27. Complete starvation. Occurrence and process.
Starvation refers to generalized deficiency of energy-containing nutrients like fat, protein and carbohydrates. There is no deficiency of minerals, water or vitamins, or the deficiency of these is not as urgent as the deficiency of calories, meaning that their deficiency does not have time to affect the person.
There are two forms of starvation: Complete starvation, where absolutely no calorie-containing foods are consumed, and partial starvation, where the amount of calorie-containing foods is lower than needed.
Complete starvation
In complete starvation are no calorie-containing foods consumed. This happens in cases of hunger strikes or natural catastrophes. The time a person can survive complete starvation depends on the fat and protein-stores in that person’s body.
There are four phases of complete starvation:
- Postprandial phase – the first 12 hours after eating
- Postabsorptive phase – 12 hours to a few days after eating
- Early, non-adapted phase – After a few days to a week
- Late, adapted phase – After the first week
The body covers its energy need from different sources in the different phases. Let’s see how.
Postprandial phase
The body covers its energy needs from the nutrients that are being absorbed. Especially carbohydrates are used for energy while proteins and fats are used for anabolism and storage, respectively.
Postabsorptive phase
In this phase are there no more nutrients from the food that the body can utilize. The body must rely on its pre-existing stores. The liver can maintain the blood sugar by breaking down glycogen for around 12 hours. After that must the liver perform gluconeogenesis to maintain the blood sugar, by using glucogenic amino acids, lactate and glycerol (from breakdown if triacylglycerols). The blood glucose level declines slowly but is still within normal range.
Early, non-adapted phase
The blood glucose level is now moderately low. The low blood glucose means that there is very little insulin present, which prevents insulin-dependent tissues like adipose tissue and muscle from using the blood glucose – these tissues must rely on free fatty acids. These fatty acids are supplied by adipose tissue, in which hormone-sensitive lipase breaks down triacylglycerols into free fatty acids.
In the liver acetyl-CoA is produced from β-oxidation of fatty acids. This acetyl-CoA should go into the TCA cycle to be consumed for energy, however the low insulin level inhibits the TCA cycle. This causes the acetyl-CoA to be used for ketone body synthesis instead.
Neural tissue and red blood cells can not switch to fat metabolism – they operate exclusively on glucose, independent of insulin levels. Maintaining the blood glucose level is essential to allow the brain to continue to operate. This blood glucose is supplied from gluconeogenesis of the liver, which uses glycogenic amino acids. The supply of these amino acids comes from increased protein breakdown. This causes blood urea nitrogen (BUN) levels to increase, and the protein loss can lead to loss of specific protein functions, which you can read more about in topic 29.
Late, adapted phase
This phase is “adapted” because at this point has the brain adapted to using ketone bodies as well for energy, in addition to glucose. This slows down the protein breakdown, which is essential for survival.
Because the brain will always need some glucose supply during starvation will there always be some protein breakdown. Because of this is the size of protein stores more important than the size of the fat stores for survival during starvation. However, when there are no fat stores left must protein breakdown increase.
Death from complete starvation occurs when so much protein has been broken down that essential functions cannot continue. Breakdown of proteins of the respiratory muscles or immune system can lead to insufficient ventilation or infection, respectively, can be causes of death. At this stage is there also hypoglycaemia as the body has exhausted the protein stores.