Insulin resistance, what is it and why does it happen?
Insulin is a hormone released by beta cells in the pancreas (a pear-shaped organ behind your stomach) into the bloodstream. I like to imagine hormones as messages received by cells with hormone receptors – for example, insulin says “please take glucose out of the bloodstream, love from the pancreas”. Body cells with insulin receptors will get this message every time insulin binds to one of those insulin receptors. Insulin is very important because glucose (a type of sugar) is the main source of fuel for cells.
This article explains glucose distribution and then insulin resistance.
PART 1: glucose distribution
Depending on the body’s situation, glucose will float about in the bloodstream or get transported into cells to provide them with fuel.
Sometimes it’s useful to have more glucose in the cells
Normality: cells break down glucose to create energy that they need for normal processes like moving and growing – the only reason that glucose is there in the first place is to supply the cells, so the cells may as well take it!
Exercise: muscles imminently need more glucose in order to contract so there’s no point in keeping the glucose in the blood when the muscles need it now!
Cells are robust. They deal with too much glucose by using the extra glucose to build stores of glycogen (made from lots of glucose molecules stuck together) or fat. Cells cope with not enough glucose by obtaining energy from breaking down stores of fat, glycogen and protein, or in extreme cases, just dying).
Sometimes it’s useful to have less glucose in the cells
If there are too many cells per unit glucose: this could happen due to undereating or surviving a famine, which means there won’t be enough glucose, or being obese, in which case there are too many cells – then each cell should not take too much glucose, even if insulin is there, in order to leave some glucose for the other cells.
Sometimes it’s useful to have more glucose in the blood
Infections: blood cells need glucose too – if you are ill, there are more white blood cells than normal in your bloodstream trying to fight the infection, hence more glucose is needed in the blood to provide for these extra cells. White blood cells release hormones called cytokines to make sure that other cells don’t take too much glucose from the blood during an infection.
Stress: there’s a bunch of reasons that bodies can be stressed, and some of them, such as chronic illness or suspected ‘fight-or-flight’ in the near future, means that the body should ‘save the glucose for later’ instead of taking it all up into the cells now. If you are stressed, your adrenal glands (there’s one adrenal gland above each of your kidneys) release a hormone called cortisol which prevents cells taking too much glucose out of the blood.
However, the bloodstream is very fragile and can’t contain too much glucose for the following reasons:
- Glucose is very sticky – this makes blood clots which block up small blood vessels and damage capillaries.
2. Glucose attracts water – if there’s too much glucose in the blood, then water will diffuse out of cells and enter the bloodstream, causing body cells to become dangerously dehydrated.
Also, the bloodstream can’t have too little glucose in it because some cells, like brain cells, do not store glycogen or fat, so get glucose exclusively from the blood. The brain gets very unhappy if there is not enough blood glucose, and makes you feel very hungry, drowsy, confused and faint!
A body cell asks itself two questions when deciding how much glucose to take up
1) How much insulin has bound to my receptors?
(more insulin detected by insulin receptors means that cells take up more glucose)
2) How many anti-insulin signals have bound to my receptors?
(more anti-insulin signals detected means that cells take up less glucose)
However, the pancreas only asks itself one question when deciding how much insulin to release
1) How much glucose is there in the blood?
(more glucose in the blood means that pancreas cells release more insulin. It also means that pancreas cells divide faster, which enables more insulin to be released faster in the future.)
PART 2: what is insulin resistance?
It’s when the pancreas releases insulin (because the blood glucose is high), but the cells do not take up glucose.
Why don’t the cells take up glucose?
Maybe insulin was not sensed by the body’s receptors
Every cell has a different number of insulin receptors. Some insulin receptors respond more to insulin than others. Cells with a greater number or larger responsiveness of insulin receptors will respond more to insulin.
Maybe there were lots of anti-insulin signals
Anti-insulin signals include cortisol and cytokines released due to stress and illness respectively. Insulin resistance will decrease again when someone recovers from their infection or stops being stressed!
What does the pancreas do to combat insulin resistance?
The pancreas gets bigger (its cells divide more) so that a larger amount of insulin can be released faster. Insulin resistant people have higher levels of insulin in their bloodstream in order to achieve the same average glucose concentration as insulin sensitive people.
How big can a pancreas get?
Big… but not that big! Some people are so extremely insulin resistant that they would need huge pancreases releasing huge amounts of insulin in order to make their cells hear the message “please take up blood glucose, love from the pancreas” and maintain safe levels of blood glucose.
However, cells that release hormones (e.g. beta cells) will commit apoptosis (i.e. kill themselves) if they sense overly high levels of stimulus telling them to divide like crazy and release loads of hormone. That’s an example of the body protecting itself against cancer (excessive, uncontrolled cell division).
If you are very insulin resistant, and your blood glucose creeps up because your pancreas is not releasing enough insulin, some of your pancreas cells might sense that they need to divide like crazy… and they might decide to die instead! If too many pancreas cells die, there won’t be enough insulin to control your levels of blood glucose. This is such a serious problem that it even has its own name: diabetes mellitus. It’s illustrated in the image below.
Diabetes mellitus can be treated using insulin injections, various medications and lifestyle modifications.