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Crash diets cut calories drastically. They often restrict entire food groups as well. Such diets can result in rapid weight loss, although that weight is mostly water. That's because to the body a crash diet feels like being starved, and its reaction to starvation is to release water. As soon as you start eating normally again, the water—and the weight—will return.
Beware of any diet that drastically cuts calories, forbids entire food groups (like fruits or grains), includes pills or potions, advises that you eat foods only in certain combinations or tells you to skip meals. In the long run, crash diets won't help you lose weight and keep it off.
To add to the previous answer, there are a number of reasons why you want to avoid a crash diet: potential loss of muscle, lower thyroid function and increased cortisol. Here's the scenario: You have 10 days until your beach vacation. What do you do? Slash the calories, live on a liquid diet, up the exercise and hope for the best, right? After all, how much damage can that low-calorie diet do? Well, a classic study from the Journal of Clinical Endocrinology & Metabolism discovered that fasting resulted in a 53 percent reduction in serum T3 levels (your active thyroid hormone that increases metabolism) and a 58 percent increase in reverse T3 (RT3) levels, which block thyroid hormone.
On a given day your liver converts T4 (the less active thyroid hormone, thought of as a storage hormone) to RT3 as a way of getting rid of the excess. In a low-calorie situation, where your body needs to conserve energy, the percentage may spike significantly, and (based on the study above) it’s common to find yourself converting 50 percent or more of your essential thyroid hormone into metabolic waste. On a starvation diet you also experience a significant increase in cortisol. The acute stress activates this surge because your body is under the impression that there’s less food available. Bottom line: Avoid starvation diets and be sure to throw in a weekly or bi-weekly cheat meal to keep your metabolism revving.
Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.