Ketogenic diets, characterized by very low carb and high fat intake, are quite popular.
A lot of attention has been given to the effects of ketogenic diets on weight and fat mass, but a recent meta-analysis sought to assess the effects of ketogenic diets on a variety of body composition outcomes in people who are lifting weights.
Compared to non-ketogenic diets, keto led to significantly larger reductions in weight (-3.67 kg), fat mass (-2.21 kg), and body-fat percentage (-2.27%). However, losses in fat-free mass were also greater (-1.26 kg).
These results line up pretty well with recent studies suggesting that ketogenic diets may lead to smaller gains in strength and hypertrophy when compared to non-keto diets (Vargas 2018, Vargas-Molina 2020, Kysel 2020, Paoli 2021).
While results are likely impacted to some extent by glycogen-related shifts in water weight, it seems hard to argue that ketogenic diets are optimal for hypertrophy. That doesn’t mean that it’s impossible to make great progress on a ketogenic diet – rather, it merely suggests that keto diets may not be the best possible option for adding new lean mass or retaining the lean mass you have.
Having said that, a ketogenic diet might be a viable option for individuals focused on weight loss who struggle with hunger management, or who simply find it preferable to restrict carbohydrate-rich foods in comparison to fat-rich foods.
Studies in this area tend to show that keto diets lead to either less favorable or similar impacts on lean mass when compared to control diets, so a more traditional macronutrient breakdown might be the way to go, particularly if you typically train in higher repetition ranges that tend to be more glycogen-dependent.