Some details.
Q. Some claim that that your body will go into 'starvation mode' if you eat too few calories, preventing you from losing weight and that trying to lose weight by eating fewer calories doesn't work. What do you think?
A. Well there is no doubt that the body slows metabolic rate when you reduce calories or lose weight/fat. There are at least two mechanisms for this.
One is simply the loss in body mass. A smaller body burns fewer calories at rest and during activity. There's not much you can do about that except maybe wear a weighted vest to offset the weight loss, this would help you burn more calories during activity.
However, there's an additional effect sometimes referred to as the adaptive component of metabolic rate. Roughly, that means that your metabolic rate has dropped more than predicted by the change in weight.
So if the change in body mass predicts a drop in metabolic rate of 100 calories and the measured drop is 150 calories, the extra 50 is the adaptive component. The mechanisms behind the drop are complex involving changes in leptin, thyroid, insulin and nervous system output (this system is discussed to some degree in all of my books except my first one).
In general, it's true that metabolic rate tends to drop more with more excessive caloric deficits (and this is true whether the effect is from eating less or exercising more); as well, people vary in how hard or fast their bodies shut down. Women's bodies tend to shut down harder and faster.
But here's the thing: in no study I've ever seen has the drop in metabolic rate been sufficient to completely offset the caloric deficit. That is, say that cutting your calories by 50% per day leads to a reduction in the metabolic rate of 10%. Starvation mode you say. Well, yes. But you still have a 40% daily deficit.
In one of the all-time classic studies (the Minnesota semi-starvation study), men were put on 50% of their maintenance calories for 6 months. It measured the largest reduction in metabolic rate I've ever seen, something like 40% below baseline. Yet at no point did the men stop losing fat until they hit 5% body fat at the end of the study.
Other studies, where people are put on strictly controlled diets have never, to my knowledge, failed to acknowledge weight or fat loss.
This goes back to the under-reporting intake issue mentioned above. I suspect that the people who say, "I'm eating 800 calories per day and not losing weight; it must be a starvation response" are actually eating far more than that and misreporting or underestimating it. Because no controlled study that I'm aware of has ever found such an occurrence.
So I think the starvation response (a drop in metabolic rate) is certainly real but somewhat overblown. At the same time, I have often seen things like re-feeds or even taking a week off a diet do some interesting things when people are stalled. One big problem is that, quite often, weekly weight or fat loss is simply obscured by the error margin in our measurements.
Losing between 0.5 and 1 pound of fat per week won't show up on the scale or calipers unless someone is very lean, and changes in water weight, etc. can easily obscure that. Women are far more sensitive to this. Their weight can swing drastically across a month's span depending on their menstrual cycle.
Thing is this, at the end of the day, to lose weight or fat, you have to create a caloric deficit, there's no magical way to make it happen without affecting energy balance. You either have to reduce food intake, increase activity, or a combination of both.
Since my Rapid Fat Loss Handbook actually uses an extremely large deficit, I discuss the issue of metabolic slowdown (and what to do about it) fairly extensively.
https://www.thefactsaboutfitness.com/research/lyle.htm
