Why can I live several weeks without having to eat food?
The laws of nature demand that life must have enough energy to do what it needs to do to survive. Your cells get the energy they need to live and work properly by cellular respiration, a process that releases the energy within a sugar molecule called glucose in the presence of oxygen. Your body can’t store oxygen and its need for it is so acute that you have to breathe in a new supply every few seconds. In fact, if you don’t breathe for about four minutes, your cells begin to die (particularly the ones in the respiratory center in your brainstem that tell you to breathe) and soon, so do you! What about glucose? It too plays a major role in providing the energy your body needs. Why, as opposed to only being able to live for four minutes without bringing in new supplies of oxygen, can you live for several weeks without taking in new supplies of glucose (or other sources of energy)? Just like a car that stores its supply of fuel in a tank and can run on gas, or diesel, or even alcohol, your body can store glucose and other molecules it uses for energy. That’s why you can live longer without eating food as compared to not breathing. Here’s how it works.
When you eat food your gastrointestinal system takes in all of the glucose and puts it into your blood. Your body uses some of this new supply of glucose for its immediate energy needs but (if you’re like me) there’s usually a lot left over. So, right after a meal, the level of glucose in your blood increases. This increase in glucose is detected by glucose sensors on the plasma membrane of beta cells in your pancreas and it tells them to release a hormone called insulin. Insulin travels in the blood and locks on to specific insulin receptors on the surface of cells in your liver (and other organs and tissues). When it does this it signals them to bring in and use glucose, and join together any left over to form a carbohydrate called glycogen. Once these organs and tissues have formed as much glycogen as they can, any glucose left over gets stored as fat, apparently without limit. The brain however, is unable to store glucose as glycogen or fat. This makes the brain totally dependent on the glucose it receives from the blood in the circulation.
Storing energy is useful for your body only if it can be released later so it can be used when needed. Your body is always using up energy, and so a few hours after a meal, during an overnight fast and especially when you’re working or playing hard, because no new supplies of glucose are coming in, the blood glucose level decreases. This decrease tells the beta cells in the pancreas to turn off the release of insulin. But, through glucose sensors on the plasma membrane of alpha cells in your pancreas, it tells them to release a hormone called glucagon. Glucagon travels in the blood and locks on to specific glucagon receptors on the surface of certain cells, particularly in the liver, and tells them to release the glucose from within glycogen. In fact, the liver has the capacity to store enough glycogen to meet all of the body’s energy needs for about twenty-four hours. Also, when not enough glucose is present, glucagon tells the liver to take certain proteins and fats and chemically change them into glucose and other molecules that the brain can use for energy.
So one can see that insulin basically tells the cells in the body that "we've just been fed and we’ve got more energy than we need right now so let's store up the extra for later use" and glucagon tells them "we haven't been fed for a while and we need more energy so release what we stored up from before". Although the body can live for several weeks without insulin, the absence of glucagon is incompatible with life. It’s the combination of insulin telling your body to store any excess molecules it uses for energy and glucagon releasing this energy when needed that allows you to live several weeks without having to eat food.
Three Questions for Mr. Darwin
- Where did the information come from to tell my body how to make the glucose sensors and place them exactly where they're needed (on the alpha and beta cells in my pancreas) and how do they work?
- How did life anticipate its need to store energy and so produce all the parts needed (glucose sensors, beta cells to make insulin and alpha cells to make glucagon and their respective receptors present on liver cells) to perform this task and how did it survive each step along the way?
- How do the beta cells in my pancreas know how much insulin, and the alpha cells know how much glucagon, to release for a given blood glucose level to get the job done right?
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Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.
Copyright 2017 Dr. Howard Glicksman. All rights reserved. International copyright secured.
