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  Last updated: 4/8/2019

Building Blocks of all Foods

Fats provide between 20 and 40% of the daily Calories in an average American diet. Almost all (nearly 95%) of dietary fat is triglycerides, a single glycerol molecule linked to three fatty acid (FA) molecules. Cholesterol and phospholipids make up the other 5%. Cholesterol and phospholipids are essential building blocks for cell growth, while triglycerides are primarily an energy source.

Fat digestion takes place in the small intestine. Triglycerides are first cleaved into their component molecules - glycerol and fatty acids. The fatty acid molecules are then transported through intestinal lining cells and into the blood. As they circulate through the body, they diffuse through cell membranes where they are available to be directly metabolized as an energy source or reconstituted back into triglycerides for storage and future use. (Note - excess dietary carbohydrates are also converted into triglycerides).

The bulk of our triglycerides are found in fat cells (lipocytes), but a small percentage are stored within the muscle cells where they are more immediately available for ATP production. Of our body's total of 50,000 - 60,000 triglyceride Calories, 2,000-3,000 are in the muscle cells. Triglycerides are a larger potential source of exercise energy than muscle and liver glycogen (about 1,500 Calories).

Although both triglycerides and glycogen provide ATP for the exercising muscle, glycogen is the preferred energy source at higher levels of exertion. What keeps us from using more fat as an energy source for high level exercise?

This article gives us the current perspective.

It is bottlenecks in intracellular metabolism that limit the production of ATP from fat to power the muscle fibers. With training you can modify the ratio of fat ATP to glycogen ATP used at any specified endurance pace (and extend your riding time before you bonk) but you cannot increase the %VO2max that can be achieved on fat alone (about 65% VO2max).

The rate at which our intracellular metabolic machinery (enzymes) can produce ATP from fat (to provide energy for exercise) is limited to about one-third the rate of ATP formation from glucose. The net effect is that using fat alone as an exercise energy source sets an upper limit for maximal performance at ~ 65% VO2max.

Endurance training increases both intracellular fat oxidation as well as the ability to extract fatty acids from the blood, but it does not increase the rate of release of fat from peripheral adipose tissue into the blood stream (that is increase the delivery of new fatty acid Calories). Thus with endurance training, athletes can replace glycogen Calories (for exercise) with fat Calories allowing the athlete to exercise longer (at endurance levels of ~ 65% VO2max) before becoming exhausted due to glycogen depletion. But training does not increase the %VO2max performance that can be achieved with fat alone as the muscle ATP energy source.

The exact reason for this metabolic preference is unclear although there have been a number of suggestions as to why this might be the case.

This graph (Romjin et al, shows us the relative contribution of Caloric energy from intracellular glycogen versus triglycerides along with the energy released from metabolism of the glucose and FFAs (in the blood) as the level of exertion increases. There is a plateau in fat Calorie production as VO2 increases, and as a result a falling percent of total expended Calories provided by fats as muscle cell energy needs increase. It is this plateau in fat ATP (Calorie) production that sets the maximal limit of %VO2max performance.

The ability to extract FFA from the blood to support exercise reaches a plateau as well and as a result the total Calories provided from FFAs as an energy source actually goes down at higher levels of exertion. The result is that the percentage of Calories provided by fats to support performance at 90 to 100% VO2 max is quite small, and above 100 %VO2 max (anaerobic sprints) basically non existent with carbohydrates alone fueling the muscle cells. The graph also shows us the reason for "the Bonk". Once your glycogen stores have been depleted, no matter how hard you try, you can only produce ~ 50 - 60 % of the total ATP (Calories) that would be possible if glycogen was still available - thus overall muscle cell performance is capped.


Brown fat versus white fat. What is the difference? There are definitely health benefits (brown fat is better) and there has been speculation that brown fat (which is generally subcutaneous (under the skin) is the reason Eskimos and other cold climate dwellers tolerate cold so well (insulation as well as access to extra calories for heat). This post from Dr. Mirkin is a nice summary of the current literature. As I read the post, it crossed my mind that the development of brown fat cells is another small piece to explain the benefits of training. Brown fat cells contain extra mitochondria, suggesting they more easily provide fat energy. And as exercise appears to stimulate the development of brown fat, this change may be one of the many training effects that leads to performance improvement (in endurance sports) by supplementing the calories available to the active muscles from your internal glycogen stores.


Multiple physiologic studies have proven that fat will not sustain aerobic activity > 65% VO2max anaerobic activity . The following study is just one of many that document the need for CHO to fuel maximum performance.

It was the lack of adequate muscle glycogen that was the basis of the limited performance - both %VO2max achieved, and longer total time to complete the time trial.

A high fat diet contributes to the lack of glycogen repletion in two ways. First, fat itself will not rebuilding glycogen stores. And second, as fats are quite effective in quelling hunger, a high fat diet leads to eating even fewer CHO Calories in the low CHO meal. And over time, this vicious cycle leads to chronic muscle glycogen depletion with associated poor performance.

How about endurance events, at 50 - 60% VO2max? Although the evidence is clear that fat cannot replace carbohydrates to fuel high level performance, we know that a trained athlete will get a greater percentage of their exercise Calories from fat when exercising at 60 - 65% VO2max. And that should translate into riding further at endurance speeds, especially if there is a lack of adequate oral CHO supplementation.

Will a ketogenic diet lead to an additional "training" of the cells metabolic machinery? That remains controversial. Various arguments have been put forward to support the ketogenic training approach:

As you read the various studies, keep in mind these two diet scenarios.

Here are a couple studies/reviews that support the possibility that eating a high fat diet will lead to adaptation of fat metabolism and improved endurance performance. My conclusion is there is limited data that training on a high fat/low carbohydrate diet beneficially changes the fat/CHO ATP fuel ratio for high VO2 max performance and any improvement may be: This is a well written summary that reiterates the limits of a low carb, high fat diet, that it works IF the athlete is happy both training AND competing at " 62 to 64 percent of their VO2max. " But if they are already competing at 62 to 64 percent of their VO2max, and want to improve their times, they have to reintroduce carbs into their diet to get better times.

To quote Dr. Mirkin: "The Flaw in Studies Used to Support LCHF for Athletes - When you correct for the diet-induced weight loss, all the oxygen capacity gains appear to be from the loss of weight and not their ability to take in more oxygen and go faster. i.e. increase their own personal 62 - 64% VO2max for fat fueled performance."

The bottom line is that if you want to run longer at a slower speeds, a LCHF diet will do the trick. If you want to run faster, you need carbs both to improve your potential (by training at a higher %VO2max) and for the event itself (which means you can then run a bit faster than those who are glycogen depleted and thus limited by a high fat diet alone.)

The comments in this New Zealand study are probably closest to a real life adaptation of a ketogenic diet for the athlete. It found that participants migrated back to a diet containing some carbohydrates even after noting an increased sense of well being on the ketogenic diet. To quote: "Being a translational study, we followed up participants informally 12 months after the study concluded. They were all still competing in endurance events, and while not eating a ketogenic diet, none of them had returned to their previous high carbohydrate, low fat style of eating. Collectively, they reported that once the study concluded they gradually increased their carbohydrate intake until the point at which they felt their performance at high intensities return. They were still restricting carbohydrate and eating more fat than mainstream guidelines recommend, and reported having discovered the optimal macro-nutrient ratio that satisfied a performance, body composition and a health goal."

It supports the idea that a low carbohydrate diet (ketogenic diet) may increase the percent of fat Calories used (and in turn lessen the need for glycogen Calories) for endurance level exercise (< 65% VO2max) but training on a carbohydrate limited diet will not significantly increase the intensity of exercise that can be achieved from fat metabolism alone. Carbohydrates will still be needed for sprints and other periods requiring more intense exercise.

So we find that the answer is not at the extremes - full ketogenic versus unrestricted carbohydrates, but in between. Applying the old adage "moderation in all things" should be interpreted as no free sugars, a lot of plants, modest meat protein (optional) and just enough carbohydrates to replace the Calories used in your more intensive athletic pursuits.

Fats are a useful part of a balanced diet for the athlete in training. But there is no proof that going the extra step and eating an extremely low carbohydrate/high fat training diet in some way forces fat metabolism efficiency. A friend of mine commented to me that training on the South Beach diet (a low carbohydrate and thus relative high fat weight loss diet) was one of the more difficult things he had ever done as he rode with a feeling of fatigue every day. And as a result, he could not push his training to levels above ~ 65% VO2max. He lost weight (his goal) but in turn felt he had lost his high end aerobic training edge.

A high fat/low carbohydrate training diet seems to offer a benefit only for a select sub group of elite ultra-endurance athletes, where long distances are to be covered at a < 65% VO2max level of exertion - and a final sprint is not part of the competition plan.

And how about the role of fats in the 4 hour period immediately preceding the event. At this time there is no evidence that fats offer any advantage to carbohydrates in the 4 hour prerace interval.

Final take aways?


In terms of overall health, several decades of research and clinical studies have led the US Surgeon General and numerous other health authorities to recommend a diet that is higher in carbohydrate, lower in fats, and rich in fruits, vegetables, and whole grains. Such a diet is associated with a lowering of the risk of major chronic diseases including hypertension, atherosclerosis and heart disease, and certain cancers. But in the past few years the validity of that recommendation has been brought into question. Volunteers fed a 60% CHO, 25% fat diet versus 40% CHO, 45% fat with Calories equal had lower HDLs without a change in LDL questioning the idea of substituting CHO for fat to lower risk. (Am J Cardiol 2000:85:45-48.) And when planning a diet to lose weight, a 33% fat diet was more effective in achieving weight loss than a 20% diet. (Int J Obesity 2001:25:1503-1511).

Consider replacing as much saturated fat as possible with mono-unsaturated fats. You also need to be sure to get adequate omega-3 and omega-6 fatty acids -- the essential fatty acids found in many foods, including fish, flax seed, walnuts, and wheat germ. Use nuts such as almonds and walnuts as a topping in your cereal or add them to salads. Substitute monounsaturated (olive oil, canola oil, and avocado) for saturated oils and eliminate trans-fatty acids.

As (1) the negative effects of an extremely high fat diet on health are well accepted, (2) carbohydrates are clearly superior to fats for high intensity events (both for training and on event day), and (3) fats may, EVEN AT THEIR BEST, only be equal to carbohydrates for lower intensity, endurance events, there is no reason to emphasize fats in a training or day of event dietary program. And for those who still aren't convinced, it should be remembered that even the leanest athlete has plenty of stored fat available (approximately 100,000 Calories worth in a 70 kg male) without any need for any dietary fat supplementation.

A variation on this theme is reflected in fat containing energy bars which are alleged to improve performance by SELECTIVELY increasing fat metabolism. While there has been some evidence that an occasional long slow recovery ride in your training program MIGHT improve the ability to metabolize or use stored fat Calories for muscle energy, there is no scientific basis for the claims made by these products that eating any particular food or food type (i.e. fat) will enhance fat metabolism.

A healthier alternative to eating more fat would be to focus on a training program that stresses more baseline miles at a relatively slow pace (60% VO2 max.) with the thought (unproven) that this might improve the muscle cells ability to use internal fat stores. Another variation on this theme is to avoid carbohydrates in the pre ride meal, and minimize carbohydrate snacks while on that long slow ride to "force" the development of metabolic pathways that use fat energy (a planned "bonk' if you will). The idea, or so goes the theory, is that when it comes time for that sprint at the end of a competitive event, having used more of your fat stores for muscle energy along the way, there will be more muscle glycogen remaining to give you the edge. Friends have told me that the agony of training on a low CHO diet is not worth it - so you will have to make up your own mind as to the benefits of this type of "metabolic training".

TYPES OF FAT (vegetable vs animal - saturated vs non saturated)


Q.I am trying to chase nutritional information down to primary references. For example, you and many others state that one pound of body fat = 3,500 calories, but give no reference. I calculate 9 calories/gram of fat x 453 grams/pound = 4046 calories per gram of fat.

A.I did find this footnote in Understanding Nutrition 4th edition, 1987, by Whitney and Hamilton; West Publishing Company (a text on nutrition) on page 90. "The reader who knows that 1 pound = 454 grams and that 1 gram of fat = 9 Calories, may wonder why a pound of body fat doesn't equal 9 x 454 Calories. The reason is that body fat contains some cell water and other materials; it's not quite pure fat."

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