In part 1 of this series on carbohydrate intake we looked at what our biology and physiology tell us about adaptation to diet and established what types of foods that fueled the evolution of our large brains and distinct human bodies. In part 2 we looked at how the healthiest populations that have ever existed eat/ate. The most important takeaways from this journey through time is that Homo sapiens sapiens is an extremely adaptable species and that populations around the world have thrived on diets with vastly different macronutrient compositions. However, a general theme is that ancestral – and to a certain extent traditional – human diets tend to be markedly lower in carbohydrate (usually in the 15-40% range) than western diets consumed today (45-65%). In this third part of the series on carbohydrate intake we’re going to build on the information in part 1 and 2 and take a closer look at what nutritional science and dietary advice tell us about optimal carbohydrate intake.
What can a critical look at nutritional science and dietary advice tell us about optimal carbohydrate intake?
Very low-carb camp: “A strict low-carb diet is the healthiest diet (<15%)”
Why do some people think that dramatically reducing the carbohydrate content of the diet is the way to go? There are several reasons, but two seem to be especially important. The first one being that plenty of anecdotal reports suggest that many people experience weight loss, more energy, better health, and a wide range of other beneficial effects when they reduce their carbohydrate intake. Second, if we look at the scientific studies that have tested high-carbohydrate diets against low-carbohydrate diets, low-carb generally comes out ahead on most points – especially in terms of fat loss (1,2,3,4). These results are undeniable, and if you do a search on PubMed, you’ll quickly find dozens of randomized trials which show that a low-carb diet is great if your goal is to lose weight and improve metabolic health.
However, as you know if you’ve been reading my articles, contrary to what a lot of people think, we can’t simply look at anecdotal reports or the conclusions of these studies and say that reducing the carbohydrate content is the key factor. To really understand why we’re seeing the study results we’re seeing, we have to dig a little deeper and ask some very important questions; such as:
- What types of foods were the participants eating?
- What types of foods did the subjects remove from their diet when they started eating a low-carb diet?
- Did their protein intake change?
I’ve previously made the case that it often isn’t the reduced carbohydrate intake per se that leads to the beneficial effects. The fact is that when people switch from a high-carb to a low-carb diet, they tend to eat more protein, less grain products, more vegetables, and less sugar and processed food. All of these things will positively impact weight loss and health. That’s not to say that carbohydrate restriction isn’t effective or that ketogenic diets are never useful, it just highlights the importance of looking at the bigger picture.
A recent example of this type of simplification was the “low-carb study” that made the rounds in the media a couple of weeks ago (e.g., here). The authors of the study conclude the following:
The low-carbohydrate diet was more effective for weight loss and cardiovascular risk factor reduction than the low-fat diet. Restricting carbohydrate may be an option for persons seeking to lose weight and reduce cardiovascular risk factors. (5).
In my opinion, the authors should have focused more on the actual food choices than on the carbohydrate component, as the low-carb group ate a diet that had several characteristics that made it superior for fat loss – including a higher protein percentage. I’m all for a reduced carbohydrate intake compared to western standards. My main point is just that we have to look beyond the conclusions of most of these meta-analyses and randomized trials and find out why we’re seeing the results we’re seeing.
Clearly, for people with insulin resistance and metabolic problems, a “low” carb diet is great. But do you really have to go very low? Here’s a quote from one of the most recent reviews on carbohydrate restriction:
… for the significant percentage of the population with insulin resistance or those classified as having metabolic syndrome or prediabetes, there is much experimental support for consumption of a moderately restricted carbohydrate diet (i.e., one providing approximately 26%-44 % of calories from carbohydrate) that emphasizes high-quality carbohydrate sources. This type of dietary pattern would likely lead to favorable changes in the aforementioned cardiovascular disease risk factors, while minimizing the potential negatives associated with consumption of the more restrictive LCDs (6).
Possible downsides of eating a diet that is very low in carbohydrate
- A very low carbohydrate diet is devoid of healthy foods such as tubers/vegetables and fruits.
- To compensate for a very low carbohydrate intake you have to eat a lot of fat. While a high-fat diet in itself isn’t problematic, a high intake of very dense sources of fat (e.g., oils, bacon, butter, GHEE) can be. Many people who end up significantly reducing their carbohydrate intake rely on these highly concentrated sources of fat to get enough energy.
- Contrary to what some people think, ketogenic diets are rare among hunter-gatherers. Drifting into ketosis for “short periods” of time is not uncommon, but few traditional populations lived exclusively on a ketogenic diet. Even the Inuits seem to have eaten enough plant food to “avoid” ketosis. Does this tell us that ketogenic diets aren’t healthy? No. However, it does show that very low carbohydrate diets are the exception rather than the rule for our species.
- A very low carbohydrate intake often leads to a low intake of fermentable fiber. New studies showing the benefits of diets high in prebiotic fiber are published pretty much every day. Contrary to most ancestral diets, modern diets are very low in prebiotic fiber, and our health suffers from it. When you go very low carb, it’s difficult to get an optimal intake of fermentable substrates, since you reduce the consumption of legumes, root tubers, and other plant foods.
- A very low carbohydrate intake can negatively impact athletic performance. It’s definitely possible to make a case for ketogenic diets in sports and fitness, but in general, eating a very low carb diet is rarely practical for the vast majority of trainees. As I’ve previously mentioned, I’ve found that getting some starch into my body is essential to perform well in the gym. That’s not to say I dig into a bowl of pasta two hours before a workout; it simply means that I add some tuberous roots into my diet.
Official health authorities: “45-65% of your daily caloric intake should come from carbohydrate”
Okay, so what about the official dietary recommendations? Why do health authorities argue that we should get as much as 45-65% of our daily energy from carbohydrate? There are many reasons of course, and to avoid making these posts very long, I won’t delve too deeply into this issue. However, we can quickly understand why the macronutrient guidelines are as they are if we look at the foods we’re recommended to eat.
The guidelines dictate that we should eat several servings of whole grains each day, a recommendation that in itself leads to a diet where fat and/or protein is diluted in favor of carbohydrate. It’s just very difficult to avoid getting more than 45% of your daily energy from carbohydrate if you’re eating grain products for every meal – and on top of that, choosing low-fat dairy products and lean meats. It would be a complete discordance if the guidelines for dietary carbohydrate intake were on the low side, while the recommendation to eat a lot of cereal grains still applied.
What are some of the other reasons we are recommended to get most of our energy from carbohydrates?
- It’s often believed that a plant-based diet is superior to a diet rich in animal source food.
- It’s often assumed that “fat is more fattening than carbohydrate”. This is primarily because fat contains more calories per gram than carbohydrate, and it’s therefore believed that it’s easier to overeat and gain weight on a diet that is rich in fatty foods.
- High-carbohydrate diets are supposedly rich in hearth-healthy fiber.
There is both some truth and untruth to all of these statements. If you want to delve into these things, check out my previous articles on these subjects.
Possible downsides of eating a diet that is very high in carbohydrate
- If you eat according to a paleo template, it’s simply very hard to get more than 40% of your daily energy from carbohydrate, unless you eat a lot of fruits and tubers. It’s definitely possible to eat a healthy high-carb ancestral diet, but a better approach is to find a more equal balance between the three macronutrients. It’s also important to mention that the fruit you get at the typical grocery store today is a lot sweeter than the fruit that was available to our ancient ancestors. That doesn’t mean that fruit is unhealthy, but snacking on foods high in “sugar” all day is not optimal.
- If you eat organic eggs, coconut products, grass-fed meats, and/or seafood rich in omega-3 and other fatty acids on a regular basis, you will naturally get plenty of protein and fat into your diet. To achieve a very high intake of carbohydrate you would have to limit your consumption of these healthy foods.
- A very high-carbohydrate intake is usually synonymous with a high intake of cereal grains. As we know, a grain-based diet is not the way to go for optimal health. You can definitely soak, ferment, or in some other way process cereal grains to get rid of the possibly harmful antinutrients and proteins, but you’re still left with food that is inferior to vegetables and fruits in terms of micronutrient profile, fiber content, etc.
- A lot of people in the western world have some type of glucose impairment and do poorly on a high-carb diet.
- While more studies in this area are needed, a very high carbohydrate diet will most likely negatively impact gene expression. Here’s a quote from the author of a recent study: “Both low-carb and high-carb diets are wrong,” says Johansen. “But a low-carb diet is closer to the right diet. A healthy diet shouldn’t be made up of more than one-third carbohydrates (up to 40 per cent of calories) in each meal, otherwise we stimulate our genes to initiate the activity that creates inflammation in the body” (7).
- A very high carbohydrate intake (especially refined grains, sugar, a lot of fruit, and fruit juices) will negatively impact the gut microbiota.
The overall theme is that both a very high and a very low intake tend to be unfavourable, and for most people, “optimal” carbohydrate intake falls somewhere in between the percentage of carbohydrate in very low carb diets and the percentage of carbohydrate in “high-carb” diets. In other words, a carbohydrate intake of 15-40% of your daily calories is a good general guideline for the vast majority of people. This is not to say that you can’t design a healthy diet that contains less than 15% carbohydrate or more than 40%, it’s just that it’s more difficult, and it’s usually not optimal.