Exercise and nutritional ketosis

Does a keto metabolism impair exercise performance?

8 min readOct 8, 2021


Photo by sporlab on Unsplash

It is known that muscles use glucose for energy, so what happens when glucose (carbohydrate) is withdrawn with a ketogenic diet (KD)— can athletes still perform?

Endurance and performance

Distance runners and other endurance athletes need to carb-load before a race, supplement their glucose supply during the race, and replenish glucose stores after a race. This is standard advice and practice. However, there is only one reason why all this is necessary — the athlete is carbohydrate (glucose) dependent.

Becoming carbohydrate-dependent is a strange thing to do, on reflection. Glucose is damaging to us, and it must be burned for energy on ingestion or stored straight away either as glycogen or locked away in fat stores under the influence of insulin. While muscles can burn fatty acids and glucose for energy, fatty acid oxidation in glucose-adapted muscle is limited — fat cannot be oxidised sufficiently for endurance exercise. That leaves the athlete dependent on glucose in their glycogen stores to power their exercise.

But, glycogen stores are minor, and nothing like the store of energy in body fat (there is at least an order of magnitude difference even for lean athletes). An athlete with only 10% body fat carries enough fuel in their fat reserves to potentially run 24 hours a day for about 3 days (in the absence of other obvious factors). In contrast, such an athlete could only run for about 2 hours on their glycogen stores — barely sufficient to get them through a marathon. Which is why carbs have to be pre-loaded, consumed and replenished before, during and after a race.

There’s another problem for exercising in a glucose-adapted state — both brain and muscle rely on glucose for energy. As glucose reserves are small and soon depleted, it is not long before brain and muscle start to compete for glucose. With longer races and without adequate glucose replenishment, muscle demand for glucose can start to bring glucose levels dangerously low for the brain. The runner becomes confused, depressed and delusional, and collapses mid-race. This is the dreaded ‘hitting the wall’ and it is a result of being glucose-adapted.

However, when keto-adapted, fat is released from adipose tissue to fuel the body. The liver breaks the fats down into fatty acids and ketones. Muscles can burn both of these for energy, however as ketosis becomes established muscles turn to fatty acids as their priority fuel. This leaves ketones to fuel the remainder of the body, particularly the brain. Given the size of fat reserves, there is no fear of depletion. This means that the brain and other systems can operate on ketones while muscle operates on fatty acids — there is no longer any competition between brain and muscle. ‘Hitting the wall’ is a thing of the past — the keto-athlete calmly and steadily accesses energy from their fat reserves as needed. In the days after a race it is easy to replenish fat stores according to hunger signals.

This is so logical that there is no doubt in my mind that this is how we are meant to function from an evolutionary perspective (over ~2 million years). Before tools, our forebears ran their food into the ground. Humans control their core temperature by sweating all over their body. However, their prey cannot sweat through their hides, and rely on panting to lower body temperature — a slow process. Thus, while prey could outrun humans, humans could outlast their prey by targeting just one animal and eventually running it to exhaustion and collapse. This could take most of the day. Our forebears managed this without carb-loading, lurid sports drinks or stopping to munch on a tuber. They almost certainly managed this remarkable feat because they were keto-adapted.

To underline this keto-adaptation, consider hunter-gatherer societies documented before westernisation. The Inuit could survive on a zero carbohydrate diet consisting of around 85% fat and 15% protein (westerners can too by the way, as demonstrated by a number of early arctic explorers). When a caribou was caught and slaughtered, the first thing that the Inuit did was drink the blood. This was a source of sodium. The next priority were the internal organs (liver, brain etc). These provided minerals, vitamins (including vitamin C) and other nutrients that make carbohydrate sources for them redundant. Next to go was the fat and fatty meat. When the Inuits got to the lean meat, such as as the fillet/tenderloin, well, they threw that to their dogs. Lean meat, much praised by modern dietary authorities, was dog food. The Maasai followed the same pattern and were remarkably healthy. Even large carnivores do something similar — when a pride of lions moves away from a carcass there can still be a lot of meat left on it, but it is lean meat and not worth the effort to digest. It’s left for the jackals.

One final word of caution. The high carbohydrate consumption of glucose-adapted athletes, and the glucose spikes from sports drinks taken during a race, may have long-term health implications. Habitual marathon runners can have significant coronary artery disease (compared to matched sedentary individuals). Repeatedly depriving brains of their glucose energy is unwise. Further, the oral health of 2012 Olympic athletes (~300 participated in the study) was found to be generally poor, perhaps because of overconsumption of sugary sports drinks to sustain performance.

Resistance exercise

Resistance exercise is often undertaken with muscle growth or toning in mind. A KD may have something to offer for this too.

Day and night, new muscle proteins are being synthesised and existing muscle proteins broken down. Whether a muscle bulks or wastes depends on the fine long-term balance between these two processes. During exercise, muscle protein breakdown dominates. Protein synthesis occurs during the post-exercise period.

An important regulator for muscle protein synthesis is the amino acid leucine. Leucine is one of 3 branched-chain amino acids (BCAAs), the others being isoleucine and valine. Leucine is an essential amino acid that must be acquired through the diet. Dietary intake is usually high. The BCAAs are relatively abundant and normally make up 15–25% of dietary protein. Leucine is high in egg yolks, dairy and meat (because leucine is in muscle cells).

An important property of leucine is that it is exclusively ketogenic (it can be used to form ketones). There is only one other amino acid that shares this property (lysine). The other amino acids are either exclusively glucogenic (used to form glucose — there are 13 of these) or both glucogenic and ketogenic (n=5). This means that 18 (of the 20) amino acids are potentially glucogenic, and this is why high protein consumption on a KD can work against ketosis — excess dietary proteins are broken down to make glucose. But that’s an aside…

Dietary leucine is not degraded by the liver and instead, the liver directs it to muscle cells where it can be transported into the cell. There, it has two possible fates — it can be burned (oxidised) to fuel the cell, or it can initiate a pathway for protein synthesis. Without leucine, significant muscle protein synthesis will not occur.

During high-level resistance training, protein synthesis decreases and protein breakdown increases, leading to a nett negative effect. This is partly because leucine is being used by muscle cells for their immediate fuel needs under conditions of high demand. As blood leucine levels rise again with post-exercise feeding and resting, protein synthesis takes over again. This is why rest periods (and sleep) are important for muscle gain.

Keto-adaption can alter this process subtly but importantly. Remember that leucine is ketogenic and a type of ketone is created as part of the oxidation of leucine for energy. However, there are already plenty of circulating ketones with a KD. This means that leucine is not in demand as a fuel and more leucine is available for protein synthesis, even during exercise. There is also more available post exercise to accelerate protein synthesis during recovery.

There is one final consideration that people who know much more about this topic than me will be thinking — what about insulin? This is because leucine needs insulin to enable protein synthesis. As insulin is kept low on a KD this looks like a problem. However, leucine can stimulate insulin production for its purposes.

The average Jo/Joe

That’s fine for athletes, but what about us mere mortals? The extra reserves of energy we enjoy from tapping into our fat stores can help with the endurances of daily living, skipped meals and other demands that life uses to challenge our fortitude. A better functioning musculature during intermittent activities such as gardening is likely to reduce injury and soreness and help with recovery. The mental alertness that comes with driving our brains on ketones can help with skilled sporting activities (golf, cricket, tennis etc). There is a spectrum of benefit that goes all the way from the elite athlete to us.

I want to finish by making one last point about exercise for the average us:

Moderate exercise is ineffective for weight loss

We are admonished by health authorities for eating too much and not exercising enough. Rather than reflecting on the veracity of their own dietary advice, authorities resort to blaming us for being overweight and weak-minded. This is to redirect attention away from their own dietary advice that has resulted in our ill-health in the first place.

Exercise guidelines, such as ‘moderate exercise for 30 mins a day 5 days a week’, are arbitrary. They are sensible from a broader health perspective, but they will not contribute significantly to weight loss.

It is important to understand this in order to have the correct expectations when starting a physical exercise program. Lean people are not lean because they exercise, rather they exercise because they are lean. They exercise because being lean means they can, and because exercising is more enjoyable when lean than when overweight.

The problem arises from being glucose-adapted in accordance with current dietary guidelines. Glucose-adapted bodies utilise glycogen stores, not fat stores, for energy during moderate exercise and so fat is not depleted. Furthermore, weight is maintained after exercise by modifying metabolic rate and sending hunger signals that we listen to. We ‘deserve’ a treat after exercise (a croissant and coffee would be nice). Little do we realise that our bodies have instructed us to eat just the right amount to make up for any calories burned, and perhaps a bit more to be on the safe side. If we don’t do it straight away, we do it incrementally during the remainder of the day. If anyone does overcome these signals, they will probably lose weight, but they are losing it because of calorie restriction, not because of exercise. The same could be achieved by reducing calories without exercise.

However, that doesn’t mean don’t exercise! There is nothing more beneficial that you can do for your total health than being as physically active as you can.




Science of cooking, eating and health. Retired neuroscientist.