How Ketogenic Diets Impact Our Body and Exercise Performance

The Truth Behind Keto Diets

Bacon and eggs in skillet
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Ketogenic diets have become one of the most popular trends in quick and dramatic weight loss. Commonly referred to as the “keto diet,” it promises drastic results in a short period of time. According to research it may even improve exercise performance in athletes and enable them to lose fat while maintaining muscle mass.

Is there enough evidence to support keto diets as an effective way to reduce the obesity epidemic and help athletes?

What exactly is the keto diet anyway, and is it safe?

What Is the Keto Diet?

The ketogenic or keto diet (KD) can be defined as a very low-carbohydrate, high fat, and adequate protein diet stimulating metabolic changes in our body. It’s said the diet forces our body to use fat for fuel instead of glucose (sugar). The keto diet theory implies if we’re not consuming carbs, our body will burn fat, making us lose fat.

Healthy caloric intake is still maintained but the carbohydrate food group is basically removed. Because it’s such a strict diet, many people aren’t able to maintain it for long periods of time. The keto diet means avoiding all grains, beans and legumes, most fruits, starchy vegetables, alcohol, and all sugars. Certain milk and dairy along with some fats are also eliminated. Because the diet removes essential nutrients, some registered clinical dietitians indicate keto diets work only short-term and can be unhealthy.

The keto diet usually contains less than 20 percent of energy from carbohydrates, variable protein and more than 50 percent of energy from fat intake. Some keto diets are dropping carbohydrate intake to as low as 5 percent of total daily calories consumed. According to research, recommended daily protein intake on keto diets should be between 1.3 to 2.5 g/kg to maintain muscle and burn fat efficiently.

How Does It Work?

The keto diet places our body into ketosis. This happens as a result of insufficient carbohydrate intake (at or below 20g daily) causing our glucose levels to become very low. This triggers a chemical response in our body to create ketone bodies as an alternative energy source. Ketone bodies are chemicals produced during the metabolism of fats and that the body uses as energy during periods of fasting and carbohydrate shortage.

So what’s happening during ketosis is a drop in our blood sugar (glucose), causing our body to utilize fat as an alternate energy source. How can this be a good thing or safe?

According to some research, ketosis is indicated to be safe for overweight individuals or people struggling with obesity. It’s also shown to be an effective treatment for children suffering from epilepsy. Other research claims keto diets may improve chronic disease in the general population. Other studies are indicating keto diets may help athletes maintain proper body composition.

Keto Diet Research

A review of ketogenic diets examined if athletic performance could be improved using low-carbohydrate-high-fat (LCHF) diets. This is surprising as decades of sports nutrition research has supported high-carbohydrate-low-fat (HCLF) diets for athletes to function at optimal levels.

The success rates of LCHF diets like the Keto or Atkins diet for weight loss and other clinical conditions has helped widen the scope of keto diet research to athletes.

According to research, keto diets may help athletes control body weight, reduce body fat, and maintain muscle mass in weight-sensitive sports. It appears endurance athletes adapt to LCHF diets and are able to burn fat more efficiently than their HCLF diet counterparts. Keto athletes have also shown similar muscle glycogen content and tissue repair at the same rate as an athlete consuming an HCLF diet. The increased fat oxidation and rate of glycogen return may be of significant benefit to endurance athletes.

Study results also discovered some negatives for athletes using the keto diet. It appears elevated levels of free fatty acids and ammonia showed up in the bloodstream during exercise. High levels of free fatty acids and ammonia contributed to an impaired metabolism and central nervous system fatigue. Apparently, several months are required for an athlete to adapt to LCHF or keto diets for positive metabolic changes and muscle glycogen to occur.

In order for endurance athletes to improve their exercise performance using a keto diet, an adaption period of several months is recommended. Without long-term adaption to the keto diet, an athlete would experience adverse effects including reduced muscle glycogen, hypoglycemia, and impaired athletic performance, according to research results. 

Other findings indicated keto diets in combination with resistance training may allow athletes to maintain muscle and burn fat.

How Does the Keto Diet Affect Strength and Endurance?

Further studies indicated elite male gymnasts using a keto diet maintained maximal strength and significantly reduced body weight and fat. Contrarily, a similar study on athletes using an LCHF diet for three weeks showed a loss in muscle mass and decline in anaerobic performance. However, endurance performance for the keto diet groups was increased compared to the HCLF group.

It appears how the keto diet is applied is what determines improved athletic performance. According to research, endurance athletes who followed the diet for 9 to 36 months could reach maximal fat oxidation (burning) better than carbohydrate adapted athletes.

Other research shows keto diets are unlikely to benefit high-intensity exercise. This is because our body requires energy from the anaerobic system for short-term high-intensity exercise. The anaerobic system relies on glycolysis or the breakdown of glucose for energy. Because the keto diet utilizes fat for energy instead of glucose, it basically has an empty tank for quick burst energy exercise.

For example, elite off-road cyclists significantly decreased maximal power output after 4 weeks using a keto diet. Researchers are claiming further studies taking decreased body weight and fat into account and impact on anaerobic performance are required to confirm these findings.

LCHF diets are also shown to affect the central nervous system (CNS) during exercise. Evidently, the metabolic changes and how our body uses fuel during exercise can affect brain function. It appears cerebral amino acid (protein) uptake and the ability of our brain to communicate with our muscles becomes impaired. This response stimulates a release of increased free tryptophan to our brain causing lethargy, tiredness, and central fatigue. Central fatigue (central nervous system fatigue) adversely affects exercise performance and muscle function.

Studies indicate the high protein content of the keto diet can also lead to elevated ammonia levels during exercise. Higher levels of ammonia in our bloodstream can negatively affect brain function and cause central nervous system fatigue. Because limited studies exist on central fatigue and keto diets, further research is recommended to increase our knowledge in this area.

Expert review conclusions include:

  • The keto diet may be safe for the general population and reduce the risk of chronic disease.
  • Low-carb-high-fat (LCHF) diets may be a great way to control body weight, reduce fat, and maintain muscle for athletes in weight-sensitive sports.
  • Keto diets appear to be beneficial for endurance athletes after an adoption period.
  • Further research is required on potential benefits of the keto diet for high-intensity athletes relying on strength and power for their sport. 
  • Further research is required on the keto diet and impact on central nervous system (CNS) fatigue.

Further Research

A 6-week study to examine the impact of the keto diet on the physical fitness, body composition, and blood was conducted on 42 healthy adults. Research participants consumed 75 percent total daily calories from fats, 15-20 percent protein, and 5-10 percent from carbohydrates. This meant carb intake was restricted to 20-40g per day with the goal of placing each person in ketosis during the trial period.

The volunteers underwent clinical testing including blood draws, body measurements, and exercise testing. The following results were reported after completion of the six- week trial:

  • Mild weight loss
  • Body fat was reduced but also a small decrease in muscle (inconsistent body fat assessments between differing measurement methods were noted)
  • The body composition changes were considered positive
  • Muscle function and muscle mass were not considered negatively affected
  • Slight decrease in physical performance
  • Significant changes to thyroid hormones were noted (decline in fT3 and increase in fT4)
  • Significant decrease in fasting insulin levels

Participants complained of the following during the testing period:

  • Headache
  • Gastrointestinal symptoms
  • General weakness

Researchers concluded:

  • Slight negative impact on exercise performance
  • Further research to examine the long-term impact of keto diets on endurance exercise performance is required
  • Keto diets appear to be safe for weight loss without compromise to muscle mass or function
  • Findings indicate keto diets acceptable on physical fitness for daily living and aerobic training
  • Keto diets may not be the best nutritional strategy for competitive athletes
  • Further studies on how keto diets impact cholesterol, especially LDL (low-density lipoprotein) are required

Does the Keto Diet Actually Work?

Obesity has become an epidemic affecting over 30 percent of the American population. This has led to increased risk factors for cardiovascular disease, hypertension, type 2 diabetes, and certain types of cancer. Although the ideal diet to reduce obesity is still in question, the ketogenic diet has become one of the most widely researched nutritional strategies for weight loss.

According to research, three important questions should be considered:

  • Does the ketogenic diet work?
  • Is there a yo-yo effect?
  • Is a ketogenic diet safe for obese individuals?

Several studies confirm the keto diet as an effective method for weight loss. What appears to be a continued subject of debate are the underlying effects on weight loss. Clinical reviews have summarized the following based on the evidence on keto diets and weight loss:

  • Keto diets are indicated to reduce appetite because of the high satiety effect of protein in the diet.
  • Keto diets appear to have a positive impact on appetite control hormones.
  • Ketone bodies are indicated to provide an appetite suppressant action.
  • Keto diets are shown to decrease fat formation and increase fat burning.
  • Greater metabolic function consuming healthy fats is indicated.
  • Increased metabolic function for glucose and protein are indicated.  

Other conclusive recommendations for keto diets and weight loss:

  • Keto diets may help control hunger by improving the way our body burns fat leading to weight loss.
  • Improving the diet by adding carbohydrate foods similar to the Mediterranean diet could make it healthier and better-tolerated long-term.
  • The renal function of individuals using the keto diet should be monitored.
  • The transition from a keto diet back to a normal healthy diet should be gradual and well-controlled.
  • Keto diets are indicated to be administered minimally for 2-3 weeks to enter the ketosis phase and maximum of 6-12 months for weight loss.
  • It’s indicated the keto diet can be a useful tool to treat obesity.
  • It’s advised individuals have a thorough understanding of the keto diet before using it and preferable to be under the guidance of a physician.

A Word From Verywell

Proper nutrition plays an essential role in our ability to lose weight and improve athletic performance. With all the new diets and nutrition information out there, knowing the right strategy to use can be difficult. Although many positive findings have been discovered about the keto diet, there are some negatives to consider. The important thing is to understand how the keto diet works and if it’s the best method for you to reach your health and fitness goals.

Chang C et al., Low-Carbohydrate-High-Fat Diet: Can it Help Exercise Performance?, Journal of Human Kinetics, 2017

Paoli A, Ketogenic Diet for Obesity: Friend or Foe?, International Journal of Environmental Research and Public Health, 2014

Urbain P et al., Impact of a 6-week non-energy-restricted ketogenic diet on physical fitness, body composition and biochemical parameters in healthy adults, Journal of Nutrition & Metabolism, 2017

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