How Ketogenic Diets Impact Your Body and Exercise Performance

The Truth Behind Keto Diets

Bacon and eggs in skillet
Jill Chen/Stocksy United

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 your body. It’s said the diet forces your body to use fat for fuel instead of glucose (sugar). The keto diet theory implies if you're not consuming carbs, your body will burn fat, making you 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 your body into ketosis. This happens as a result of insufficient carbohydrate intake (at or below 20g daily) causing your glucose levels to become very low. This triggers a chemical response in your 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 your blood sugar (glucose), causing your 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.

Is it Considered a Fad Diet?

According to Jennifer McDaniel, MS, RDN, CSSD, LD, Registered Dietitian and Spokesperson for the Academy of Nutrition and Dietetics, fad diets over promise weight loss but usually end up under delivering (or leaving the dieter in worse condition). For the majority of individuals seeking long-term weight loss or long-term improvements in health or physical performance, the keto diet would fall into this “fad diet” category. 
 
The keto diet has been used for decades in conjunction with other medical therapies to treat epilepsy, especially in children. There are circumstances in which doctors will recommend the keto diet before weight loss surgery, particularly if the patient must lose weight to safely undergo surgery. In both of these cases, patients are typically monitored by physicians and registered dietitians, and they’re usually limited to only 20-30 grams of carbohydrate, says McDaniel.

Are Ketogenic Diets Sustainable and Realistic?

The Keto Diet is very restrictive and eliminates essential nutrients coming from grains, beans and legumes, most fruits, and some vegetables. Dairy products like milk are also avoided. Removing important nutrients from your diet is considered not sustainable or realistic.

Many people who claim to follow the keto diet actually aren’t, according to registered dietitian and nutrition expert Jennifer McDaniel. It’s an extremely low carb, high-fat diet with less than 10% of your calories coming from carbohydrates. The remaining 70-80% of your calories come from fat and very hard to maintain. Enjoying a piece of cake at your child’s birthday or grabbing a granola bar before a long flight would quickly kick yourself out of ketosis. Extreme restriction with any kind of diet usually backfires sending the dieter into a downward spiral of guilt and frustration from another failed diet attempt.

Are Ketogenic Diets Beneficial for Athletes?

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 your 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 your body uses fuel during exercise can affect brain function. It appears cerebral amino acid (protein) uptake and the ability of your brain to communicate with your muscles becomes impaired. This response stimulates a release of increased free tryptophan to your 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 your 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 in 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 your 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.

Are Keto Diets Safe?

The ketogenic diet appears to be safe in the short term based on small studies, according to registered dietitian and Spokesperson for the Academy of Nutrition and Dietetics, Jennifer McDaniel. However, we don't have much evidence that it's safe in the long-term (or sustainable!) Studies that have focused on safety have either been small or founded on animal studies, says McDaniel.

McDaniel indicates the Academy of Nutrition and Dietetics currently doesn’t recommend keto for people with or at high risk for kidney disease, osteoporosis, or high LDL cholesterol. 
 
Additionally, there’s always a risk for nutrient deficiencies when entire groups of foods are eliminated from the diet. One example with keto would be removing fiber, because the diet is low in fruits and grains. 
 
Some other potential risks from being in long-term ketosis include the loss of muscle mass, bone weakness, and impaired kidney function, according to McDaniel. 
  
That being said, low carb diets can help people lose weight, says McDaniel. They just don’t have to be this extreme. For example, adding back some high-quality carbs like a serving of fruit with breakfast, beans on a salad with lunch, and a serving of whole grains with dinner, you are adding back a ton of good nutrition while still maintaining a low carb diet.

McDaniel also recommends that you make sure the fats you choose are healthy, so let’s replace unhealthy fats: the bacon, beef and butter with avocado at breakfast, olive oil on your salad, and fish fats at dinner.

The following meal improvement recommendations will be helpful:

  • Breakfast – scrambled eggs made with heavy cream and bacon. Improve it - Lose the heavy cream and bacon and add avocado with a side of fruit.
  • Lunch – salad greens with chicken breast on top, avocado, oil and vinegar dressing; celery sticks and ranch dressing. Improve it - add some fiber-rich beans and a little almond butter to the celery instead of ranch.
  • Dinner – grilled ribeye with ranch dressing (butter, heavy cream, mushrooms), a small cup of broccoli. Improve it - swap out fish with the steak, add a small portion of whole grains, and keep the broccoli.
  • Snack – macadamia nuts – keep!

Should I Use the Keto Diet?

In looking at some of the studies on keto, it appears that weight loss is rapid in the first weeks or months, according to McDaniel. But over time, the results are often pretty comparable to more traditional low-calorie weight loss plans. This may be due, at least in part, to the restrictive nature of the keto diet.

Diets that are restrictive often result in rebound weight gain, because many people eventually start to crave the foods they’re avoiding. If you are set on trying a keto diet, McDaniel highly recommends speaking with a doctor first and also working with a registered dietitian, who can help ensure that you don’t become deficient in any nutrients.

The bottom line, according to McDaniel, is no matter what diet plan you try to follow, the key is that it’s a plan that you can stay on for life.

A Word From Verywell Fit

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.

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