Fats

More than 95% of dietary fat is in the form of triglycerides which are constructed of three fatty acid (FA) molecules linked to a single glycerol molecule. Cholesterol and phospholipids make up the other 5%. Cholesterol and phospholipids are important as building blocks for cell growth while triglycerides are an important energy source.

Fat digestion takes place 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 throughout the body, they diffuse through cell membranes where they can either be metabolized as an energy source or reconstituted into triglycerides for storage and future use. The bulk of the body's triglycerides are found in fat cells (lipocytes), and a small percent are stored in muscle cells and are more readily available for the production of ATP to power muscle cell contraction. Of the body's 50,000 - 60,000 stored triglyceride Calories, 2,000-3,000 are in the muscle cells. Stored triglycerides are a bigger potential energy source for the muscles than the glycogen in muscle and liver combined (about 1,500 Calories).

Although both triglycerides and glycogen can be metabolized into ATP, only glycogen supports exertion approaching VO2max. As VO2 increases above 50% VO2max, the percentage of exercise Calories provided by glycogen increases until it approaches 100% at VO2max.

What keeps us from using more fat as an energy source for high level exercise? This article suggests it is from bottlenecks in intracellular metabolism, in the conversion of fat into the ATP. There is evidence that training increases the energy provided by fat (and in turn less from glycogen) for any specific riding speed. The result? More riding time before you run out of glycogen and bonk. But even though it can be delayed, you cannot increase your "bonk speed", the %VO2max you can achieve on fat ATP alone. This level of performance is ultimately limited by the rate at which cells can produce ATP from fat, which is about one-third the maximal rate of ATP formation from glycogen/glucose.

For those interested, this article provides further details on fat metabolism in skeletal muscle cells.

This graph (from Romjin et al, http://jap.physiology.org/content/88/5/1707.long) we see the relative energy contribution from intracellular glycogen versus triglycerides (along with additional energy released from metabolism of blood glucose and FFAs) as the level of exertion increases. You can see the plateau in total Calories from fat, and as a result its providing a lower percent of total expended Calories as muscle cell energy needs increase. It is this plateau in fat ATP (Calorie) production that limits the maximal %VO2max that can be sustained by fat alone.

The ability to extract FFA from the blood to support exercise reaches a plateau as well and as a result the percentage of total Calories provided from FFAs actually goes down at higher levels of exertion.

The result of these limits in intracellular fat and blood FFA metabolism is a progressive decrease in the percentage of Calories provided by fats as the level of exertion approaches 90 to 100% VO2 max. And above 100 %VO2 max (anaerobic sprints as an example) the contribution of fat Calories is 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, fat alone provides only 50 - 60 % of the total ATP (Calories) available from a combination of fat and glycogen metabolism. And this caps the upper level of muscle cell performance.

GOING KETOGENIC - CAN YOU FORCE A CHANGE IN THE MUSCLE FUEL SOURCE?

The idea of increasing the use of fat for cellular fuel was picked up by endurance athletes as it implied the body's cells could be coaxed or trained toward a fat based metabolism and, as a result, improve endurance performance. That is the muscles would use more of the bodies vast store of fat Calories (and less of the limited glycogen stores) for those long, medium intensity rides.

Let's review the physiology and specific studies that might help us decide if this is a possibility.

There is little question, based on multiple physiologic studies, that fat will not sustain aerobic activity much above 65% VO2max. The following is an example, demonstrating that carbohydrates are needed for performance at higher aerobic (and all anaerobic) levels.

• 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).
• Then they were randomly placed on one of two equi-Caloric diets for 24 hours. One high in CHO (83% CHO, 5% fat). The other high fat (16% CHO, 68% fat).
• Muscle biopsies were done a second time at 24 hours. These showed that those on the high carbohydrate diet had replenished 93% of their original muscle glycogen stores. But those on a high fat diet were at only 13% of baseline. The researchers also noted that muscle triglycerides levels were 60% higher in those on the high fat diet.
• THEN both groups cycled at their maximum self paced time trial pace (75 - 80% VO2max) until they had completed a specified amount of work (1600 kJoules). The high fat group could not maintain their usual level of exertion and slowly dropped to 55% VO2max while the high carbohydrate group was able to maintain at 75 - 80% VO2max throughout the ride.
• As a result, the high carbohydrate group finished at 117 min vs 139 min for the high fat group - a 20% difference.

In addition to being unable to replace carbohydrates as a muscle energy source for high level aerobic (and sprint events), a high fat/ low carbohydrate diet can also result in inadequate (or suboptimal) muscle glycogen stores as

• fat itself will not rebuilding these glycogen stores
• and as they (fats) are quite effective in quelling hunger, a high fat diet results in eating even fewer total CHO Calories in the daily diet.

Fats can't substitute for carbohydrates to fuel high level aerobic and sprint events, but what about those endurance events ridden at 50 - 60% VO2max? We know that a trained athlete gets a significant percentage of their energy Calories from fat when exercising at 60 - 65% VO2max. Will a high fat, low carbohydrate (ketogenic) diet provide additional "training" of cell's metabolic machinery, increasing the number of fat Calories used for any specific endurance riding pace, and thus ride further on their muscle glycogen stores? That remains controversial.

As you read the various studies, think about them as being in one of two groups:

• ACUTE studies where diet is manipulated (usually high fat, low carbohydrate) for a few days or a week before assessing performance.
• CHRONIC studies where the training period lasts weeks to months, with the presumption it may take time to adapt to a high fat diet.

• A summary of a presentation at a recent conference reprinted from Medscape.com. It suggests that
  chronic
  training on a high fat diet does lead to improved fat metabolism at moderate levels of endurance exertion. It also highlights the down side, the lack of adequate glycogen stores to support a kick or sprint at the end of an event. But for ultra-marathons, it promises more Calories from fat and fewer from carbohydrates which might delay or even blunt the slump that occurs when muscle glycogen depletion occurs in these endurance events.

• This interview on RoadBikeRider.com

• This study suggesting an improvement in endurance performance on a carbohydrate restricted diet (although I interpreted the data in the article differently.)

  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 100 K time trial times.

  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 you.

• This well written summary of the performance risks of a ketogenic diet, that if an athlete wants to improve their times, they have to reintroduce carbs into their diet to get better times.

  To quote Dr. Mirkin: "When you correct for the diet-induced weight loss, all the oxygen uptake gains appear to be from a loss of weight (oxygen uptake is generally normalized per kg body weight) and not an ability to take in more oxygen and go faster. i.e. increase their own personal bests... for fat fueled performance."

1. There is limited data that training on a high fat/low carbohydrate diet beneficially changes the fat/CHO fuel ratio for ATP production for endurance events.

2. If you want to ride faster, you need carbs to train at a higher %VO2max as well as for the event itself. If you've been training on a ketogenic diet, consider pre-loading with just enough carbohydrates to cover the Calories you expect to expend on the ride.

3. Any anecdotal (self reported) improvements from a ketogenic diet may be:
   • a placebo effect from the sense of well being we all experience after eating foods containing a higher percentage of fat
   • acclimatisation to the discomfort of carbohydrate depletion (that is approaching "the bonk") during training, and learning to push through it
   • the feeling of well being (again a placebo type effect) that is often described on a ketogenic diet. It is speculated this is not from ketosis itself but instead linked in some way to the elimination of sugar.

4. If you want to improve your performance in competitive events (even a weekend ride with your cycling group) you should think about pre-loading enough carbohydrates to cover the Calories you expect to expend on the ride.

It is supportive of the suggestion 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 limiting dietary carbohydrates did 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.

It suggests (to me) 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 reasonable fat for a palatable diet, modest protein (meat or plant) and just enough carbohydrates to replace the Calories used in highly intensive athletic pursuits - both for training as well as the events themselves.

A final point to consider is training.

We know that you cannot increase the upper limits of fat based performance (%VO2max) on a carbohydrate free diet. Physiologic barriers in fat based ATP production do not change with training on a carbohydrate free diet. But what if you train on a ketogenic diet and then add back carbohydrates just for that special ride or event, allowing short term VO2max level performance?

If you are competing with equals, who trained on a balanced diet, you will lose. A friend of mine commented 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. 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.

Fasted cardio training is another example of a recent training trend, that just as with a South Beach diet, or a ketogenic diet, means you won't have the glycogen stores required to stress (and improve) your VO2max.

You need to push your VO2max in training if you hope to use its improvement to win an event. And if you limit your training, any slight improvement in fat metabolism will not make up the difference. Thus carbohydrates are needed for both training as well as competition. The only exception might be an ultraendurance event where you will never need to perform at > 65% VO2max. For that 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 planned as part of the competition plan.

Final take aways?

• muscle glycogen stores are essential to maximum performance above 65% VO2max
  • fats do not a substitute for carbohydrates in rebuilding muscle glycogen stores
  • you must have adequate carbohydrate Calories to optimizing muscle glycogen stores at the start of the ride or event.
• fats may extend performance at moderate levels of activity (< 65% VO2max)
• only carbohydrates can provide the energy to maximize training and improve the VO2max

1. While you have glycogen stores available, the body metabolizes a combination of glycogen and fat to produce ATP energy for muscle contractions. At lower VO2max, a higher percentage of that total energy is from fat (than from glycogen). But above ~ 65%VO2max almost all ATP comes from glycogen stores and very little if any from fat.
2. Once glycogen stores are depleted, you produce your ATP from fat alone. But bottlenecks in the cell enzyme "machinery" limit ATP production which in turn caps aerobic performance at ~65% VO2max.
3. If you train glycogen deficient, there is little evidence you will change either
   • the ratio of fat/glycogen for ATP production at less than 65% VO2max
   • the top %VO2max you can achieve on fat energy alone.

AND A FULL KETOGENIC DIET WILL DECREASE YOUR MUSCLE MASS.

The basis of the Atkins low carbohydrate, and more severely carbohydrate restricted ketogenic diet, was the thought that restricting carbohydrates would force fatty acid metabolism and combined with caloric restriction increase the rate of weight loss. A reasonable assumption supported by the logic of fat/carbohydrate based energy physiology. The blind spot is that we forgot about protein, usually a source of cell energy only in starvation situations.

. Initially, a ketogenic diet gives dramatic results. On a standard calorie restricted diet, a study group lost about a pound a week. Switched to a ketogenic diet this increased to 3 1/2 pounds the first week. But on further investigation of their body composition, it was found that the rate of fat loss had decreased by 50% with the bulk of the extra weight loss from water weight. It seemed that the with carbohydrate restriction, the body began cannibalizing its own protein instead of using fat. They lost less fat mass and more fat-free (protein and water) mass.

In a real life scenario, a group of CrossFit trainees switched to a ketogenic diet based on the assumption that a switch to a fat based metabolism would help their endurance performance. They found their leg muscle shrank by 8%. Not the trade off they were expecting.

My take-away from the referenced study?

Fat metabolism decreases on a ketogenic diet with the needed calories to support daily activities now being derived from protein (muscle) breakdown. Fat alone does not fill the energy deficit.

You could argue that if you ate a pre competition diet with enough carbohydrate calories for the planned activity, there might be a tiny bit of extra fat utilized for every calorie of muscle performance. But does this potential offset the risks of a poorer performance from both a less effective training program (see the section above) and loss of muscle tissue?

Your body evolved to use carbohydrates to fuel muscle activity. Excess carbs (beyond those needed to support that work) are a problem as they will be converted into fat. But too few carbs is also bad as the body scavenges needed calories from muscle mass.

And if you have any doubt, this article directly addresses the question of using a ketogenic diet for high level (more than endurance at 50 - 60% VO2max) performance.

Even without addressing the additional risks of osteoporosis, kidney stones, and the rare case of pancreatitis.

A ketogenic diet is not the ultimate diet for a serious athlete.

KETONE SUPPLEMENTS AS A PERFORMANCE ENHANCER

Ketones have historically been considered an important alternative energy source when blood glucose levels are low, and organs like the brain need an alternative energy source. But they also appear to modulate inflammation, oxidative stress, cell breakdown processes, and even gene expression.

Energy physiologist suggest ketones might improve cycling and other athletic performance by

1. providing an alternative (to glucose) energy source
2. improving the efficiency of adenosine triphosphate (ATP) production
3. reducing reliance on CHO utilization during exercise.

• In the first, there was a decrease in power output and time to completion of the test task.
• Three demonstrated no effect on power output
• Two showed improved performance, BUT were problematic in that they used a multi ingredient supplement (which included caffeine).
• One showed improvement in power output, but differed from the others by providing a daily supplement for 10 days rather than just a single dose pre test.

One paper suggested that ketones might positively affect the post exercise recovery of stressed muscle tissue itself. But another more focused study failed to show a difference in the recovery of muscle function, inflammation, or soreness at 24 and 48 hours. There is no evidence that ketone ingestion improves recovery better than a good protein/carbohydrate post exercise refeeding program.

Finally, one recent study suggested an alternative potential for chronic supplementation - elevated EPO level which over time might increase the total circulating red blood cell mass and oxygen carrying capacity of the blood. But without hard data from real life athletes, this remains speculation at best.

Is there a role for ketones in cycling? Aside from a still to be studied impact on red cell mass, any benefit might well be just a placebo effect. So no acute benefit on performance or recovery, but a possible positive effect from chronic use. Time will tell.

BROWN FAT

FATS AND YOUR HEALTH

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) carbohydrates are superior to fats for high intensity events (both for training and on event day), and (2) fats may, EVEN AT THEIR BEST, only be equal to carbohydrates for lower intensity, endurance events, there is no reason to emphasize fats for training or on the day of an event. 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.

TYPES OF FAT (vegetable vs animal, saturated vs non saturated, LDL vs HDL).

READER'S QUESTIONS

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."