by Bret Contreras January 25, 2014
“To learn about health, one must study health”
– Albert Einstein
There are few topics in the health and fitness community that are as controversial as carbohydrate consumption. While low-carb dieters sometimes claim that a high carbohydrate intake drives insulin resistance, weight gain and obesity, dietitians and proponents of low-fat diets on the other end of the spectrum often say that everyone should get 40-60% of their daily energy from carbohydrate to fuel the body. In this post I’ll take a step back and look at macronutrient intake in an evolutionary perspective and then discuss carbohydrate consumption in the context of overweight and obesity.
When studying macronutrient intake through history it’s natural to start with the human diet during the paleolithic era, which lasted from the earliest known use of stone tools about 2.6 million years ago, up until about 10,000 years before present. Although there is a lot we don’t know about the diet of ancestral populations, it’s well established that our paleolithic ancestors primarily ate fish, meat, eggs, vegetables, fruit, fungi, roots, and nuts. While it’s believed that ancestors of Homo Sapiens evolved into modern humans in Africa, we slowly migrated out into Europe and the rest of the world, and access to fat, carbohydrate and protein depended on geographical location, food availability, and season.
Loren Cordain – the world’s foremost expert on paleolithic nutrition – and colleagues have estimated that our ancestors subsided on a wide range of “paleolithic diets”, but that the most plausible percentages of total energy would be 19–35% for dietary protein, 22–40% for carbohydrate, and 28–58% for fat (1,2). While these numbers have later been questioned by other researchers, it’s widely accepted that paleolithic diets were typically higher in protein and lower in carbohydrate than are current western diets or dietary guidelines. Hunter-gatherers also consumed the most nutrient dense parts of the animal and would therefore have a moderate-high fat intake when possible.
Although hunter-gatherers certainly ate a nutrient-rich diet, this doesn’t imply that we necessarily have to emulate the eating habits of our paleolithic ancestors to be healthy. However, it does provide us with a useful framework for understanding the mismatch between the basic human nutritional template and the modern diets consumed in the world today. Even the most hardcore paleo advocates recognize that the paleo diet has to be tailored to the modern environment and often acknowledge that grass-fed dairy, red wine and some other foods unknown to the paleolithic man can be a part of a healthy diet.
The advent of agriculture about 10,000 years ago marks the beginning of a substantial shift in the human diet. Up until this time, humans had subsided primarily on wild foods, but we now began domesticating plants and animals (3,4).
The first agricultural revolution is sometimes blamed for introducing harmful food into the human diet, and proponents of the paleo diet often claim that our genetics haven’t changed much since the paleolithic era and that we therefore haven’t had enough time to adapt to “neolithic foods”. This premise is highly debated among scientists, and we have also learned that although our human genome changes fairly slowly, the “second” genome in our body – the human microbiome – can be altered fairly rapidly. Micro-organisms in our body provide metabolic functions that stretch far beyond the capabilities of the human host and can adapt to break down a wide range of food ingredients. Both lactose intolerance and gluten sensitivity are often classified as permanent conditions, but the fact is that bacteria in the gut can provide the needed enzymes to break down lactose and are also able to degrade harmful gluten peptides.
Although archaeological data show that the adoption of grain-based diets coincides with a shortening of stature and increased incidence of tooth decay (5), we know that several cultures have maintained good health with cereal grains, legumes and dairy as staple foods, and this suggests that introducing “neolithic foods” in the human diet isn’t necessarily problematic.
Weston A. Price was a dentist that travelled the world during the 1930’s visiting populations that were cut off from the western world. Dr. Price primarily wanted to investigate the oral health of people still eating traditional human diets, but he quickly learned that dental arch deformities and tooth decay are just symptoms of overall poor health. The primitive peoples he visited had very low incidence of non-communicable disease as long as they ate “natural”, unrefined food, but their health quickly detoriated when they moved and began eating refined flours, vegetable oils, sugar, and other western foods. Some of the non-westernized societies Dr. Price visited didn’t even have a name for cancer, and although they made little efforts to maintain good oral hygiene, dental cavities were virtually absent (6).
While the work of Dr. Weston A. Price has several shortcoming, it does show that humans can be lean and healthy on a wide range of diets and macronutrient intakes. Traditional people such as those in the Lötschental Valley in Switzerland, and the Scottish and Gaelic living in the Outer Hebrides relied heavily on grains as staple foods, while other non-westernized societies got more of their energy from fat. His work also suggests that part of the problem with grain consumption in the modern world is that we’re no longer taking the time to neutralize some of the potentially harmful components found in grain fiber (e.g., gluten, lectins) through traditional processing techniques such as soaking, sprouting, and fermentation.
Several other researchers have also documented the health of non-westernized people that are lean, healthy, and fit on diets with widely different macronutrient compositions (7):
While the average lifespan of humans today is probably higher than in any other part of human evolution, it’s clear that several factors such as decline in infant mortality, lower rates of infectious disease, and pharmaceutical use must be considered when comparing life expectancy (13).
The energy homeostasis system in our body regulates body fatness and energy balance on a long term basis. The increased rates of overweight and obesity in affluent nations have led to the general belief that this homeostatic system is maladapted to food excess and a sedentary lifestyle, and people are therefore advised to exercise more and to count calories in an attempt to overcome the shortcomings of the thermostat in our body.
However, the fact that non-westernized people such as The Kitavans are lean and healthy despite an abundance of food and with only a moderate amount of exercise, suggests that the energy homeostasis system functions properly when we live in the right ecological niche, eat simple whole-foods, and avoid westernized foods such as grains (refined?), sugar, and vegetable oils (14).
Although looking at protein-, fat- and carbohydrate intake in hunter-gatherer societies and healthy non-westernized populations only gives us hints about the “optimal” macronutrient ratio and diet in terms of weight loss, health, and longevity, it does provide a useful framework for understanding human nutrition because we learn the following:
Statistics show that energy consumption in the U.S. has increased from 3,100 kcal/day in 1965 to 3,900 kcal/day in 2012 (15), and although this is on the extreme end of the spectrum, similar trends are seen around the world and especially in affluent nations. It’s no doubt that overweight and obesity are caused by an imbalance between energy intake and energy expenditure, but this doesn’t tell us anything about why we overeat, how the different macronutrients impact our body, and the mechanisms that lead to weight gain. Are gluttony and inactivity really the only reasons why we get fat?
Just like our body tries to keep things like blood pressure and concentration of ions within a certain range, scientists have known for more than a century that the amount of body fat we carry is regulated by areas in the brain. We essentially have a fat mass “setpoint” that the brain defends through a number of mechanisms, such as increasing or decreasing hunger and metabolic rate (16,17).
A study published in the American Journal of Clinical Nutrition clearly illustrates the idea behind the body fat setpoint. They gave lean and overweight subjects a diet that contained 50% more calories than they naturally consumed, and as expected, all of the participants gained a significant amount of weight during the six weeks of overfeeding. The brain tried to “defend” the body fat setpoint by increasing metabolic rate and body heat production, but these compensatory mechanisms can only prevent a certain degree of fat gain. The interesting thing happened when the overfeeding phase was over and subjects were allowed to eat as much as they wanted for the following six weeks. All of the participants lost the majority of the weight they had gained although they didn’t restrict calories (18).
Other overfeeding studies both in humans and animals have shown the same thing, subjects lose most of the weight when they are no longer overfed and usually stabilize at the same, or a slightly higher setpoint than before.
These mechanisms also apply to underfeeding. Yes, off course we lose weight by consuming less energy than we expend, but the brain signals to increase energy expenditure and hunger, and we usually gain most of the weight back as soon as we’re no longer restricting calories. We’re basically fighting the body instead of working with it. These poor long-term results are seen again and again in scientific trials, but some dietitians and personal trainers still cling to the idea that we just have to exercise more and eat less (of the same food) to permanently shed the fat.
While conscious calorie restriction is necessary for someone who is already lean and wants to go down to single digits of body fat (male), it’s not a long-term solution for those that are overweight and obese. What we really have to do is acknowledge that fat mass is homeostatically regulated and look for the mechanisms that cause overeating and elevated fat setpoint. To permanently shed the fat we have to address the factors that cause the body to want to store more weight! Only then can we design a diet that allows us to lose weight without chronic calorie restriction and hunger.
Several theories have been proposed as to why the setpoint is elevated in overweight and obesity, and signaling hormones seem to play an essential role. Leptin is a hormone that’s produced by fat cells in the body and allows the brain to monitor the size of the fat stores by varying appetite and energy expenditure. The fact that leptin production correlates with the size of the fat stores would suggest that overweight and obese people produce more leptin and that the brain should respond to these signals by reducing appetite. However, obesity is characterized by leptin resistance, a condition where leptin produces a smaller response at the receptors in the brain. The brain has essentially become resistant to the signals from leptin and “believes” that the body carries much less body fat than it actually does (19,20).
In general, most westerners have leptin levels that are many times higher than non-westernized people eating “ancestral” diets (14,21). One of the reasons people who are overweight and obese have an elevated fat mass setpoint is that the feedback signal from the fat tissue only partially gets to the hypothalamus in the brain. So, the brain triggers hunger and decreased energy expenditure in an attempt to increase fat storage, boost leptin production, and overcome leptin resistance.
Although a lot of the underlying mechanisms associated with an elevated fat-mass setpoint and increased circulating leptin concentrations aren’t fully established, we do know that inflammation in the brain plays an essential role.
As I talked about earlier, the energy homeostasis system in our body seems to function properly when we eat “simple” foods such as meat, fish, fowl, vegetables, and eggs. This is clearly seen in non-westernized populations where overweight is rare and obesity is unheard of, and suggests that it’s not the brain’s hard-wired mechanisms for regulating hunger and energy expenditure that are not functioning properly, but rather that some aspects of our modern lifestyle dysregulate the energy homeostasis system. Diet seems to be the most important factor involved, and although carbohydrates per se aren’t necessarily problematic, specific types of carbohydrates play an essential role in this whole process of inflammation, leptin resistance, weight gain, and elevated body fat setpoint.
Here I’ll briefly discuss some of the common hypotheses and established facts regarding carbohydrates and weight regulation.
Most of the carbohydrates we eat are broken down into glucose in the body and then absorbed into cells with the help of a hormone called insulin. Simply put, the insulin hypothesis basically states that carbohydrates (especially refined carbs) elevate circulating insulin and that insulin drives us to store more fat. I’m not going to spend a lot of time discussing the hypothesis for the following reasons:
Bottom line: Insulin does play a role in the modern obesity epidemic, and the fact is that most people in the modern world aren’t as metabolically healthy as non-westernized people eating traditional diets, but to suggest that all carbohydrates are bad because they elevate insulin makes little sense.
Regular physical activity is associated with several improvements in health, and resistance training is great for building the body. When we lived as hunter-gatherers we typically had to expend a fair amount of energy to get the food we needed, and the cost to obtain energy was therefore much higher than it is today. Since physical activity increases energy expenditure it’s often believed that regular exercise is beneficial for weight loss. However, the brain homeostatically guards the fat mass we carry, and many of people compensate for the increased energy expenditure during exercise by eating more and/or being less active during the rest of the day (25).
The fact is that exercise doesn’t burn that many calories compared to what is achievable through diet, and although physical activity can influence body weight through several other mechanisms, such as decreasing low-grade chronic inflammation and improving insulin and leptin sensitivity, studies show that for most people, exercise alone is not a very effective strategy for losing weight (25).
The fact that several cultures with a high carbohydrate intake are fit and healthy even though they aren’t especially active also supports the notion that we don’t necessarily have to exercise to be lean.
While it’s often believed that we are less active today than in any other part of human evolution, statistics show that physical activity expenditure has not declined over the same period that obesity rates have increased dramatically (26).
Bottom line: Although physical activity improves our metabolic health and allows us to consume a larger amount of energy and carbohydrates without getting fat, studies show that for most people, isolated anaerobic or aerobic exercise is not very effective for losing weight.
The Protein-Leverage Hypothesis (PLH) suggests that humans regulate their intake of macronutrients and that protein is prioritized over fat, carbohydrate, and total energy intake. The notion that animals and humans have a target protein intake and continue eating in an attempt to reach this setpoint, actually has a fair amount of support in the scientific literature. Sources of carbohydrate and fat such as vegetables, grains and vegetable oils are cheap compared to high-quality meat, seafood, eggs, and grass-fed dairy, and protein is therefore often substituted for the other two macronutrients.
The western dietary pattern is characterized by high intakes of refined flours, vegetable oils, and sugars, and although meat and dairy products are usually a part of the diet of most westerners, modern diets are typically lower in protein and higher in carbohydrate compared to the diet we have been eating through most of human evolution. It’s estimated that our hunter-gatherer ancestors got between 19-35% of their energy from protein, while the average protein intake in the U.S. today is around 15% (27).
A recent randomized controlled study in humans showed that subjects eating a diet with 10% protein had a higher total energy intake compared to participants eating 15% or 25% protein, but that increasing protein from 15% to 25% did not alter energy intake (28). This would imply that the protein intake in the “western diet” is sufficient to avoid overconsumption of fat and carbohydrate. But, the skyrocketing rates of overweight and obesity in westernized countries suggest that people routinely consume more energy than they expend and that the percentage of protein in the diet therefore reflects protein intake on a diet that contains more food than we need to sustain bodyweight. So, although the daily excess energy intake isn’t a lot, one of the reasons we consume more food could be because we’re aiming for a targeted protein intake…
A recent review of 28 experimental trials shows that percent dietary protein in the diet is negatively associated with total energy intake regardless of fat and carbohydrate content of the diet, and strongly supports a role for protein leverage in lean, overweight, and obese humans (29).
Another study from 2012 also shows that body-weight loss and weight-maintenance on a low-carbohydrate diet seems to depend more on the protein consumption than the carbohydrate intake (30).
Bottom line: Protein is a very satiating macronutrient, and studies show that a low protein intake can lead to overeating.
One of the basic premises of the paleo diet is that the first agricultural revolution introduced “toxic” food ingredients such as lectins, gluten and phytic acid into the human food chain. Both plants and animals have their own defense system that protects them from invading predators, and while fish and land mammals are able to flee, hide or fight, plants develop secondary metabolites that protect them against pathogens and herbivores. These secondary metabolites are either benign, toxic, antinutritional or somewhere in between, depending on the animal that eats them and the quantity in which they are consumed.
“Neolithic foods” are especially rich in antinutrients, and one of the reasons paleo advocates argue that humans should avoid cereal grains, beans and white potatoes is that we haven’t had adequate time to develop resistance to antinutritional compounds that interfere with the absorption of nutrients and possibly cause other harmful effects in the body.
The toxic effects of secondary metabolites seem to depend on the antinutrient in question, how much we ingest, and how well we tolerate these compounds. There’s a lot of debate regarding the harmful effects of antinutrients, and although animal models show that phytic acid, lectins and protease inhibitors can disrupt normal gut physiology, the available data in humans are scant (31,32,33). Some researchers have proposed that the human leptin system isn’t adapted to a cereal-based diet and that lectins in grains could cause leptin resistance (34), but more research is needed to know if there is a connection between secondary metabolites such as lectins and weight gain.
One of the problems with whole grains in the western diet is that we no longer remove or deactivate antinutrients by using traditional food processing techniques. Healthy grain-based cultures usually soak, grind and/or ferment cereal grains to remove or deactivate many of the defensive substances found in grain fiber.
Bottom line: Many people experience better digestion and health when they remove wheat and other cereal grains from their diet, and this probably has to do with a sensitivity to proteins and secondary metabolites in these foods. Using traditional food processing techniques make grains and legumes more nutritious and easier to digest.
We need more human studies to say anything concrete about the connection between antinutrients, human health, and weight regulation.
In my last guest post at BretContreras.com I talked about the trillions of microorganisms that live in and on the human body and how imbalances (dysbiosis) in the microbial communities can initiate a vicious cycle of increased intestinal permeability, inflammation, and weight gain. Bacteria in the body provide a wide range of metabolic functions that stretch far beyond the physiological capabilities of the human host, and one of the primary jobs of the microorganisms in our digestive system is to break down indigestible (to the human host) food components. While most of the critters live in the large intestine, we also have bacteria in the mouth and small intestine, and the composition of microbes throughout our gastrointestinal tract depends on several factors such as diet, hygiene, and pharmaceutical use.
Plant foods such as vegetables, tubers, fruits, and functional plant parts store their carbohydrates in living cells that stay largely intact during cooking and are first breached during the digestive process. These fiber-walled living cells only allow for a maximum density of around 23% non-fibrous carbohydrate by mass, which explains why “ancestral” sources of carbohydrates such as fruits and vegetables have a relatively low-carbohydrate density compared to the most common sources of carbohydrate in the western diet (14).
Flour, sugar and processed plant foods don’t have this cellular storage and contain a considerably higher percentage of carbohydrate than anything else we have been eating throughout our evolutionary history. These “acellular” carbohydrates are essentially already broken down through the production process and provide an evolutionary unprecedented high concentration of carbohydrates in the semifluid mass of partly digested food that pass from the stomach into the small intestine.
It’s well established that a high intake of refined carbohydrates increases the growth of harmful bacteria in the mouth and often produces tooth decay. A new hypothesis of obesity suggests that dense sources of carbohydrates such as sugar and flours (possibly also whole grains) change the balance of bacteria in the small intestine and that this inflammatory microbiota initiates leptin resistance, weight gain, and obesity (14).
We also know that people frequently substitute fruits, vegetables and nuts for processed foods containing refined grains and/or sugar and therefore consume less dietary fiber. Some types of dietary fibers – prebiotics – are utilized by beneficial bacteria in our digestive systems, and a lack of these fermentable substrates can also be a part of the reason why obesity is characterized a dysbiotic gut microbiota (37).
All the microorganisms that live in the human body – the human microbiome – don’t only impact our physiological health, but also influence our mood, behaviour, and thoughts. This is relevant in terms of carbohydrates and weight gain, because gut bacteria could play a role in controlling our appetite (38). So, when our gut microbiota changes due to a high intake of highly dense (acellular?) carbohydrates and/or by a lack of prebiotic fiber, it could affect the food preferences of the human host through a positive feedback loop. It might sound far-fetched that the microorganisms in our body can influence the preferences of the host for a particular dietary regime to their own advantage, but the fact is that we are actually 99% microbe from a genetic perspective and that this microbial rainforest impacts most aspects of both our physiological and mental health (39).
Bottom line: The trillions of microbes that live in and on the human body play an essential role in regulating our body weight, and overweight and obesity is characterized by imbalances in the microbial communities. Refined carbohydrates (and possibly even whole grains) and a low intake of prebiotic fiber could drive weight gain by promoting dysbiosis, inflammation, and insulin and leptin resistance. A recent hypothesis suggests that this is a self-sustaining cycle where highly-dense sources of carbohydrates promote the growth of proinflammatory microorganisms in the gut and that these microbes increase our satiety for substrates that benefit their growth and survival.
The brain has it’s own value/reward system that reinforces or discourages certain types of behaviors. For example if we burn ourselves on a hot stove we quickly learn that the reward value of that behavior is low, and we probably won’t do it again. On the contrary, if we eat foods that are rich in rewarding qualities such as fat, starch, sugar and salt, we learn that these foods are safe, palatable and possibly energy-dense, and we’re motivated to seek out these products again and again.
The problem is that the reward system evolved to deal with natural environmental stimuli. When we lived as simple farmers and hunter-gatherers it motivated us to seek out nutrient-dense sources of energy to be able to survive and store fat for scarcer times. However, in affluent nations we now have access to an abundance of food, and food manufacturers hire scientists to design products with the most rewarding and palatable combination of fat, starch, salt, glutamate, and sugar. Compared to fruits, nuts, vegetables, and meats, modern processed food is hyper-rewarding in the sense that it contains a broad spectrum of reward factors that aren’t found in “natural”, whole foods. Some people essentially become “addicted” to fast food (40).
One of the most interesting studies on food reward in humans was published in 1965 and involved feeding lean and obese humans through a liquid feeding device. Participants were allowed to eat as much of the liquid food as they wanted, but couldn’t consume anything else. While lean subjects ate their normal amount of energy and maintained body weight, obese subjects lowered their food intake dramatically, and although they only ate between 200-300 calories a day (yes, you read that right), they didn’t experience fatigue or hunger, and quickly started losing massive amounts of weight. It seemed like their bodies responded to the monotonous, low-rewarding diet of liquid food by lowering the body fat set point and increasing the use of stored body fat for energy (41).
The brain’s mechanisms for regulating food intake and body fatness aren’t sufficient to protect against processed western food. It seems that highly-rewarding food can increase the body fat setpoint in susceptible people and cause abnormalities in the parts of the brain that regulate metabolism, fat storage, and reward (42,43,44,45).
Many obesity researchers now consider the food reward hypothesis to be a dominant factor in the modern obesity epidemic.
Highly-palatable and energy-dense foods typically contain little water, fiber and protein, and are therefore less satiating per calorie compared to simple, unprocessed foods such as root tubers, nuts, and fruits (46). This is probably one of the reasons why many people unconsciously eat fewer calories when they start eating a diet based on nutrient-rich, whole foods – they feel fuller on less energy.
Sucrose, commonly know as table sugar, is made up of the simple sugars glucose and fructose. While a healthy human body can handle a significant amount of glucose, excessive consumption of fructose from juice, sweetened beverages and other processed foods is linked to leptin resistance and weight gain (47,48).
Bottom line: One of the problems with sugar, flours, and other refined sources of carbohydrate is that they are commonly found in processed foods that also contain plenty of fat, salt, and glutamate. These foods are engineered to be palatable and rewarding in the sense that we seek them out again and again. Western foods such as sweetened beverages and biscuits are less satiating than simple, whole foods and often contain sugars such as fructose that can be harmful if eaten in excess.
A common practice employed by proponents of a low-carbohydrate eating style is to cite some of the numerous studies which show that diets with a relatively low percentage of carbohydrates (typically 5-20% of daily energy from carbs) are superior to low fat diets (40-60% energy from carbs) for weight loss.
The majority of studies show that when participants are allowed to eat as much as they want (ad libitum) from either a low-carbohydrate or low-fat diet, subjects in the low-carbohydrate group unconsciously reduce their energy intake and lose more weight than those in the low-fat group (49,50,51).
However, in energy restricted trials where participants are instructed to eat the same amount of calories from either a low-carbohydrate or low-fat diet, they lose an equal amount of weight (52,53,54). This indicates that if there is a “metabolic advantage” to low-carbohydrate diets, it’s quite small.
The fact is that these studies tell us little about the underlying mechanisms and why carbohydrate restriction is beneficial on an ad libitum diet. Is it the macronutrient intake in itself that regulates satiety, total energy intake and fat storage, or are there other factors involved? Why do people who eat low-carbohydrate diets tend to reduce their energy intake?
If you’ve read this entire post you can probably answer that question by now, and just to recap, here are some of the reasons why people tend to unconsciously reduce their energy intake – indicating a lowered body fat setpoint – when they start eating a low-carb diet:
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