Cutting Fast Doesn't Cost
What You Think.

The standard advice is to cut slowly. Half a percent of bodyweight a week, keep your muscle, be patient. The standard advice is right about the muscle and wrong about why that matters, and it misses the two costs that actually decide whether a cut works.

What a fast cut costs, measured

The best trial on this put elite athletes on either 0.7% or 1.4% of bodyweight a week, with four lifting sessions a week and matched protein. The slow group finished in about eight and a half weeks, the fast group in five. Per week, the fast group actually shed fat faster. But the slow group gained lean mass and strength while dieting, and the fast group only held theirs.^[1]

That's the real difference. It isn't that a fast cut strips muscle. In trained people with protein and hard lifting in place, it mostly doesn't. It's that a slow cut leaves room to keep building, and a fast one doesn't.

And then it washes out

The same researchers followed those athletes for a year. At twelve months there was no difference between the groups in body composition or performance. Their own conclusion was that the rate of loss is not the most important factor in keeping what you built.^[2] So the muscle argument for slow cutting is a temporary one. Whatever the fast group gave up, they got back.

The cost that doesn't wash out

For a 90 kg lifter maintaining on about 2,900 kcal, half a percent a week is a 500 kcal deficit, eating 2,400. One percent a week is a 1,000 kcal deficit, eating 1,900. That's double the deficit and a third of your food gone. People don't fail cuts because of muscle loss. They fail them because 1,900 kcal a day with a full-time job and a gym habit is miserable, and the diet ends before the fat does.

Slow costs you time. Fast costs you adherence. Neither costs you muscle for good.

The limit most people don't know about

There's a physical ceiling on how fast your body can pull energy out of fat: roughly 31 kcal per pound of fat per day, and possibly closer to 22.^[3] Past that, the deficit comes from somewhere else, and the somewhere else is you. The ceiling scales with how much fat you carry, which means a lean person can't sustain a fast cut at all. At 75 kg and 12% body fat, 1% a week needs more energy than your fat can supply on either estimate. At 90 kg and 25% it clears easily. So "how fast should I cut" has a different answer depending on how much fat there is to burn, and the leaner you are, the slower you have to go.

The deficit size itself has a limit too. Across trials of lifting during a deficit, lean-mass gains hold up until the deficit reaches roughly 500 kcal a day and fall off beyond it.^[4] For most people that's the practical top end.

What the app does

Coast sets your cut rate from your own fat mass. It works out how much energy your fat can supply each day and only offers rates your body can run without eating into muscle, with the slowest preselected. If you're lean you'll see one option. If you're carrying more, you'll see a faster one too, with what it changes stated plainly: fewer weeks, a harder diet, and no chance to build on the way down. Cut rates and timelines sit in The Lab, running on my own numbers.