Ketogenic diets limit carb intake (generally to <50 grams/day or <10% of caloric intake) to induce ketosis. When in ketosis, your body uses ketone bodies (acetone, acetoacetate, and β-hydroxybutyrate) instead of carbs to fuel cells, including muscle cells.
You are in ketosis (also known medically as hyperketonemia) when your blood levels of ketones are high. How high?
Under normal, mixed-diet conditions, you usually have up to 0.1 millimoles of ketones per liter of blood (≤0.1 mmol/L). A state of ketosis is generally defined as >0.2 mmol/L, but most studies use ≥0.5 mmol/L as their practical threshold.
Your ketone levels can vary even if your diet doesn’t, because they depend on your ketone production (ketogenesis) and utilization (uptake by your tissues). Higher levels could be due to increased production or decreased utilization, and likewise, lower levels could be due to decreased production or increased utilization.
Nutritional ketosis is a kind of “low level” ketosis (0.2–3.0 mmol/L, typically) anyone can reach through carb restriction. This kind of ketosis shouldn’t be confused with ketoacidosis, a life-threatening condition characterized by elevated blood ketones, elevated blood glucose, and marked changes to the blood pH. Ketoacidosis is mostly seen in some cases of diabetes and alcoholism, and a handful of case reports have associated it with hyperthyroidism, starvation, and lactation.
Endurance athletes may use a keto diet for its glycogen-sparing effect, whereas athletes looking to lose weight before a competition may use a keto diet to suppress their appetite and empty their glycogen stores. But what if you want to get strong and build muscle?
This was a narrative review of 9 randomized controlled trials (RCTs) totaling 188 healthy, trained people. It summarized the evidence on the effects of keto diets (>2 weeks long, with <10% of calories from carbs or <50 grams of carbs per day) on body composition (body weight, fat mass, muscle mass) and various measures of strength or power.
With regard to body composition, the authors of the present review start by presenting evidence of keto diets promoting weight loss in the general population via appetite suppression (caused by ketosis and an increase in protein intake) and glycogen depletion. Then, they mention a 2021 meta-analysis that found that keto diets reduced body weight, fat mass, and fat-free mass (FFM) more than other diets. This meta-analysis found that keto diets caused greater fat loss than FFM loss (−2.21 vs. −1.26 kg), and as we explain in the “Fat-free mass ≠ muscle mass” box below, FFM loss on a keto diet can be a loss of glycogen and water rather than muscle.
Of the 9 RCTs in the present review, 7 matched caloric intake between the keto diet and the control diet, and 2 let the participants eat ad libitum. In 6 RCTs (two of which let their participants eat ad libitium), the keto group lost more weight and fat than the control group; in the 3 other RCTs (all of which matched caloric intake between groups), both groups lost the same amount of weight and fat. However, in 5 RCTs (one of which let its participants eat ad libitum), the keto group lost more (or gained less) FFM.
The authors also described 4 nonrandomized controlled trials: in 3 trials, keto diets promoted weight or fat loss, but in 2 trials, keto diets had a negative effect on FFM.
While many publications (including the present review) assume that a gain or loss of fat-free mass (FFM) is a gain or loss of muscle mass, FFM actually includes everything that isn’t fat — so your muscles, yes, but also your bones and other organs, your glycogen, and your body water.
Glycogen cannot be stored on its own: it must be bound to water. Each gram of glycogen stored in your liver is bound to 2.4 grams of water, and each gram of glycogen stored in your muscles is bound to ≥3 grams of water. The average man carries an estimated 341–593 grams of glycogen (85–127 in the liver, 256–466 in the muscles), and this glycogen is bound to thrice its weight in water. Since, when you start a keto diet, you deplete your glycogen stores, losing the bound water at the same time as the glycogen, you can easily lose several pounds of FFM without losing any muscle at all.
As to why keto diets might impair muscle growth, the authors proposed a few hypotheses, notably these three:
By reducing carb intake, keto diets reduce insulin levels, thereby inhibiting mTOR, thereby reducing muscle protein synthesis.
Keto diets might promote AMPK phosphorylation, which can inhibit anabolic pathways.
Keto diets can suppress appetite, thereby reducing caloric intake, thereby making it harder to build and maintain muscle.
With regard to strength and power, the authors of the present review start by reporting the results of a 2021 systematic review: of 16 studies, 11 saw no effect, 3 saw a performance decrease, and 2 saw a performance increase. However, the two studies that saw a performance increase tested cycling power, which might actually reflect muscle endurance more than muscle power.
Of the 9 RCTs in the present review, 7 assessed the effects of keto diets on strength or power in strength-trained individuals. Of these 7 RCTs, 6 matched caloric intake between the keto diet and the control diet, and 1 let its participants eat ad libitum. In 3 RCTs, the keto group improved less than the control group; in the 4 other RCTs (one of which let its participants eat ad libitum), both groups improved similarly.
The big picture
The findings of the present review suggest that, while keto diets might promote fat loss in strength and power athletes, they have a neutral or negative effect on muscle mass and performance.
Furthermore, keto diets had no effect on endurance performance in most studies on this topic. And when they did have an effect, it was more often negative than positive.
From a practical standpoint, the literature suggests that, for fat loss, the best diet is the healthy diet you can stick to. Along these lines, some evidence suggests that people with lower insulin sensitivity (higher insulin resistance) are more likely to adhere to a low-carb diet than people with higher insulin sensitivity (lower insulin resistance). The latter might want to try a low-fat diet.
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