The last few decades have seen a rise in rates of overweight and obesity. Average duration of sleep has declined in the Western world too. Could the two be connected? Actually, yes. There are a number of mechanisms through which reduced sleep time might be somehow contributing to the obesity ‘epidemic’, and these were summarised in a review published recently in the journal Cell Biochemistry and Function .
The review, from scientists at Federal University in Sao Paulo, Brazil, refers specifically to a range of potential mechanisms here including:
1. Reduced insulin sensitivity
One of insulin’s jobs is to assist the transfer of nutrients in the bloodstream into the cells. If insulin does not do this effectively, cells can be effectively ‘starved’ of fuel. If, say, brain and muscle cells, are under-fuelled, the end result could be mental and physical fatigue. And under-fuelled brain would tend to predispose to heightened hunger too.
When the body is broadly resistant to insulin’s effects, then the pancreas will generally pump out more insulin in an effort to lower blood sugar levels. The problem here is that insulin has a range of biochemical effects that we’d expect to translate into increased fat accumulation in the fat tissues. Here are some of insulin key effects in this respect:
Insulin stimulates the uptake of fat into the fat cells
Insulin activates the enzyme ‘lipoprotein lipase’ that catalyses the conversion of fat in the form of triglcyerides into smaller molecules known as fatty acids. These fatty acids, unlikely triglyceride, can make their way into the fat cells.
Insulin increases the supply of glycerol for the ‘fixing’ of fat in the fat cell
Insulin also facilitates the uptake of sugar into cells where it can be converted into glycerol In combination with fatty acids, glycerol forms triglyceride again, effectively ‘fixing’ fat in the fat cells.
Insulin inhibits breakdown and release from the fat cells
Triglyceride in the fat cells is disassembled through the action of an enzyme known as ‘hormone-sensitive lipase’. Insulin inhibits this enzyme, and therefore slows fat release from the fat cells (lipolysis).
2. Raised levels of cortisol
Cortisol is a major ‘stress’ hormone secreted by the adrenal glands. It’s essential to life, but as with everything, too much can be a bad thing. Cortisol antagonises insulin, and may therefore contribute to ‘insulin resistance’ (see above). It also has the ability to predispose to fatty accumulation, particularly around the midriff.
3. Raised levels of ghrelin
Ghrelin is a hormone secreted by the stomach and the pancreas. It stimulates appetite thorough and effect on the brain. Higher levels of this hormone may drive us to overeat.
4. Lowered levels of leptin
Leptin is a hormone secrete by fat cells. It acts on the brain to quell hunger and stimulate the metabolism. Lower levels of this hormone, as induced by short sleep duration, might therefore increase hunger and put a brake on the metabolism – not ideal for someone seeking to maintain or attain a healthy weight.
Short sleep can cause us to be tired, which might contribute to obesity by reducing overall levels of activity.
The authors of this review conclude that: “…present literature highlights the importance of getting enough good sleep for metabolic health.” Here, here to that. Tomorrow, I’m going to write a post on some of the simple strategies I’ve found to be most useful for ensuring better sleep.
1. Zimberg IZ, et al. Short sleep duration and obesity: mechanisms and future perspectives. Cell Biochem Funct. 2012 Apr 4 [Epub ahead of print]