Fasting Can Boost Endurance (#5)

Intermittent fasting has become an increasingly intriguing tool for athletes aiming to enhance their endurance and the body's ability to utilize fat as fuel. In this article, we delve into what fasting entails, how the body is affected, and what the research actually shows. The focus is on whether fasting can contribute to better performance or if it is mainly a training tool for long-term adaptation. The emerging picture is nuanced: fasting can strengthen the body's energy systems and make training more efficient, but it should be used thoughtfully and not in competitive situations.

First and foremost, there were significant individual variations from fasting. Most test subjects experienced an eightfold increase in key enzymes affecting fat utilization. However, a couple of test subjects experienced a hundredfold increase in the same enzymes. That's more than ten times the effect, which might explain why some (~20%) respond very well to fasting and increased fat utilization. There are several steps to consider, and the researchers have investigated the various steps active in the fat oxidation chain. First, we have lipoprotein lipase (LPL), the enzyme that initiates the breakdown of fat molecules. Then comes carnitine palmitoyltransferase-1 (CPT-1), the transporter that moves the fatty acid into the mitochondria where energy is extracted. CPT-1 appears to be the main limiting factor when it comes to how much energy we can extract from fatty acids during activity.

The researchers discovered that the levels of CPT-1 remained elevated even after participants broke their fast and resumed normal eating. The effects of fasting are thus not negated by eating properly afterward. In practice, this means we can do our morning workouts on an empty stomach, then eat a proper carbohydrate meal when we get home, and start preparing the body for upcoming high-intensity quality training. We can thus achieve fat adaptation and quality training on the same day = Awesome! The adaptation to increased fat oxidation remains even when you're on the starting line fully loaded with muscle glycogen after carbohydrate loading.

In summary, you become better at using fat as fuel by training fasted after a 24-hour short-term fast once a week.

Short-term fasting and performance is an exciting area. First and foremost, it's important to know that fasting should be done during training periods, never before competition. There is no evidence that short-term fasting has any acute positive effect, so you should not compete while fasting. Here, the majority of researchers agree, including Volek and Phinney, who are pioneers in the field. They demonstrate that the adaptation to fat usage increases with various forms of fasting, but you should obviously not line up depleted of carbohydrates. It's a bit like training on your road bike for the entire season before the Vätternrundan and then starting the race on a mountain bike with poorly inflated tires.

It is primarily you as an endurance athlete who can benefit greatly from improving your body's ability to utilize fatty acids as fuel to a greater extent and at higher intensity. Volek and Phinney found in one of their studies that after four weeks on a ketogenic diet, participants could oxidize as much as 90 grams of fat per hour at medium intensity, which is really exciting. So, there are indeed advantages to optimizing fat utilization, but this should not come at the expense of downregulating carbohydrate combustion, which is still the body's primary fuel in sports. You can read more about the chemistry behind this statement at the bottom of the article. If your goal is to maintain fitness, it works great to both train and compete while fasting. But as the researchers in this study write, it's not advisable if the goal is to boost performance:

“These findings are of interest to persons who exercise to maintain and enhance health and are not concerned with elite performance.”

Fasting leads to reduced levels of both liver and muscle glycogen, which in turn lowers the anabolic signals for muscle repair and growth, which is negative. However, as an endurance athlete, the goal of short-term fasting is rather to increase our metabolic flexibility and thereby improve our ability to use fatty acids as fuel. So, we accept a slight negative impact on muscle building and recovery, in favor of boosting our fat oxidation, which will likely benefit us in the long run.

In a simple and straightforward way—fasting has an edge over a ketogenic diet. This is mainly because you can gain the same benefits as with a ketogenic/low-carb diet, but you avoid many of the downsides of training with low glycogen stores (such as impaired recovery and a higher heart rate). However, we haven't come across any performance studies comparing fasting or energy-fueled performance within the same test group. There are some studies conducted during Ramadan, where results consistently show reduced performance in energy-demanding sports if fasting is started. But we've already discussed that: you shouldn't be at the start line without loaded energy reserves.

Fasting can be a practical way for you as an athlete to maintain or reduce body weight. By removing a day's eating, you effectively cut about 15 percent of your weekly energy intake. As long as you avoid overeating in the days that follow or rewarding yourself with extra cake or pastries just because you fasted, fasting one day a week can be very beneficial. However, it's important to balance performance and health. It's generally found that endurance athletes perform better at a lower body weight.

During our college years, we practiced "intermittent fasting" ourselves, but it wasn't a full 24-hour fast; we simply delayed breakfast until the afternoon and then ate between 1 PM and 9 PM or so. This straightforward approach worked well for us at the time. The specific food choices mattered less, but with fewer meals per day, it becomes crucial that each one is nutritious to avoid nutrient deficiencies. This holds true for intermittent fasting too. Plus, you actually have more time for other activities on the day you're not eating anything, which can boost your productivity.

Catabolic stress hormones increase gradually during fasting, mainly to convert fat into fatty acids and glycogen into glucose. This process also gives you a boost of energy and alertness, which can be a positive effect. However, don't extend the fast beyond 24 hours, especially if you plan to train soon and need to replenish your glycogen stores for upcoming high-quality sessions. You might also consider implementing more intermittent fasting during the off-season, particularly for strength sessions where your performance doesn't seem affected.

There's a lot going on in this field. As it's quite new, you might hear claims that aren't entirely accurate or even close to the truth. We're eagerly anticipating upcoming research. Currently, we're intrigued by ketone salts, particularly ketone monoesters, which are beginning to be used more and more in healthcare. For athletes, there's a hypothesis that taking these ketone salts could teach the body to utilize fat without the need for a ketogenic diet. On that point, everyone agrees: a ketogenic diet falls short of fasting in almost every aspect of athletic performance. If we can avoid fasting while training the body to become efficient at using fat, regardless of our carbohydrate intake or how much we eat, that would be groundbreaking. Imagine training with full glycogen reserves throughout the season, maximizing your workouts, and then starting a competition in a physiological state of ketosis, where the body excels at using fatty acids and ketones as fuel.

We also envisage that direct intake of ketone salts (BHB) during activity could enhance performance. Free-form ketones even appear to be more energy-efficient than glucose in combustion. Could we add ketones as fuel in a sports drink? We don't have the answer yet, but we're monitoring this area closely.

Two very excited people are writing this. However, we also know there are always challenges, and maybe the practical benefits for an athlete will be minimal at best. We'll just have to wait for the outcomes of the ongoing research.

We now have our two fuels, fatty acids and glucose (excluding protein from the equation). Both require energy to break down (the energy investment phase), but ultimately produce energy, referred to as the "energy generation phase." If we calculate based on the same number of carbon units in both substances (it's from the carbon that the energy comes), we see that: a fatty acid (stearic acid with 18 carbon units) provides 120 ATP at a cost of 26 oxygen molecules. Ratio 4.6:1. Glucose – three glucose molecules with 6 carbon units each, which totals 18 carbon units – delivers 96 ATP at a cost of 18 oxygen molecules. Ratio 5.3:1. The same amount of oxygen required for fat breakdown would have yielded 139 ATP (about 13% more energy) if glucose had been the fuel instead. You get 4.62 ATP (120/26) per oxygen molecule when oxidizing fat. You get 5.33 ATP (96/18) per oxygen molecule when oxidizing the same amount of energy from glucose. The result is that each energy unit from fat costs 13.3 percent more oxygen to extract compared to glucose (4.62/5.33). However, this formula is purely theoretical and when tested in practice where more parameters affect it, it turns out that each energy unit of fat still costs more energy to extract, but the difference is only 7.2% instead of 13.3 percent as the formula indicated.

Here we just want to show an underlying reason why carbohydrates/glucose are more energy-efficient as a fuel compared to fat/fatty acids. This is one of the reasons why the body can't keep up with using fat as an energy source when the intensity increases. And thus a factor in why you should train but not compete fasted, because the energy must be extracted from fat, which can be compared to competing uphill while your opponents have flat terrain. It felt like one or two years ago that we delved into this knowledge in college, so our memory needed refreshing. It was far longer ago that we sat in the mandatory school benches. It's very good to revisit the basics sometimes to gain a more fundamental understanding.