How to Know If Your Keto Diet Is Costing You Muscle

The research on ketogenic diets and muscle mass is genuinely mixed. Some studies show muscle staying steady, or even increasing slightly, alongside fat loss. Others show meaningful muscle loss on the same type of diet. The difference usually comes down to protein intake, resistance training, and how the diet is structured, not the "keto" label itself.

This matters because the number on the scale can't tell you which outcome you actually got. A DEXA scan can. This article covers what the evidence actually shows, why the scale (and even some body composition tools) can be misleading on keto specifically, and how to time a DEXA scan properly if you want an accurate read on your own results.

What Does the Research Actually Show?

Across controlled trials, ketogenic diets consistently reduce fat mass. What happens to muscle alongside that fat loss varies considerably by study:

• Some trials show real muscle loss. A randomised controlled feeding trial in healthy young women found that four weeks on a ketogenic diet reduced lean mass by 1.45kg, more than the 0.66kg of fat mass lost, despite matched energy intake and activity levels against a control diet. The researchers concluded the diet disproportionately reduced lean mass over fat mass, and recommended concurrent strength training to help offset it.
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• Some trials show muscle held steady. An 8-week trial in trained men doing resistance training, measured by DEXA, found the ketogenic group lost fat mass and visceral fat with no significant change in lean mass. Notably, though, a comparison group on a non-ketogenic diet in the same calorie surplus did gain muscle over the same period, the ketogenic group didn't.
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• One trial found a genuine increase in lean mass. Men following a very-low-carbohydrate diet for six weeks, with energy intake designed to maintain body weight, gained 1.1kg of lean mass on DEXA while losing 3.4kg of fat mass.
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• A more extreme case exists too. A study following Olympic-level weightlifters on an unrestricted (ad libitum) ketogenic diet over three months found that roughly 77% of the total weight lost was muscle, not fat, a genuinely concerning result for anyone using bodyweight alone as a progress marker.

Put together, this is not a diet that reliably preserves muscle, nor one that reliably costs you muscle. It depends heavily on how it's implemented.

Why the Scale Can't Tell You the Difference

Ketogenic diets cause a rapid drop in body weight in the first one to two weeks, largely from glycogen and water loss, not fat. A controlled study following breast cancer patients through a ketogenic diet during radiotherapy found that after this initial water-driven drop, weight and fat mass continued to decrease gradually, while fat-free and skeletal muscle mass stayed largely stable. The early rapid loss looked dramatic on the scale, but it wasn't fat, and it wasn't the ongoing trend either.

This is exactly the kind of result a scale, or your own sense of "feeling leaner," can't distinguish. You need an actual measurement of what changed, fat or muscle, not just a change in total weight.

What Actually Moves the Needle

Across the studies, a few factors show up repeatedly as the real difference-makers, more than the ketogenic diet itself:

• Protein intake. Trials that explicitly controlled for adequate protein, rather than letting fat and protein both rise freely, tended to show better muscle retention.
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• Resistance training. Every trial that showed muscle preservation or gain included a structured resistance training program. None of the muscle-loss findings came from a purely dietary intervention with training held constant and adequate.
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• Energy balance. Diets designed to maintain or slightly restrict calories, rather than large unrestricted deficits, showed more favourable muscle outcomes.
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• Duration and diet structure. Very short, highly restrictive protocols (under two weeks) showed some of the more concerning muscle loss results; more moderate, longer, structured approaches generally showed steadier outcomes.

Why DEXA Is the Right Tool to Check Your Own Result

A DEXA (Dual-Energy X-ray Absorptiometry) scan directly measures fat mass, lean mass, and bone density, rather than estimating them from a single number on a scale. It's considered a gold standard for body composition measurement, which is exactly why it's the tool used in the clinical trials above, not a bathroom scale or a tape measure.

One honest caveat worth knowing: DEXA itself can be temporarily affected by the same glycogen and water shifts that make the scale unreliable in the first couple of weeks of a ketogenic diet. A sudden drop in glycogen and hydration can cause a DEXA scan to slightly underestimate lean mass during that early window. This doesn't make DEXA inaccurate, it means timing matters.

How to Time a DEXA Scan If You're On Keto

• Avoid scanning right in the first one to two weeks of starting (or significantly changing) a ketogenic diet, this is when water and glycogen shifts are largest and least stable.
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• Once your weight and hydration have stabilised, generally after the initial adjustment period, a scan gives a much more reliable read of your actual fat and lean mass.
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• For tracking change over time, scan at consistent points, for example every few months, rather than immediately after a diet change, so you're comparing stable readings to stable readings.

What Do Everlab's Doctors and Dietitians Say About Keto?

Everlab's nutrition team doesn't recommend a ketogenic diet as a first-choice approach for the general population. The diet's high fat intake and low fibre content (a consequence of restricting carbohydrates) are associated with increased risk of high cholesterol, high blood pressure, and, over time, heart disease.

If you're already following a ketogenic diet, or considering one, that's a conversation worth having with your own doctor or dietitian, since the right approach depends heavily on your individual health profile, not a blanket recommendation either way.

Frequently Asked Questions

1. Does keto cause muscle loss?

It can, but it doesn't always. The evidence is genuinely mixed: some trials show real muscle loss, others show muscle held steady or even slightly increased. Protein intake, resistance training, and how restrictive the diet is all matter more than the ketogenic label itself.

2. Why does keto cause rapid weight loss in the first week?

Restricting carbohydrates depletes glycogen stores, and each gram of glycogen is stored with several grams of water. Losing that glycogen and water causes a rapid initial drop on the scale that isn't fat loss, and isn't representative of your ongoing trend.

3. Can a DEXA scan tell me if I'm losing fat or muscle on keto?

Yes, that's exactly what it's designed to measure directly, rather than estimating it from total weight. Timing matters, though, scanning in the first one to two weeks of a ketogenic diet can be affected by the same water and glycogen shifts that make the scale unreliable in that window.

4. Is keto a good diet for building or preserving muscle?

On its own, the diet doesn't reliably preserve or build muscle either way, outcomes vary considerably by study. Resistance training and adequate protein intake appear to matter more than the ketogenic diet itself for muscle outcomes.

Explore More From Everlab

• DEXA Scan: measure your actual bone density and body composition, fat, muscle, and all. Learn more.
• Visceral Fat: why it matters and how it connects to metabolic health. Read more.