Keto Diet and Weight Loss: What Happens to Muscle Mass?

The ketogenic diet has become increasingly popular for weight loss and body transformation. While some studies suggest that ketogenic diets may promote rapid short-term weight loss, an important question remains: What kind of weight is actually being lost?

Originally developed as a medical therapy for drug-resistant epilepsy in children, the ketogenic diet is a very low-carbohydrate, high-fat eating pattern designed to shift the body into a metabolic state called ketosis.

A typical ketogenic diet consists of approximately:

• 70–80% fat

• 15–25% protein

• 5–10% carbohydrate

  
  

To maintain ketosis, carbohydrate intake is often restricted to approximately 20–50 grams per day. Because even moderate amounts of carbohydrates may interrupt ketone production, some individuals monitor blood, breath, or urine ketone levels to confirm they remain in ketosis.

Keto Diet, Weight Loss, and Muscle Mass

One frequently cited study published in the American Journal of Clinical Nutrition examined 17 overweight and obese men living in metabolic wards while consuming diets with identical calorie intake.

Participants followed:

• 4 weeks on a high-carbohydrate diet (50% carbohydrate, 35% fat, 15% protein)

followed by:

• 4 weeks on a ketogenic diet (5% carbohydrate, 80% fat, 15% protein)

Findings

Participants lost weight more rapidly after switching to the ketogenic diet.

However

• body fat loss slowed significantly,

• glycogen stores became depleted,

• and much of the early weight reduction appeared to come from water loss and reductions in lean body mass rather than body fat alone.

These findings highlight an important distinction between overall weight loss and improvements in body composition.

What Happens When Keto Is Compared With a Low-Fat Plant-Based Diet?

Another controlled feeding study compared an animal-based ketogenic diet with a low-fat, plant-based diet in 20 overweight adults.

Participants consumed:

• 2 weeks on a low-fat, unrestricted plant-based diet

followed immediately by:

• 2 weeks on an animal-based ketogenic diet

Findings

Ketogenic Diet

• Increased ketone production

• Reduced calorie intake

• Impaired glucose tolerance

• Increased loss of lean body mass

  
  

Low-Fat Plant-Based Diet

• Greater body fat loss

• Lower calorie intake

• Improved blood pressure

• Lower cholesterol levels

The researchers concluded that both diets led to reduced calorie intake and weight loss, but the plant-based diet resulted in greater fat loss despite participants consuming larger amounts of carbohydrates.

Looking Beyond the Scale

Rapid changes on the scale do not always reflect improvements in long-term health or body composition. Ketogenic diets often trigger a quick reduction in water weight because glycogen stored in muscles and the liver binds water.

For some individuals, ketogenic diets may help reduce appetite and promote short-term weight loss. However, concerns remain regarding long-term sustainability, cardiovascular health, nutrient intake, fiber intake, and maintenance of lean body mass.

Goals matter. If the primary objective is rapid short-term weight loss, ketogenic diets may appear attractive. However, for long-term health, metabolic function, muscle preservation, and sustainable body composition, overall dietary quality and resistance training may matter far more than ketosis itself.

A well-planned, minimally processed plant-based diet combined with regular strength training can support healthy body composition while also providing fiber, antioxidants, vitamins, minerals, and other beneficial plant compounds associated with long-term health.

Documentaries such as "The Game Changers" have also contributed to the growing discussion about plant-based nutrition and athletic performance.

Resistance training plays a central role in building and maintaining muscle mass — regardless of dietary approach.

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References:

Hall KD, Chen KY, Guo J, et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. Am  J Clin Nutr. 2016;104(2):324-333. doi:10.3945/ajcn.116.133561

Hall KD, Guo J, Courville AB, et al. Effect of a plant-based, low-fat diet versus an animal-based, ketogenic diet on ad libitum energy intake. Nat Med. 2021;27(2):344-353. doi:10.1038/s41591-020-01209-1