Stress, Fasting and Starvation

Study break is over so it's back to classes. Woohoo...

Body energy stores

We have a small amount of free glucose in our blood, but this will only last us around half an hour. Thankfully, we can also store other nutrients so that we don't starve to death between meals. We have glycogen stores, which can last for around 18 hours, triacylglycerol (a.k.a. fats), which can last for around 55 days, and protein stores (basically all the tissues in our body can be broken down for fuel), which can last around 21 days.

Short term fasting

In short-term fasting (i.e. before glycogen stores have been depleted), plasma glucose decreases. Since glucose levels decrease, insulin decreases and glucagon increases. The actions of these two hormones promote glycogenolysis (breakdown of glycogen to form glucose) and inhibit glycogenesis (formation of glycogen from glucose), which helps to keep glucose levels high enough to sustain the various body tissues. I have described the functions of insulin in more detail here. Glucagon pretty much just does the opposite of insulin.

Long term starvation

As I mentioned above, glycogen stores only last for around 18 hours. Therefore, if you starve for a while, fats and protein become more important fuel sources.

Fats are our largest energy source and can keep us going for around 55 days. A large proportion of this fat is stored in adipose tissue. Unfortunately, not all tissues can derive energy from fats directly. These tissues include the brain, heart, erythrocytes and renal cortex. Therefore, some alternatives are necessary: energy derived from breaking down fat can be used to power gluconeogenesis (discussed here), or acetyl CoA can be converted into ketone bodies (discussed here). Gluconeogenesis depletes supplies of oxaloacetate (a component of the citric acid cycle), which prevents acetyl CoA from entering the citric acid cycle, resulting in production of ketone bodies instead. Ketone bodies are released into the blood (blood ketones reach around 6-8mmol/L) and enter other cells, where they are converted back to acetyl CoA and oxidised to produce ATP via the citric acid cycle.

During starvation, our metabolism also decreases. This is due to a decrease in leptin secretion, which in turn decreases TSH secretion, which in turn decreases thyroid hormone secretion.

Summary

When we stop eating, insulin decreases and glucagon increases. As glycogen stores are depleted, tissues that are capable of metabolising fatty acids start doing that (so as to save glucose for tissues that need it, like the brain). The liver also performs gluconeogenesis and ketone production in order to power tissues that can't oxidise fatty acids. Metabolism also decreases. During long-term starvation, there is a kind of steady state with gradual depletion of protein and fat stores. Blood ketones reach around 6-8mmol/L, of which around 2/3 is used by the brain. When fats are depleted, protein degradation increases dramatically, ultimately resulting in organ failure.