Contributed by Dr. Steven T. Devor – Director of Performance Physiology for MIT and OhioHealth, and Associate Professor of Exercise Physiology, Department of Human Sciences, and Department of Physiology and Cell Biology, The Ohio State University
A high fat, low carbohydrate diet for potentially improved endurance performance has received a lot of attention in the media lately. However, in spite of all the popular press coverage and internet blogging, the peer-reviewed published scientific data to fully support this diet, and subsequently apply it widely for all levels of endurance athletes, is sparse. And the scientific literature that does exist is equivocal in nature. Additionally, no published studies to date have provided conclusive evidence that a high fat, low carbohydrate diet results in an actual improvement in endurance performance. Importantly, these same studies do indicate that a high fat, low carbohydrate diet actually decreases performance in shorter length higher intensity activities when carbohydrate is absolutely necessary for optimal performance.
The one clear conclusion from the scientific literature is that endurance athletes adhering to high fat, low carbohydrate diets become highly proficient at utilizing fat as a fuel source. Make no mistake, if your diet consists of almost exclusively fat that is what your body will adapt to and burn as your primary fuel source. Your body will become efficient at burning fat as a primary fuel source for all activities. For ultra-marathon length events, there may be some advantage to such a diet.
Advocates of the high fat, low carbohydrate diet argue that early human civilizations lived a hunter-gatherer lifestyle and their diets consisted of primarily fat. While there is clear evidence to support this, recall early humans were on their feet moving for many hours every single day. These early humans were often engaged in what is known as continual pursuit hunting, which is the process of chasing after prey until the target animal was completely exhausted and essentially stopped trying to escape. We know these early humans covered ultra-marathon length distances quite frequently.
Today many endurance athletes believe that reducing daily dietary carbohydrate intake to less than 25 percent of total calories, and increasing dietary fat to approximately 65 percent of daily total calories is a model for the high fat, low carbohydrate lifestyle. And if your diet has this sort of makeup the thought is you will burn nearly all fat during endurance exercise. However, in order to have your body enter a state of nutritional ketosis, which is required to burn fat as a primary fuel source, a diet must consist of closer to 85% fat, and almost no carbohydrates.
As mentioned, in order for your body to burn primarily fat as a fuel source, you have to enter a metabolic state known as nutritional ketosis, or simply ketosis. To have your body fully enter nutritional ketosis takes approximately three to four weeks. And during this time period you need to reduce your daily carbohydrate intake down to approximately 50 grams per day, or even lower for some people, perhaps as low as 35 grams per day. Keep in mind that one medium banana has approximately 25 grams of carbohydrate. Accordingly, your total carbohydrate intake for the day while trying to enter nutritional ketosis could consist of two bananas, or some other combination of carbohydrates, but no more than 50 grams. If carbohydrate intake is to high while trying to enter a state of nutritional ketosis you will never fully get there.
To accurately determine if your body has fully entered what is also known as a ketogenic state you must measure ketone levels in the blood. This is accomplished in a similar manner to how a diabetic measures blood glucose, via a finger stick to obtain a drop of blood and the use of a small hand held electronic monitoring device. When you have entered nutritional ketosis your body will have fully depleted your glycogen storage (glycogen is the storage form of glucose). Recall glycogen is stored in your skeletal muscles (primarily, approximately 1,400 calories worth) and your liver (secondarily, approximately 600 calories worth).
In a more traditional dietary state, when carbohydrate is abundant, the only fuel your brain and motor nerves (the nerves that connect to your skeletal muscles and allow contractions) can utilize to obtain ATP is glucose. Your body will not choose to utilize ketones for fuel, which are the byproduct of fat metabolism; they are produced when your body burns fat as fuel. However, when dietary carbohydrate intake is extremely low your body adapts, and is able to use the ketones that are produced as a byproduct of fat breakdown for fuel. Accordingly, once you have fully entered nutritional ketosis your daily dietary intake will need to remain very high fat and low carbohydrate. At this point your body is fully adapted to burning ketones as fuel.
Both dietary fat and carbohydrate are able to supply your skeletal muscles with ATP, the fuel for all body functions. There is no question that fat can be burned as fuel, but the issue related to running performance is you only get the ATP from fat breakdown half as fast as you get it from carbohydrate breakdown. Therefore, high intensity exercise becomes very difficult. When you "hit the wall" or "bonk" it is the result of your body being completely out of carbohydrate (glycogen storage is fully depleted), and you have to rely on fat as fuel. You are still able to run, but it feels extremely labored and hard. This is due to your fuel supply (ATP) only being delivered half as fast to your working skeletal muscles when you have to rely on fat as the way you obtain the ATP. My favorite car analogy for this concept is to imagine if the diameter of your gasoline supply line has been cut in half. Yes, your car will still move forward, but driving performance and speed are severely compromised.
Where there appears to be advantages to an extremely high fat diet, and being in a state of nutritional ketosis, is for athletes competing in ultra-endurance events. Running events that are at a minimum of 50 miles, and the longer the better. The published data indicate that athletes competing in ultra-marathon races that are 100 miles plus seem to benefit the most. In ultra-endurance races the intensity of exercise, the speed you are moving, is quite slow compared with a half or full marathon. And there are often several times in ultra-endurance running events where you stop running completely for nutritional or medical aid stations. These sorts of ultra-endurance events mimic the early hunter-gatherer continual pursuit hunting. Because the intensity of exercise in an ultra-endurance race is low (relatively speaking) burning fat as a fuel is not an issue. Obtaining your ATP only half as fast is still fast enough. Again, it is because your exercise intensity, your running pace, is so low compared with the pace per mile during a half or full marathon.
It is also important I believe to consider what living in a state of nutritional ketosis means from a long-term sustainable lifestyle and daily dietary perspective. Once your body is in a state of nutritional ketosis maintaining it requires that you consume very little carbohydrate every day. Typically carbohydrate intake can be no more than 15% of daily total calories, or less for some people. To achieve a daily carbohydrate intake that is less than 10% to 15% of total calories means you will have to give up consumption of many vegetables, fruits, all grains, most beans, legumes, and many forms of dairy.
If you remove nearly all the vegetables and fruits from your diet your body will not naturally obtain a large number of essential vitamins, minerals, bioflavonoids, phytochemicals, and carotenoids that are found in these foods. All of those nutrients are important for optimum health. Simply put, I do not believe long-term nutritional ketosis is a sustainable lifestyle or diet for most people. And given the large number of dietary restrictions that are necessary to remain in nutritional ketosis it becomes more of a challenge to maintain optimal health and wellness.
Let there be no question, and I cannot emphasize this point strongly enough. Most people could, and should, eat less carbohydrate every day. But what they need to cut out is the “junk” carbohydrate. Completely eliminate from your diet the processed and packaged high carbohydrate foods many Americans consume. Further, strive to completely eliminate all of the added sugars in prepared and processed foods. By added sugars I am referring to table sugar, brown sugar, all varieties of syrups, honey, confectioners glaze, dried cane extract, dextrose, high-fructose corn syrup, agave nectar, molasses, and all other calorie containing sweeteners found in prepared and processed foods and beverages. The amount of added sugars in your diet per day should not exceed more than 5% of your overall caloric intake.
Do not be fooled into thinking that because something is sweetened with, for example, honey or agave that it is “natural and healthy”. It is not, and those sweeteners still count as added sugars in your daily caloric intake. The elimination of sugars does not include those that are naturally occurring and found in fruit, fruit juice, vegetables, and milk and dairy products.
Replace all the carbohydrate you eliminate with nutrient dense, healthy fat choices and sources of high quality protein. Healthy fat choices include many varieties of nuts (e.g., almonds, walnuts, peanuts, cashews, pecans, macadamia, and pistachios), and those same nuts ground into nut butters, olives, avocados, coconut, and sunflower, sesame, and pumpkin seeds.
Sources of high quality protein can be vegetarian and vegan or from animal sources, and many protein sources also include healthy fats. Vegetarian and vegan sources of high quality protein include quinoa, many varieties of beans (e.g., black, pinto, red, navy, kidney, lentils, lima, and great northern), chickpeas, non-GMO tofu and tempeh, hemp, chia seeds, seitan, and non-dairy milks (e.g., almond, soy, rice, and coconut). Animal sources of high quality proteins include many cuts of grass fed beef and pork, responsibly farmed fish (e.g., salmon, tuna, mackerel, herring, trout, and sardines), cage free skinless-prepared chicken and turkey, bison, pheasant, and ostrich.
In summary, yes I believe nutritional ketosis is a very interesting experimental paradigm. Yes, I believe being in a state of nutritional ketosis can offer benefits for some athletes in ultra-endurance running events. However, a half or full marathon event is likely not long enough to see a performance benefit, as relying on fat as an exclusive fuel source for an event of 13.1 or 26.2 miles will almost certainly not permit a high enough running intensity. Finally, I do not believe a consuming an extremely high fat, low carbohydrate diet is a health conscious long-term sustainable way to eat and live in order to maintain optimal health and wellness.Hgi