When weight loss is a goal, the focus should be on boosting metabolism in order speed weight loss. Metabolism is the rate at which your body uses energy, or ATP. When it is slow, it causes bodily functions to slow down, so as you can imagine, the associated symptoms of a slow metabolism are fatigue, depression, feeling cold, and being sluggish. A slow metabolism represents bodily functions being in conservation mode, making it very difficult to lose weight.
There are many factors which can slow metabolism, including lack of sleep, nutrient deficiencies, sedentary lifestyle and dehydration. Another major factor is low thyroid function. Thyroid hormone, specifically T3, controls the metabolic rate in the cells of the body and determines the rate of energy expenditure. Low thyroid hormone reduces the rate of lipolysis (fat breakdown). Therefore, in the case of low thyroid hormone expression, metabolism and energy expenditure slows and weight loss is very challenging.
Ironically, in some circumstances, lowering caloric intake can lead to a slowing of the metabolism. Many people reduce the amount of food they consume in order to lose weight, but they end up with the opposite effect of a slower metabolism. Consequently, the weight loss is only temporary and tends to comes back over time.
Research on the topic of calorie restriction and sluggish metabolism is not new; there are studies that go as far back as the mid-seventies which demonstrated this fact. In one small study, 9 obese patients with normal thyroid function used severe caloric restriction in an attempt to lose weight. After four weeks, researchers found that there was a significant decrease in active T3 thyroid hormone. These patients also were shown to have an increase in reverse T3 (rT3), which is significant because reverse T3 is created when the body is attempting to conserve energy. Instead of converting T4 (the inactive form), to active T3 (which speeds up metabolism), T4 is converted to the inactive form rT3. High levels of rT3 indicate that metabolism is in conservation mode, exactly the opposite intention when attempting weight loss.
Clearly, severe caloric restriction can sometimes cause the body to save its reserve as a primitive reaction in case food becomes even more scarce. In this study, after resuming normal caloric intake for 5 days, the body reverts back to normal metabolism. However, other studies have shown that the slowing of the metabolic rate can persist for over a year.
Physiol Rev. 2014 Apr; 94(2): 355–382. doi: 10.1152/physrev.00030.2013 Thyroid Hormone Regulation of Metabolism. Rashmi Mullur, Yan-Yun Liu, and Gregory A. Bren
J Clin Endocrinol Metab. 1975 Jul;41(1):191-4. Diversion of peripheral thyroxine metabolism from activating to inactivating pathways during complete fasting. Vagenakis AG, Burger A, Portnary GI, Rudolph M, O’Brian JR, Azizi F, Arky RA, Nicod P, Ingbar SH, Braverman LE.