After thinking about this for maybe four or five years I still do not understand the main argument people make against starvation diets. Malnutrition and sustainability aside, as well as the safety and efficacy of transitioning off the diet - those things are not directly related to the actual starvation diet and can be coped with.
The main argument goes as follows: When you eat significanly less than you should your metabolism slows down to a crawl and you negate any positive effects.
All right, I can understand that the metabolism slows down but I don’t see how it could possibly work out mathematically that you negate any positive effects.
Let’s look at this in numbers. Suppose your total daily metabolic rate is 4500 calories - an obese person with a light to moderate level of activity. Suppose you start eating 3000 calories a day without altering your activity level, thereby giving yourself a 1500 calorie deficit. Good, you’ll lose 2-3 lbs a week give a take a few.
Now suppose the same person starts eating 200 calories a day without changing the activity level. Their initial calorie deficit will be 4300 calories, but supposedly their metabolic rate will plunge because the body will try to conserve energy. However, for the 200 kcal a day diet to be actually WORSE than the 3000 kcal a day diet the metabolic rate would have to plunge by (4300 - 1500) = 2800kcal. With the same activity level and same body weight, that seems compeltely unrealistic. I can understand a starvation mode where everything that can be conserved is conserved, but I fail to see what magic principle of biology or thermodynamics will allow the body to suddenly be over 50% more efficient. I’ll buy 5%, you might even convince me 10% if you give me a cite, but 50%?
Then comes the second factor, that for a lot of people eating nothing is a lot easier than eating less. Same reason most people who quit smoking do so cold turkey. If you had the will power to eat less you already would’ve and wouldn’t be overweight. That’s what diets are for - you need to find a trick that makes eating less easier, be it avoiding carbs, avoiding breads, eating only a subset of foods, adding a ritiual, breaking ritiual, or simply not eating anything at all.
I mean, don’t get me wrong, I’m not advocating starvation diets - they can and will kill or cripple you if you don’t take everything into account. However, I think the argument against them should not be centered around metabolic rate, or at least “starvation mode” and efficiency. Your metabolic rate might plunge because you find it impossible to maintain same activity level without any food, but that’s not what people are talking about when they mention “starvation mode”.