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Is Metabolic Damage Real?

As you are dieting and trying to lose weight, your metabolism decreases even faster mainly because of the lower thermionic effect of food, since you are eating less, and lower basal metabolic rate since you have less body mass. This is called adaptive thermogenesis.

During this period, your body does not have an idea that you are just trying to get in shape and look good, but instead implements a defensive anti starvation mechanism where it down-regulates hormones like leptin and thyroid hormone. This, together with increased efficiency of movement throughout a day can significantly increase metabolic efficiency and reduce energy expenditure. This is called non exercise adaptive thermogenesis. Very lean individuals can often be very ''sluggish'' as their nervous system is in ''low power mode''. This can save 10-20% of total energy on average, which is enough to stop any fat loss progress if you are not careful enough and too aggressive with your diet.

Now some people believe that this metabolic damage, where everything is slowed down in your body is permanent, and once you reach this state, you simply cannot go back to the healthy and normal condition ever again. A perfect excuse not to try to lose fat ever again, right? Well, fortunately, we have a lot of research about this, and it turns out that this is a complete myth.

It is true that an adaptive thermogenesis, where your experience significant reduction in energy expenditure, is permanent at a very low body fat percentages. But this is fully recovered as soon as you go up in your total body fat percentage. So let's say you diet down all the way to 6%, which is how bodybuilders on stage look like, indeed your metabolism would slow down so much that it would affect your daily life. However, as soon as you go up to a healthy range, 10-15%, your metabolism would increase along with it.

The best study that we have for this is Minessota Starvation Experiment from 1944, lasting 13 months in total. The study was designed to understand how to treat people coming back from war that went through a severe episode of starvation and to understand what are the psychological effects of it.

The study had 4 phases:

  • Control period (12 weeks) This was a standardization period when the subjects received a controlled diet of approximately 3,200 kcal of food each day. The diet of the subjects who were close to their "ideal" weight was adjusted so as to maintain caloric balance.

  • Semi - starvation period (24 weeks) During the 6-month semi-starvation period, each subject's dietary intake was cut to approximately 1,560 kilocalories per day.

  • Restricted rehabilitation period (12 weeks) The participants were divided into four groups of eight men; each group received a strictly-controlled rehabilitation diet, consisting of one of four different caloric energy levels. In each energy-level group, the men were further subdivided into subgroups receiving differing protein and vitamin supplements regimes.

  • Unrestricted Rehabilitation Period (8 weeks): For the final rehabilitation period, caloric intake and food content was unrestricted but carefully recorded and monitored.

After they initially lost 25% of their body weight in the starvation period, which is close to 20 kg for an average male of 75 kg, they managed to regain all the weight back in the last two phases with the same caloric intake that they had before the experiment. Metabolism was back to where it was before. Keep in mind that these participants did not engage in any exercise, which also contributes even more to the down-regulation of hormones as the lean mass is also lost. This means that the secretion of your hormones like leptin, ghrelin and thyroid hormones are closely related to your body composition. If you lean down, these hormones will decrease which increases your hunger levels and slows down your metabolism. If your body fat goes up, these hormones increase, which stops you from eating too much.

What about yo-yo dieting?

This is a phenomenon where people initially lose a lot of weight really fast, usually with crash diets where they eat very little to none, and then regain it all back very quickly after their weight loss.

The explanation for this is that these individuals do not engage in strength training, and as a result, they lose of lot of muscle tissue during the weight loss period, which in turn slows down metabolism even more. As a result, after the initial weight loss, as soon as they go back to their normal eating, which they do because crash dieting is not sustainable, they regain all the weight back and some more on top of it.

The problem here is that the defensive mechanism for anti starvation makes your body primed for fat storage after an episode of crash dieting. This is why most people have absolutely no problem losing weight, but the majority simply cannot keep that weight off for a long time.

Can re-feeds help?

Re-feeding is based around higher carbohydrate intake to bring your leptin hormone to a normal level after a period of losing weight. When you lose a lot of weight, your leptin hormone, decreases, which increases your hunger as a defensive mechanism against starvation. The theory is, that increasing your carbohydrate intake for a few days may trick your brain into believing that you are well nourished which will increase your leptin levels and decrease your hunger levels.

Unfortunately it is secreted by fat cells and is almost entirely determined by your body composition. Tricking your body by consuming a lot of carbs for a few days will not do much as your body composition will stay the same. From the study done by Dirlewanger et al (2000), 3 days of overeating by 40% surplus of energy intake caused a moderate increase of 28% in leptin levels. This also did not cause major increase in energy expenditure levels which was only 7%. As a result, metabolism did not increase and there was plenty of energy for fat storage instead.

Your appetite also does not decrease after re feeding because it primarily determined by your body fat percentage. If you are super lean, over eating for 2-3 days might only cause you to be even more hungry as it has psychological effects when you start to eat a lot more than you are used to at very lean levels.

To summarize, your leptin levels, which control your hunger, are strongly correlated with your body fat percentage. While you might increase your leptin levels temporarily by overfeeding, it will return to the levels prior to overfeeding within days.

Re feeds are a great concept in theory, but in practice I just see it as an excuse for binging.

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