In case you’ve missed my last few post, I’ve been looking into some of the physiological effects (besides intended ones eg weight loss) of intermittent fasting (IF) and caloric reduction. I was incredulous when I heard IF increases net growth hormone (GH) secretion, so I decided to investigate.
For those who aren’t familiar, GH aka HGH aka somatotropin is a peptide hormone secreted by the anterior pituitary that acts on almost all body cells to increase general growth by increasing the rate of gene transcription. Competitive athletics ban the use of “doping” on GH since taking supplements of the hormone encourages metabolic anabolism to make it easier to get bigger and more muscular faster. Notably GH also acts strongly on bone, increasing lengthwise growth (diaphyseal) and increasing bone density. The cow homolog BGH is known to increase milk production.
Turns out there is some validity to the contested claim (fasting–>more GH). Ho et al. tracked venous GH levels in fasting rats and found that the regular pulses of GH secretion became more frequent the longer rats had been on the fasted diet. Specifically:
The GH data were analyzed using two distinct methods: a discrete pulse detection algorithm (Cluster analysis) and Fourier expansion time-series, which allows fixed periodicities of secretory activity to be resolved. The 5-d fast resulted in a significant increase in discrete GH pulse frequency (5.8 +/- 0.7 vs. 9.9 +/- 0.7 pulses/24 h, P = 0.028), 24 h integrated GH concentration (2.82 +/- 0.50 vs. 8.75 +/- 0.82 micrograms.min/ml; P = 0.0002), and maximal pulse amplitude (5.9 +/- 1.1 vs. 12.3 +/- 1.6 ng/ml, P less than 0.005).
However, researches noted that serum somatomedin C levels had declined significantly during the same span of time, which is a bit alarming. Somatomedin also called IGFs (insulin-like growth factors) act as intermediaries between somatotropin and bone (like the mnemonic I used to first learn this; on an unrelated note there are a lot of somatos: somatotropin, somatomedin, somatostatin…). Less somatomedin means GH has reduced effects.
A number of other researchers confirm the finding; fasting diets in rats decrease serum somatomedin levels, probably through a posttranscriptional method of control. Interestingly, Straus et al. found that the maternal rats who had fasted ended up giving birth to progeny who had retarded growth. Intracellular analyses suggested stunted growth was largely due to decreased IGF exposure.
Undoubtedly due to costs and how long we live (and thus how long these experiments would take), there is a lack of long-term studies of the effects of fasting on human growth. Tracking venous GH and IGF levels reveal much of what we’d expect from animal experiments. Though there’s a distinct lack of evidence that fasting can stunt growth in humans, the rat experiments are concerning.