The point of this post, and related ones, isn’t to discourage weight loss completely, but to raise awareness that there is much more to the human body than “calories in minus calories out,” and that things just aren’t so simple.
Weight loss usually doesn’t last for example. One of the reasons why may be the long term change in satiety hormones such as leptin, peptide YY, cholecystokinin, amylin, insulin, ghrelin, gastic inhibitory polypeptide, and pancreatic polypeptide. Ghrelin is the main “hunger hormone,” and in that NEJM study, it was found to be reduced long term after weight loss.
In that study, subjects went on a harsh 10 week diet, but levels of these hormones were measured at week 10 and at week 62. Subjects lost an average of 13.5 kg by week 10, and gained 5.5 kg back by week 62. What was interesting as you can see in these two figures is that ghrelin levels were higher a full year after this weight loss before, during, and after the meal. Amylin, a satiety promoting hormone, in contrast, was lower in response to meals at week 10 and week 62.
They also asked the participants their subjective hunger rating, and they found that their hunger was also elevated for longer even after eating. It’s not this simple though, and this post isn’t intended to be a lesson on these hormones, because we haven’t discussed sensitivity to these hormones yet. Anorexics may have higher ghrelin and administering ghrelin doesn’t make them eat more because they’re not as sensitive to it, while obese people may have lower ghrelin levels. In another study, lower, and not higher ghrelin levels predicted weight regain after an 8 week diet.
The implications of all this are profound. Lower leptin levels mean that the thyroid may be lower as well, since leptin signals the hypothalamus to secrete thyrotropin-releasing hormone (TRH). The entire organism may be in a fatigued state afterwards, which numerous anecdotes can buttress. This low-energy state can persist a year after the weight loss, longer expected. In rats, this restoration of energy expenditure lags behind weight regain.
I’m particularly interested in how all this affects the thyroid now since I have learned that dieting and overexercising can cause lower thyroid and adrenal function. I’m sure leptin has something to do with it, along with micronutrient deficiencies. Many people are struggling with restoring their thyroid back to normal after a period of dieting. The safest option for moderate weight loss seems to be not worrying about calories in minus calories out and doing some HIIT, because studies have found you don’t really need to change your eating habits and the weight is still lost. It may be the least stressful unless you end up doing crossfit in which case you can expect adrenal problems quickly.
There are a lot of other questions. Some people may be able to lose weight healthfully, and the exact mechanisms are unclear via standard internet research. Next time I’ll take a stab at discussing how weight loss and such affects the thyroid more directly. Till then, I hope you can find some energy.