Crash diets do work, he says - but only at first. They can lead to significant improvements in cardiovascular risk markers, like cholesterol level and blood sugar levels, and the weight loss may help lower blood pressure. Where they fall short is their inability to foment real habit change.

Is crash diet effective?

“They're not going to help you burn calories or fat in the long run. You may see a decrease in the number on the scale, but it won't improve your overall health outcomes.” Weight lost during a crash diet is likely from lean muscle and water, not body fat, according to the AAFP.

Can you maintain weight after crash diet?

Crash Dieters Are Not More Likely to Regain Weight, Study Says.