OK (mentally rolling up my sleeves). I will try to make this as basic as possible, mainly not to confuse myself. To the biochemistry geeks: don’t pick on me. If you are a visual learner and don’t like reading too many words (good luck tackling “War and Peace”), scroll halfway down the page for the Insulin Action summary diagram.
It can be a revelation to realise that your metabolism (and therefore your weight) is regulated by hormones. Here you are, religiously counting your calories, looking up nutrition charts and calculating your necessary protein portions, and your own body doesn’t care one bit. I am sorry to break it to you but an uneducated old woman in a remote Russian village is doing a better job in controlling her weight than some of us are. I should know, she is my grandmother.
Insulin could be the most important hormone in our body.
How to keep a tight leash on glucose
The first thing you need to understand is that our blood sugar (=blood glucose) is kept in a narrow range. If it gets too low your body will start sweating, hands slightly shaking, heart rate goes up and you might become less pleasant to be around . If you are thinking that this sounds a lot like a first date, you are right. The same forces are at work here: hormones adrenaline and noradrenaline fire up your sympathetic autonomic nervous system, responsible for our ancient “fight or flight response”.
Other players like glucagon, growth hormone and thyroid hormone get involved. Their combined action is to raise your blood glucose back to normal by breaking down glycogen (stored glucose in the liver). Your body switches to preferentially burning fat for energy and ketones, produced from this process, are fed to the brain and to the heart.
Normally this would be the end of your troubles: blood sugar normalises, your body starts happily eating away on all that fat conveniently stored for such purposes. However, things can go wrong if you have injected yourself with a whopping dose of insulin or if your sympathetic nervous system doesn’t work. Blood sugar dips even lower, brain starts to starve without an energy source in the form of glucose or ketones, and you end up in a coma. Not nice.
High blood sugar is toxic
If your blood sugar gets too high, your body doesn’t like it either. To protect your brain and other cells from toxic effects of glucose, your pancreas release insulin. The primary job of insulin is to take glucose out of the blood and dump it into the cells which are prepared to take it. It sort of reminds me of when my boyfriend tries to clean the room. His main concern is only the mess that he can see: clothes get shoved into drawers, papers shuffled into a pile out of the way. (Don’t get me started on not vacuuming around random objects like a shoe box which because they are clearly too heavy for lifting).
Insulin sweeps the blood with the same nonchalant efficiency. It acts on specific receptors on the surface of muscle and fat cells, effectively opening the door and letting glucose into the cell. In a muscle cell, glucose can be used for energy or stored as muscle glycogen for later. In a fat cell, you guessed it, it is used for forming triglycerides, a.k.a fat. Insulin does not care that the “drawers” are full and you would prefer NOT to store any fat. All it cares about is neutralising the sugary sweet wave of glucose. If insulin did not do its job (like if you have type I or advanced type II diabetes), any high carbohydrate meal would leave you nauseated, confused, delirious and end up in a coma. All roads lead to Rome.
So to re-cap (biochemical terms in brackets).
High insulin =>blood sugar pushed into liver, muscle and fat cells (↑ glucose uptake)
=>blood sugar converted into its storage form the liver and muscle (↑ glycogen synthesis)
=>liver makes fatty acids and releases them as triglycerides (↑ fatty acid synthesis in the liver)
=>fat cells get fatter (↑ triglycerides synthesis in adipose tissue)
Incidentally, does anybody else think it’s interesting that we have a whole gang of hormones dealing with hypoglycemia (=low blood sugar) and only one dealing with hyperglycemia (=high blood sugar)? Could this mean that sky high glucose was a rare event from the evolutionary point of view and we didn’t need much back up? Hmmmmm…
So it’s actually quite simple. Let’s say you are a healthy 20-something with bulging biceps and metabolism buzzing like an Energizer bunny. You start your day with 7 weat-bix (because a paid athlete said so) with low-fat milk (because your girlfriend follows nutrition advice from Cleo magazine), washed down with a glass of orange juice. Almost 100g of carbohydrates makes your blood glucose go up, insulin rushes into the blood like a knight in shining armour. Blood glucose gets swept into the cells, and blood sugar levels settle back down. Insulin sticks around for a few more hours though. It tries to keep fat locked inside the fat cells and glycogen stored safely away in the liver and muscles. You have your protein shake “to prevent muscle breakdown” at 10.30am, raise your blood glucose again, your insulin is still up. You conscientiously top your blood glucose up every 2-3 hours because it is the healthy thing to do. Your pancreas happily spits out insulin necessary to cover this continuous influx of carbohydrate-rich food.
If you are young, fit and healthy, it doesn’t seem like a big deal. Multiply this scenario by 365 in a year, add a few spikes of glucose around Christmas parties, birthdays, a huge sugar tsunami for your mate’s wedding, reduce physical activity, add liver inflammation from alcohol, fructose, vegetable oils. Then continue for the next 15, 20, 25 years. (Any of this reminds you of a family member or a friend?)
How long would it take your cells to develop resistance to these massive doses of insulin?
How long before your pancreas runs out of steam to push more insulin out?
How long before your knight-in-shining-armor-insulin becomes your worst enemy?