Part 2: What Happens To Your Body When You Fast? – Playing With Hormones

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Last time, we started looking at the physiological effects of fasting. I first looked at it from the perspective of energy production. As we saw, fasting has several effects on hormones, such as glucagon, epinephrine, and adrenocorticotropic hormone (ACTH). These hormonal effects allow the body to tap into its stored energy reserves so you can keep on trucking. Today, I want to look at a few additional hormonal effects. Part of the problem we run into is that studies thus far have been done on long-term fasting of multiple days, whereas we’re more interested in the short-term effects of Intermittent Fasting. So we’re going to have to do some educated theorizing.

The Thyroid
The thyroid is an all-important gland located in your neck. Among the various roles this endocrine gland plays are controlling the metabolic rate, making proteins, and setting sensitivity levels to other hormones. The three main hormones produced are thyroxine (T4), triiodothyronine (T3), and calcitonin, a hormone responsible for calcium metabolism. T4 and T3 are the two hormones that we’re interested in today, however, as they are in charge of metabolism, growth rates, and the rate of function of bodily processes.

T3 is the most powerful of the thyroid hormones, increasing the basal metabolic rate and the rate of protein synthesis and degradation. It further increases the rate of glucose breakdown and synthesis through gluconeogenesis, along with increasing the rate of lipolysis by increasing the number of LDL receptors. This is one busy little hormone.

Now that we have a little background, let’s look at how fasting affects T3. T3 decreases slightly early in the fasting process, an effect that is thought to decrease the clearing rate of glucagon, helping to regulate blood glucose. To go with this, urinary excretion of nitrogen and an amino acid associated with muscle catabolism decreases as well. It seems that the body is protecting the extremely valuable muscles and directing energy production to come from other sources, specifically fat.

I’m assuming that the decrease in metabolic activity due to the decrease in T3 floating around in the blood stream is also responsible for the decreased body temperature associated with fasting as the body tries to use as little energy as possible. Of course, the question is how much is fasting reducing T3 function in the body and is this a good thing? One thing I’ve found says that the fall in T3 is not associated with clinical hypothyroidism. Also, it looks like a hypocaloric diet with at least 50g of carbohydrates prevented the fall in T3 levels.

I’d like to find some information on when T3 levels begin to drop since the “fasting” state starts at 12-18 hours post-meal and continues for approximately 48 hours. I’m assuming that it is part of the process of the shift from largely gluconeogenesis for energy to lipolysis. Someone grab that for homework! Interestingly, T4 levels do not fall, in contrast to all other thyroid hormones. In fact, T4 levels tend to normalize (if low) or remain the same (if normal) during a fast.

Other Hormonal Effects
Thyroid hormones aren’t the only ones affected by fasting however. Issue 6 of The Performance Menu was where Robb Wolf first laid out his thoughts on Intermittent Fasting. I went back and reread that article and came up with a lot of good information.

Beta-hydroxy butyrate (BHB), the principal ketone body resulting from fat metabolism, increases during a fast. Little surprise here since the body switches from glucose metabolism to a more fat-based metabolism. But here’s where the fun begins. First, BHB has shown to produce more energy than either glucose or fatty acids. But wait, there’s more! BHB has also shown to be beneficial against several ailments: toxins associated with Parkinson’s and Alzheimer’s, hypoxia (low oxygen levels), insulin resistance, and free radical damage.

If we look at growth hormone (hGH) and insulin-like growth factor (IGF), we also see increases during fasting. As Robb pointed out, these two hormones “are crucial in actions such as tissue repair, hypertrophy, DNA repair, lypolysis, and, to some degree, strength.” That seems to bode well for those of us engaged in intense activity and could explain why so many people report improved workout recovery while IFing. And while IGF has been linked to some forms of cancer, animals tested on IF protocols have shown increased IGF levels, but decreased levels of disease.

Now, let’s have a look at two chemicals responsible for cellular protection: heat shock proteins and apoptosis. Stressors like high temperature or oxygen deprivation make it more difficult for cellular proteins to fold properly and function properly. Heat shock proteins help these proteins achieve and maintain their proper form. Apoptosis is programmed cell death and helps clear out old dysfunctional cells.

Wrapping Up
Whew! Exciting stuff going on here. Based on what we’ve seen so far, it looks like Intermittent Fasting brings a lot of goodness to the table, from stress resistance at the cellular level to protein protection, from increases in hormones responsible for cellular repair and fat metabolism to protection of muscle protein during fasting. Next time I’m going to look at the digestive process itself, along with changes in the blood.

Feel free to drop any questions, observations, or corrections in the comments!