So you decide to diet, reducing carbs, calories or both. Vary rapidly, blood glucose and insulin levels are going to be reduced. This is good as it releases the "block" on fat mobilization. Additionally, catecholamine release typically goes up (at least initially), further increasing fat mobilization from fat cells. This causes blood fatty acid levels to increase. This is also good, as it tends to promote fat burning in tissues such as liver and muscle. This effect is facilitated if you deplete liver and muscle glycogen, as glycogen depletion tends to increase the use of fatty acids for fuel. The increase in blood fatty acid levels also has the short-term effect of causing insulin resistance. As I mentioned, this is a good thing on a diet since it spares glucose and helps promote fat oxidation. So far, so good, right?

Unfortunately, along with these good effects, a lot of bad things start to happen. I already described many of the central adaptations above: changes in leptin, ghrelin, Peptide YY (and certainly other hormones) "tell" your brain that you're not eating enough. This causes changes in the various neurochemicals stimulating a number of negative adaptations. I want to note that the response is not immediate, there is a lag time between the changes in all of these hormones and the body's response. But that's not all.

There are also many other adaptations which occur when you diet, so let's look at some of those. First and foremost, the drop in leptin directly affects liver, skeletal muscle and fat cell metabolism, mostly for the worse.

While the drop in insulin mentioned above causes better fat mobilization, it causes other problems. One is that testosterone will bind to sex-hormone binding globulin (SHBG) better, lowering free testosterone levels (this is in addition to the drop in total testosterone). As well, insulin is anti-catabolic to muscle, inhibiting muscle breakdown. The increase in cortisol that occurs with dieting enhances protein breakdown as well as stimulating the conversion of protein to glucose in the liver. Additionally, a fall in energy state of the muscle impairs protein synthesis (although it increases fatty acid oxidation). The mechanism behind this is more detail than I want to get into here. But the combined effect of these processes is that protein synthesis is decreased and breakdown is accelerated; this causes muscle loss.

On top of that, high blood fatty acid levels tend to impair the uptake of T4 (inactive thyroid) into the liver. There are also changes in liver metabolism that impair the conversion of T4 to T3 (active thyroid). Both of these processes cause decreased blood levels of T3. There is some evidence that high blood fatty acid levels causes tissues to become resistant to thyroid hormone itself (this is part of why just taking extra thyroid on a diet doesn't fix all of the problems). After the initial increase, there is also a drop in nervous system output (that can occur in as little as 3-4 days after you start a diet). Along with the drop in thyroid, insulin and leptin, this explains a majority of the metabolic slowdown that occurs. The change in liver metabolism (and the reduction in insulin) also impairs the production of IGF-1 from GH.

All of these adaptations serve two main purposes. The first is to slow the rate of fat loss, as this will ensure your survival as long as possible. Related to that, the body tends to shut down calorically costly activities. This includes protein synthesis, reproduction and immune function; there's little point keeping any of these functioning when you're starving to death. The drop in leptin, and the changes in hormones that occur are a huge part of why men tend to lose their sex drive (and ability) and women lose their period when they get lean/diet hard.

The second is to prime your body to put fat back on at an accelerated rate when calories become available again. Decreased metabolic rate and fat burning, along with improved caloric storage all conspire to put the fat back on when you start eating again. As I mentioned earlier, this makes perfect evolutionary sense, even if it presents a huge pain in the ass to us.

I haven't even mentioned the hunger and appetite issue which is a topic worthy of an entire book. The combination signal sent by leptin, ghrelin, insulin, glucose, and a host of other hormones (cholecystokinin, glucagon-like peptide 1 and 2, bombesin and many many others) are all involved in both hunger and appetite. The changes that occur with dieting tend to make both shoot through the roof: you tend to get and stay hungry, thinking about food nearly constantly. Bodybuilders and athletes may have unbelievable food control but it still sucks being hungry constantly when you try to diet.

Ok, enough about dieting, what about overfeeding?

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