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  Last updated: 5/21/2018

Building Blocks of all Foods - Fats

Fats provide between 20 and 40% of the total daily Calories in the average American diet. Approximately 95% of dietary fat is in the form of triglycerides, fats composed of a glycerol molecule and 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 used primarily as a source of energy. Fats are a significant energy source for the endurance cyclist, providing more than 50% of the total Calories expended for activities performed at an intensity of less than 50% VO2 max.

Almost all fat digestion occurs in the small intestine where triglycerides are 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, they diffuse through cell membranes and are either directly metabolized as an energy source or reconstituted into triglycerides for storage. (Excess carbohydrates in the diet are also converted into triglycerides and stored in the same fat cells).

The bulk of our triglycerides are stored in fat cells (lipocytes), but a small amount are also stored within the muscle cells themselves. These intracellular triglycerides are immediately available for metabolic breakdown and ATP production. They contain 2,000-3,000 Calories of energy, making them a larger source of potential energy than muscle glycogen (which contains only about 1,500 Calories of potential energy). Although both triglycerides and glycogen could provide significant amounts of ATP to power the exercising muscle, glycogen is a preferred energy source at higher levels of exertion. 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 to power the muscles derived from intracellular glycogen and triglycerides along with the energy released from metabolism of the glucose and FFAs (in the blood) as the level of exertion increases. What I find most interesting is the plateau in fat Calories production as VO2 increases, and as a result the falling percent of total expended Calories provided by fats as muscle cell energy needs increase. The ability to extract FFA to support exercise is limited 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 Calories that would be possible if glycogen was still available - thus overall muscle cell performance is limited.


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.


A number of physiologic studies have proven that fat CANNOT sustain high level (80 - 100% VO2 max.) aerobic or anaerobic activity . Can training increase our ability to use fat as a fuel source for lower levels of exertion? There are two ways this might happen.

  1. You train while on a high fat diet and thus force the cells to use fat for energy. As a result they adapt to this fuel source. As you read the various studies, keep in mind that there are two diet scenarios. Acute studies where the diet is manipulated (usually high fat) for only a few days or a week before the measured performance, and others which use a longer training period (weeks to months). As it may take time to adapt to a high fat diet, those that only ate high fat for a few days are not comparable to the longer studies.
  2. Training on a standard diet increases the use of fat Calories to power performance.

Recently a very nice study once again demonstrated that CHO, not fat, is necessary for maximum performance. Seven trained athletes (in a cross over study) rode for 2 hours at 65% VO2 max to deplete muscle glycogen stores (proven by biopsy before and after the 2 hour ride). They then ate an equal Caloric diet which was high CHO (83% CHO, 5% fat) or high fat (16% CHO, 68% fat) for the next 24 hours. Muscle biopsies were again done at 24 hours and demonstrated that the high carbohydrate diet had replenished 93% of the muscle glycogen vs only 13% for the high fat diet, and also that muscle triglycerides were 60% higher in those on the high fat diet. THEN they all cycled at their maximum self paced rate (time trial level of 75 - 80% VO2max) until they had completed a set amount of work (1600 kJoules). The high fat group could not maintain their VO2 and slowly dropped to 55% VO2max while the high carbohydrate group maintained at 75 - 80% throughout the ride. And the high carbohydrate group finished at 117min vs 139min for the high fat group - almost 20% better in terms of time.

This study clearly documented that fats do NOT replete muscle glycogen, and it was muscle glycogen that limited maximum performance. Thus a high fat diet cannot adequately replace CHO in rebuilding glycogen stores, and as fats are quite effective in quelling hunger and replacing carbohydrate Calories in the diet, athletes on a high fat training diet run the risk of chronic muscle glycogen depletion and poor performance.

How about endurance events, at 50 - 60% VO2 max? Although the evidence is clear that fat cannot replace carbohydrates to fuel high level performance, there remains an ongoing controversy as to the ability of the endurance athlete to improve the use of fat to fuel muscle cells (and increase the ratio of fat to carbohydrate as fuel for competition) by prolonged training on a high fat/low carbohydrate diet. Various arguments have been put forward to support this idea (that eating a fat based i.e.low carbohydrate diet might benefit an endurance athlete):

Several recent studies/reviews have once again raised the possibility that eating a high fat diet will lead to adaptation of fat metabolism and improved endurance performance. This is a summary of a presentation at a recent conference and is reprinted from It suggests that chronic training on a high fat diet does lead to improved fat metabolism at moderate levels of endurance exertion. It also points out the down side, the lack of glycogen stores to support a kick or sprint at the end of an event. But for ultra-marathons, it seems to offer the promise that more Calories would come from fat and fewer from carbohydrates allowing one to get past the slump that occurs with muscle glycogen depletion in these endurance events.

A second study looked at training on carbohydrate deficient diets. And although it suggested an improvement in endurance performance, the data in the article suggested otherwise.

These comments are from my FaceBook comments in the article: "It has been argued that you can stress and improve fat metabolism pathways by exercising on a low carbohydrate/high fat diet, and in this way make more Calories available for those long rides. The title of this study suggests that indeed you can. But let's look at the details. If you improve the ability to metabolize fat for fuel (after running out of muscle/liver glycogen) it follows that your endurance performance should improve. Although the study did show an increase in fat metabolism (more fat Calories used for energy there was NO improvement in the 100 K time trial times. Instead there was an unexpected improvement in the peak power produced in 6 second sprint, but no numbers on how that translated into actual times. Why should the power increase? It isn't logical, but the numbers tell the tale. So if you are working on endurance, I see no reason to suffer through those carb deficient training sessions (basically riding in a bonked state). For sprinters? I'll leave the decision up to them."

In conclusion, there is limited data that training on a high fat/low carbohydrate diet beneficially changes the fuel ratio to support high VO2 max performance. It has been suggested that any improvement may be a placebo effect from that sense of well being we all notice after eating foods containing a higher percentage of fat, or perhaps due to acclimatisation to the feeling of nearing carbohydrate depletion (that is approaching "the bonk") during training, and learning to push oneself through it.

Fats are OK, and indeed useful if they are eaten in a balanced diet that encourages adequate Caloric intake for the athlete in training. But there is no proof that eating a 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 the feeling of fatigue every day. So I'd consider the high fat/low carbohydrate training diet as specifically helpful only for elite ultra-endurance athletes, where long distances are going to be covered, carbohydrate snacks are not going to be readily available, and a final sprint is not part of the plan.

A final question has to do with 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 away points?

Fats and your health

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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