A new UC Davis Health study found that a ketogenic (keto) diet improves muscle function in older animals by preventing muscle mass loss due to age (sarcopenia). The lead author of the study is the molecular exercise physiologist Keith Baar, a professor in the Departments of Neurobiology, Physiology and Behavior and of Physiology and Membrane Biology.

In this Q&A, he shares his latest research on the keto diet, its effect on muscle growth and brain health, and its potential side effects.

What is a ketogenic or a keto diet?

A ketogenic or keto diet includes eating high-fat, low-carbohydrate foods to reach a state of ketosis. In ketosis, the body burns more fat for fuel, instead of sugar, and produces acids known as ketones.

The keto diet is widely known as a weight-loss diet, but it also has a far-reaching effect on the muscles and the brain.

You have done multiple studies on the impact of the ketogenic diet on muscles. What did you find?

First, there is a difference between what we’re doing in our studies and the keto diet used for weight loss. While we give a high-fat, low-carb diet, we provide the same number of calories to maintain weight neutrality (no gain or loss). So, our studies are not looking to control body weight or body fat percentage, but to improve body function by mimicking the effect of exercise with diet.

Naturally, people lose muscle mass and endurance with age. Older muscles lose their mitochondria, the power engines of the cells. Mitochondria not only produce energy, they also help the body break down harmful metabolites known as kynurenines.

Both exercise and a keto diet can increase the number of mitochondria in muscles. The more fit we are, the more muscle mitochondria we have, and the more energy our muscles can make from fat, the better we are at breaking down potential neurotoxins. These are chemicals that destroy the nerve tissues and harm the nervous system.

Our studies show that when we give animals a ketogenic diet, they maintain their muscle strength and their endurance into old age. They also maintain their muscle mass. A keto diet therefore prevents muscle deterioration and even restores muscle function.

Does a keto diet improve brain functions, such as learning and memory?

A keto diet has also shown positive results as far as how the brain works. It has been used for many years to treat epilepsy and other neurocognitive diseases. Now, we are looking to see if a keto diet can help treat age-related neurological decline, and potentially Alzheimer's disease.

One way we think the keto diet works is, the enzymes that the muscle produces break down neurotoxins and prevent them from getting into the brain. These enzymes protect the brain from cell loss and improve brain function.

We also know that neurocognitive decline is different between men and women. It tends to have a stronger and earlier impact on women. In our studies, we had seen that a keto diet in older male mouse models was beneficial for the brains, but for those at midlife, there was a minimal effect. When we did the same study in females, it was interesting to find that middle aged females had learning and memory benefits. In humans, that would be closer to age 50. So, there's the potential for a keto intervention to provide a greater benefit for females at a younger age.

Is a keto diet good for athletes?

The issue with athletes is that you can only compete as fast as you can make energy from fat or sugar. When you only consume fat, your muscles adapt to using it as a fuel. That's totally fine when you're at rest. But when you try to sprint, your body needs to produce energy faster and more efficiently. Our bodies are less efficient at burning fat.

The ketogenic diet therefore slows the maximal rate of energy production. If you're an athlete, that's the curse of the whole process. To compete, you need to be able to produce energy as fast as possible and that means you need to use sugar as your fuel.

What about using keto diet for endurance sports?

When an athlete goes for endurance sports, they’re still better off using carbohydrates. The best series of studies was done on race walkers. While they seem slow, race walkers are actually going under seven minutes per mile for around 50 kilometers - that's more than a marathon. Even so, the studies show that they go faster on carbs than on fat.

So, most endurance athletes, such as those who run sub 3-hour marathons, still use carbs for energy. It is much more efficient to produce energy from carbs than fat, whether that is for a sprint or a marathon.

There are many ketone supplements in the market. Are they good for the body?

The data comes with mixed results. A ketone ester is a supplement with two ketones locked together that can be broken down quickly to increase blood ketone levels. Ketone esters have some beneficial effects. Our colleagues have shown that ketone esters improve brain function in ultramarathon runners. As we run for a long time, our brains don't function as well, and we don't make good decisions toward the end. With ketone ester, the brain functions better for longer.

We have also shown that ketone esters taken for a month can decrease neurotoxins, just like a full keto diet, even in young people.

Ketone salts are a different supplement that doesn't seem to work as well. It's probably just a matter of chemistry and how they're absorbed and digested. The levels of ketones (the beta hydroxybutyrate) in the blood increases a little bit with ketone salts and a lot with the ketone ester. One reason that people like the ketone salts is they can better mask the bad taste of ketones. Ketone esters taste horrible, more like gasoline. They're also very expensive. Over time the price is expected to come down.

What are some side effects of the keto diet?

It is important that people considering a keto diet to get a blood test and look at their lipids before they switch their diet. They need to take another one after three to six months on the keto diet. Most people will actually find that their high-density lipoproteins (HDL) - that's the good cholesterol - go up and their triglycerides will go down. But some people won't see a positive shift in those markers.

There can be issues with bone health when people switch to low carbohydrate ketogenic diets. We see that markers for bone breakdown are higher and markers for bone building are lower. This is a common side effect in kids who are fed a ketogenic diet to treat epilepsy. They have lower bone mass and become osteopenic (weak bones).

Also, when people start a keto diet, they go through what's called the keto flu. Keto flu is a set of symptoms that appear 2-7 days after starting the diet. Symptoms include headaches, fatigue, foggy brain, constipation and difficulty sleeping. The body basically goes from relying on sugar to eliminating sugar. That means losing 3-5kg (6.5-11 lbs.) from shedding glycogen molecules that were used to store sugar. As glycogen is kept in cells with a lot of water, your body may get dehydrated and you will feel tired. Things will get better when you start making the proteins and enzymes that allow you to use more fat to produce energy.