As a sports med doctor, part of my job is to keep up to date with injury and training developments, and stay abreast of the latest research and expert opinion. A topic that is very much at the forefront of endurance sport right now, is the role of the High Fat/Low Carb (HFLC) diet. It's also very topical in non-sporting circles, and there is a lot of interest in the health benefits and performance enhancements this dietary approach can confer.

First let me step back a bit.

It's been generally held that carbohydrates are the main fuel used by endurance athletes in their training and racing. This advice has been around for ions, and you just need to look at all the pre race 'pasta dinner' events going on, the concept of carbo loading, and the amount of sugar based sports nutrition products out there to see that this is a very widely held belief. Populations have been urged to consume low fat diets for decades in the hope that we can reduce rates of obesity, diabetes and coronary artery disease. Now there are clearly 2 very different angles here - one of living a healthy and disease free life, and one of maximizing athletic performance, and I will address both, but I think that the times are a changin', and we are seeing a paradigm shift in the way we advise endurance athletes and in the way that physicians advise their patients.

First let's look at the population health side of things. If you look at obesity and type 2 diabetes rates over the last number of decades, there has been a steady rise. The graph below highlights obesity rates changes in US adults. Type 2 diabetes rates show an identical trend. It seems that the advice to cut down on the amount of fat in our diets, has backfired. It doesn't take a degree in nutrition to see why this may be the case.

Obesity graph

Humans have evolved over millions of years to exist on a diet that is relatively sparse in carbohydrate. Historically, we survived by eating food that we caught when hunting and fishing, or gathered from nature. These food sources certainly didn't include starches or refined sugars. When the developed world became fat-phobic in the 1980s, the fat that was removed from our diet was replaced with highly refined carbohydrates in the form of pure sugar and white flour. The industrial revolution had given us easy access to these new forms of food; unfortunately a hundred or so years is simply not enough time for our species to genetically adapt to our new diet, and consequently we don't handle these products too well.

Refined carbohydrates don't satiate like natural fats do, and the large spike in insulin that follows a high carbohydrate meal, simply makes us hungry, sooner, as our blood sugar levels drop rapidly. As a result we eat more. It's essentially an addiction or a strong craving for more, in order to satisfy our appetites. 

Now I am the first to admit fully that this is a very simplified model, and there are many other complex hormonal mechanisms at play, but the underlying principle is the same - refined carbohydrates cause insulin spikes, don't satiate, and contribute to overeating and are ultimately very damaging to our health. Obesity, diabetes and heart disease are the endpoint of chronic and excess carbohydrate consumption, and these are the biggest health issues in the world today.

What about fat - isn't that bad for us? Doesn't it elevate our cholesterol levels? The answer is complex, and it depends on what types of fats we are talking about, and it depends on what exactly you are measuring. Lipid levels do change on a high fat diet, but usually in a good way. HDL levels rise and triglycerides levels drop - both good things; but often LDL cholesterol levels will climb (although sometimes the opposite is true )and this is concerning to my patients at times. However, all LDL particles are not created equal! LDL particles have sub-types, and it seems that the LDL rise which can occur with the HFLC diet, tends to consist of the larger less atherogenic (artery clocking) sub-type of LDL particle, rather than the more sinister and dangerous smaller LDL particles, so we can reassure patients that a rise in LDL is not really a huge big deal. Increasingly I look at HDL and triglyceride levels as being better markers for health/disease risk.

Having said all of this, it really is important to consume the right sorts of fats and oils in order to maximize the health benefits.  Fresh whole foods which are naturally high in saturated fats and monounsaturated fats will make up a large portion of the HFLC diet. Examples include: fresh meat (including organ meats), poultry, fish and eggs. Dairy sources rich in healthful saturated fat includes cream, sour cream, butter, whole milk yogurt and cheese. Plant sources of good fats include avocado, coconut oil and coconut milk. Highly processed vegetable oils such as canola, corn, safflower and sunflower oils which are very high in pro-inflammatory omega-6 fatty acids are best avoided. The health benefits of the HFLC approach make total sense to me as a physician, and I have seen many patients reap the benefits.

High fat low carb foods


So now that we've dabbled a little in some of the health issues, let's look a bit at athletic performance. I will post again on this topic, but as an ultrarunner there are a few things that are very important to me when I consider the ideal fuel for my running performance.

1) I want to eat as little as possible when I race and train - I have to carry that stuff with me!

2) Ideally, my fuel source should be abundant in my body already, so that I don't need to worry about running out of it.

3) I want to be lean as a runner - I climb a lot of hills when training and racing, and excess body fat slows me down. My diet should promote lean-ness.

Ultrarunning is a primarily aerobic activity, performed at relatively slow paces. In fact almost all running is aerobic, except for short dashes like the 100 or 200m sprint events. Intensely anaerobic efforts such as sprinting or heavy power lifting rely almost entirely on carbohydrate as the energy substrate, in the form of stored muscle and liver glycogen, and no one really doubts that carbs are absolutely vital for sprint performance. But it turns out that as our activity becomes more aerobic, and as the pace mellows out, we use increasing percentages of fat to fuel the activity.  In the past, data suggested that even at slower paces, a large proportion of our energy was derived from stored glycogen. But recent research suggests that we can fuel our race efforts almost entirely off our body fat stores. What's even more exciting, is that it appears that we can train our bodies to metabolize fat very efficiently, even at much higher intensities than was thought possible! Imagine if we could exercise at near maximal efforts and still be using fat as our main fuel source.

The human body is designed to use fat as it's main fuel source. Throughout evolution, fat was abundant to the hunter/gatherer, whereas carbohydrate was scarce. To survive, humans have had to adapt to utilize a readily available fuel source. Even when we maximize our muscle and liver glycogen stores through carbohydrate consumption, we only carry enough stored calories for about an hour and a half of moderate intensity exercise. That would not have fueled an all day hunt or trek, and it certainly isn't going to get me through an ultra!  

But guess what, even the leanest athlete carries with him about 40-50,000 calories stored in fat. It's more than enough to get us through all day efforts. We just need to train ourselves to be very efficient fat burners, and that's really where the HFLC diet comes in. Combined with the right training program we can become much more efficient at tapping into our own fat stores, and by mobilizing our own fat stores as fuel, we lose weight and become leaner. Of course, all of this is win-win for the ultra endurance runner.

But we still need some carbs, I hear you say, and that is absolutely true. Without carbohydrate we are unable to fully tap into our fat stores during an all day effort, but it's actually a lot less than you might imagine; and less carb requirements means less gels, means less GI distress and less to carry on race day.

In my next post on this topic, I will delve a little bit into how I use a HFLC approach and how carbs can be used strategically to maximize performance;  I will also discuss some of the latest research findings in elite ultra athletes.

Zach Bitter, elite ultra runner who uses a HFLC approach. Picture taken from his blog at:    

Zach Bitter, elite ultra runner who uses a HFLC approach. Picture taken from his blog at: