed in the small intestine. USOs are rich in dietary fiber and not easy to digest in their raw form, but when cooked, the starch is released. There’s good data which show that animal products were very important, while for tubers and honey, the research is less conclusive (4,8,9,10).
The ability to cook food is one of the things that separate us from other animals and helps us exact large amounts of energy from foods that are otherwise hard to digest. Basically, while other animals primarily process food inside their body, humans have developed more advanced ways of processing our food outside of our body. This ability was certainly helpful in ancestral times, where hunter-gatherers were able to extract more energy from meat and tubers through cooking – and in traditional populations, fruits and vegetables could be stored for long periods of time through the use of lacto-fermentation. However, in today’s society it could be argued that our capability to process our food is working to our disadvantage in the sense that we over-process a lot of the stuff we eat.
Let’s take a quick look at the hypotheses surrounding meat, honey, and tubers…
Four of the problems with the USO hypothesis are:
- We really don’t know how far back cooking goes. Most data suggest that the ability to control fire and cook food came after the brain growth started to spur about 1.5-2 MYA (4,11).
- Brain growth of mammals is dependent upon sufficient amounts of docosahexaenoic acid (DHA) and arachidonic acid (AA); long-chain fatty acids that are primarily found in animal source food (4).
- Although we don’t have studies to say for sure, it seems that the increase in AMY1 gene copy number in humans didn’t occur until later in human evolution; indicating that starch wasn’t a major energy source for early hominins (2).
- Compared to animal source foods, wild tubers and other underground storage organs are relatively low in calories.
What about the “recent” idea that honey could be an important food supporting the brain growth of early Homo? Honey definitely scores high in terms of calorie-density, and as I’ve previously discussed, humans are naturally drawn to the taste of sweetness. All in all, honey could have been an important energy source that helped fuel the growth of the large hominin brain.
The data as a whole suggest that animal products – mostly meat, and perhaps some marine foods – were the primary foods supporting the increased brain growth of early Homo, while USOs and honey were – at best – additional contributors. It’s important to note that a high-quality diet wasn’t necessarily a driving force behind our massive brain growth, but rather a necessary condition for supporting the metabolic demands associated with evolving larger brains.
In the period that followed from Homo erectus started evolving a larger brain about 1.5-2 MYA up until the agricultural revolution about 1.0000 years ago, environmental shifts and colonization of new parts of the world led to dietary shifts – and as we know, hominins ended up eating many different types of paleolithic diets. However, animal source food, honey, and/or USO continued being important sources of energy for most populations. Everyone ate some animal products, while access to tubers, honey, and other ancestral, carbohydrate-rich plant foods depended on the environment. In part 2 we’ll look into what hunter-gatherers, traditional populations, and other healthy cultures – both ancestral and contemporary – actually ate…