In part 1 of this series on carbohydrate intake we looked at what our physiology and biology tell us about adaptation to diet and delved into some of the history behind the evolution of our large brain. In part 2 we established that ancestral diets and traditional diets tend to be lower in carbohydrate than western diets consumed today. In part 3 I highlighted some of the potential downsides of going very high or very low on the carbohydrate curve and made the case that for most people, a carbohydrate intake that falls somewhere in between the percentage of carbohydrate recommended by official health authorities and the percentage of carbohydrate in very low carb diets is “optimal”. Basically, this is the carbohydrate range you get when you eat a balanced, ancestral-type diet rich in both high-quality animal source food and plants foods. In this last part we’ll take a look at what the literature tells us about carbohydrate intake and longevity, summarize what we’ve learned, and wrap things up with some practical applications.
Does carbohydrate intake impact longevity?
I’ve already made the case that humans can be lean and healthy on both high-carbohydrate and low-carbohydrate diets as long as we eat the types of foods we’re best adapted to eat. However, this clearly doesn’t say that it’s irrelevant how much carbohydrate you eat. Perhaps the long-lived Okinawans of Japan would have lived even longer if they substituted some of their rice for more animal source food, perhaps the extremely healthy Kitavans on the Island of Kitava would be even healthier if they replaced some of their fruit with organ meats, or perhaps the Inuits would be even more robust if they substituted some of the marine food they eat with vegetables. Also, as I explained in section 1 on biology and adaptation to diet, some have many copies of the AMY1 gene and are therefore better suited for a high carbohydrate consumption than others. Other again will have genetic traits that make them better adapted for a meat-heavy diet. In other words, we can learn a lot from studying these populations, but we should be cautious about drawing firm conclusions about the connection between nutrition and health/longevity from these types of studies. Even though we can eat “extreme” diets with no apparent ill effects doesn’t mean we should.
This leads us to one of the final questions we’re going to briefly look into in this series: Will carbohydrate intake per se impact longevity? Clearly, studies looking into this question are extremely difficult to perform – at least in humans – as they would have to be long and very expensive. Also, it would be almost impossible to account for confounding variables, and since decreasing the overall carbohydrate intake would mean a change in the types of foods you’re eating, it’s difficult to know whether the observed effects are due to the change in dietary composition or due to the carbohydrate intake per se.
Some have claimed that the less glucose you burn during your lifetime, the better off you are. However, at this point, there really isn’t much conclusive data to support this statement. Several studies have shown that a decrease in insulin/IGF-I signaling can extend longevity in roundworms, mice, and some other organisms, but the data in humans are definitely more scant (1,2,3,4). One of the problems with decreasing the carbohydrate intake dramatically is that your diet will have to be very high in fat. As we know, a high fat intake in itself isn’t necessarily a problem, but when you go very low in carbohydrate you typically end up with eating more foods that have an “unnaturally” high fat density. If the choice is between processed products such as GHEE, oils, and butter and “natural”, whole foods such as root vegetables, I tend to favor the latter.
I would say there is conclusive evidence showing the benefits of eating plenty of vegetables. Adding some fruits and berries into your diet is also associated with many health benefits – as long as you don’t overdo it. But what about more concentrated sources of starch? This is really where the question lies. As you know from reading these articles, I think the starch content of the western diet is absurdly high, and there’s no doubt that those people eating a processed, western diet would be better of replacing some of the bread and breakfast cereals they’re eating with protein and fat. However, as I explained in part 2, going very low isn’t necessarily a good idea. At this point we really can’t say for sure whether going from what I consider a normal carb intake (e.g., 30%) down to a low carb intake (e.g., 10-15%) will have an effect on lifespan. It will be interesting to see what the research shows in this area in the future.
On the flip side, the great health of the Okinwans and the Kitavans, in combination with some studies that hint of longer lifespan for people eating high-carbohydrate diets, can speak in favor of a relatively high starch intake.
Ultimately, at this point, convincing evidence showing that strict carbohydrate restriction will increase your lifespan is lacking. The same applies to the claims that eating a “high-carbohydrate diet” will make you live longer.
We depend on both human and microbial genes to digest the carbohydrates we eat, and the types and amount of carbohydrate we’re adapted to handle largely depends on our genetics. Compared to most large-bodied primates, humans have small gut volumes for our size, with a relatively enlarged small intestines and a smaller colon. This gut morphology reflects adaptation to an easily digested, nutrient-dense diet. Although there’s a lot of controversy as to how humans developed such large brains, increased consumption of animal source food definitely played an important role. More honey and/or tubers in the diet and the ability to cook and control fire could also have been important contributing factors.
Homo sapiens sapiens is an extremely adaptable species. Populations around the world have “thrived” on diets both high and low in carbohydrate, and the data clearly show that food choices should be the number one priority, not macronutrient intake. This is one of the reasons the carbohydrate range I tend to prefer is fairly broad. However, a general theme is that the healthiest populations that have ever been studied tend to eat diets that are markedly lower in carbohydrate than western diets.
The fact that humans can thrive on diets with widely different macronutrient compositions doesn’t mean that carbohydrate intake is irrelevant, as there are several possible downsides to eating a diet that is either very high or very low in carbohydrate.
Both strict carbohydrate restriction and increased carbohydrate intake have been proposed to increase lifespan, but the data in humans at this point are scant.
When we combine an evolutionary outlook with he scientific literature and put everything together (e.g., protein intake, fiber intake, fat intake), a percentage of carbohydrate in the range of 15-40% is a very good fit for the vast majority of people. From an evolutionary perspective, fluctuations between periods of lower (e.g., drifting into ketosis) and higher carb intake – due to changing seasons and varying food availability – were a normal occurrence.
The exact amount of carbohydrate that you should be eating depends on several factors – such as your activity levels, metabolic health, and goals. If you’re an overweight individual with impaired glucose tolerance or insulin resistance, eating a lot of carbohydrates is clearly not optimal, while if you’re a hard-training endurance athlete, you tend to need more of this energy source. However, my belief is that for the vast majority of people, there is no need to go any higher than 40%. This applies even for hard-training endurance athletes, as a lot of activity results in a higher total caloric intake.
What does this mean in terms of actual carbs you’ll be eating? Let’s turn the information from all of the posts in this series into some quick practical tips. Here are the main categories I feel deserve the most emphasis:
- Monosaccharides and disaccharides: Some fruits/berries are great, but no need to go overboard. Fermented dairy (preferably grass-fed, full-fat, and unpasteurized) and other dairy products low in lactose tend to be better than milk. Although honey (including larvae) has been an important part of the diet of many hunter-gatherers, most westerners shouldn’t make this food a major part of their diet.
- Fermentable carbohydrates (e.g., resistant starch, inulin-type fructans): The vast majority of people benefit from eating more prebiotic compounds. Examples of foods especially rich in these carbohydrates include green bananas, traditionally prepared legumes, leeks, onions, and tubers (cooked and cooled starchy foods are especially rich in resistant starch 3). For people who lack microbial diversity in the gut, fermentable substrates can cause GI problems, and the microbiota has to be manipulated before microbiota-accessible carbohydrates can be tolerated.
- Starch: Intake depends on goals, health status, and number of AMY1 copies. Tuberous roots and vegetables are especially great sources of starch. If you’re someone eating a lot of healthy, fatty foods, such as coconut, avocado, eggs, meat, and seafood, you probably don’t need a lot of starch to get enough energy for the day.
What about the actual grams you’ll be eating? It’s difficult to say, as it depends on your body size and total energy needs. 15-40% of daily total energy intake is a lot more for an active endurance athlete, than it is for a sedentary person. For someone eating 2000 kcals a day (typical female intake), 15-40% equals to about 75-200 grams per day, while for someone eating 2500 kcals per day (typical male intake), 15-40% equals to about 93,75-250 grams per day. As I’ve discussed; a somewhat higher or lower intake than this doesn’t have to be problematic, as long as it’s done correctly.
The problem with a “high-carb” diet tends to be that it consists of a lot of grain products, fruit juices, and/or large amounts of sugary fruits. However, this doesn’t have to be the case. If you eat a diet that is high in tubers and/or traditionally prepared legumes, you’ll usually end up with a carbohydrate intake close – or even somewhat surpassing 40%. As long as you make sure you’re also eating enough high quality animal products, essential fatty acids, etc., this can be a perfectly healthy diet.
On the flip side, a very low carbohydrate diet doesn’t have to be unhealthy either. If you eat a diet that is largely based around animal source food (e.g., eggs, seafood, meats) and foods rich in fat (e.g., avocados, coconuts), you’ll usually end up with an average carbohydrate intake that is more in the range of 15-20%. As long as you are able to fuel your training efforts and make sure you eat enough plant foods to get an adequate amount of fiber, this can be a perfectly healthy diet.
For me personally, the percentage of carbohydrate in my diet fluctuates somewhat. In periods where my carbohydrate intake is relatively “high” (25-35%), I’ll eat more fruits and tubers (e.g., potatoes) at the expense of foods rich in fat, while in periods where my carbohydrate intake is relatively low (15-25%), I’ll eat more coconut products, dark chocolate, fatty animal products, avocados, and other foods rich in fat at the expense of starchy foods.
Where does this leave us in terms of the actual diet you’ll be eating? As always, a healthy diet consists of both animal products and plants, and it’s rich in foods such as grass-fed meats, eggs, seafood, vegetables, certain types of legumes, and fruits. If you eat according to a paleolithic template and combine animal products with tubers and other vegetables, fruits, and/or legumes for most meals, you’ll naturally get a healthy amount of carbohydrate.