At any given moment, there is about 4.5g of glucose circulating in your blood (5mmol/l x 180g x 5l). As the brain alone uses about 6g of glucose per hour in the absence of ketones, BG could fall to zero within an hour if we ate no sugary/starchy carbs. If we ate a mere 5g of glucose, BG level could double. As low BGs are fatal and high BGs damage proteins by a process called glycosylation (a bit like caramelisation), the body keeps BG levels within fairly tight limits by the use of a negative feedback (NFB) control system.
NFB systems consist of a non-inverting (more in → more out) part, which in this case are the islet cells of Langerhans (a.k.a. pancreatic beta cells), as increasing BG level results in increasing insulin level. It's actually more sophisticated than that. Beta cells can store insulin and dump it into the blood if there is a sudden increase in BG level. This is analogous to the accelerator pump in a carburettor, which dumps petrol into the engine if you slam your foot on the accelerator pedal, i.e. it produces a rapid response. The dumping of insulin from beta cell storage is known as the Phase 1 insulin response. If this (or the accelerator pump) fails, there is a lag in the response; this will become significant below.
Increasing BG level results in increasing insulin secretion from beta cells and is known as the Phase 2 insulin response
The other part of a NFB system is the inverting (more in → less out) feedback part, which in this case is split into three parts, all working in parallel. They are:
There are three main types of diabetes:
1) Type 2 diabetes. This is by far the most common (about 95% of all cases) and is usually caused by abdominal obesity. Type 2 diabetes has two main mechanisms going on. The first is a progressive insulin resistance of target tissues (firstly liver, then muscles and then fat cells in that order) possibly caused by increased levels of saturated fatty acids being fed to the liver from abdominal fat stores, and chronically-high BG and insulin levels caused by chronically over-consuming high glycaemic load carbohydrates, possibly accompanied by large amounts of saturated fat and/or large amounts of omega-6 fat. A sedentary lifestyle lowers the sensitivity of muscle cells to insulin. Insulin resistance also has a hereditary link. It may also be linked to a Vitamin D deficiency, see here.
Insulin resistance weakens the feedback in the NFB system, resulting in further increased BG and insulin levels. Increased BG level causes increased damage to beta cells by glycosylation. Increased insulin level causes further insulin resistance as target tissues become increasingly insensitive (a bit like louder and louder music making you progressively deafer and deafer). Eventually, beta cells become too damaged to secrete sufficient insulin and insulin levels begin to fall. This results in a massive rise in BG level and this is now full-blown Type 2 diabetes.
There are five main treatments for Type 2 diabetes:
2) Type 1 diabetes. This is much less common (about 5% of all cases) and is caused by an autoimmune disease. One possible mechanism is as follows: Due to an increase in Zonulin, the gut becomes more permeable than it should (Leaky Gut Syndrome) and allows protein fragments to pass into the blood. These are destroyed by antibodies. However, if a protein fragment happens to have the same sequence of amino acids as a protein in our own body, the antibodies then set about destroying parts of our own body. Examples of this are gluten (proteins found in wheat, rye, barley and oats) producing antibodies in the blood that can destroy the gut causing Coeliac Disease or skin cells causing Dermatitis Herpetiformis or mucous membranes causing Sjogren's Syndrome or brain cells causing Cerebellar Ataxia. As there is an association between the consumption of cows' milk and the incidence of type 1 diabetes (see here), it is quite possible that, in susceptible individuals, fragments of casein protein enter the blood in this way resulting in antibodies that destroy pancreatic beta cells.
Another possible mechanism is autoimmune attack after a viral infection:
Once all of the beta cells are destroyed, no insulin is secreted and insulin injections are required. If some beta cells survive, there is a possibility that normal BG level can be maintained if sugary/starchy carbohydrate intake is much reduced.
3) Latent Autoimmune Diabetes of Adulthood (LADA). The percentage of cases with this is unknown as it is often misdiagnosed as type 2 diabetes. This is a slow developing diabetes that is more like type 1 in origin (autoimmune with antibodies) but is often misdiagnosed as type 2 because of the age at diagnosis and the relatively slow progression of the disease (slow compared to type 1 but fast compared to type 2). It is believed that Sir Steven Redrave has this type of diabetes. Whether his autoimmune disease was triggered by a huge intake of milk (to build those Olympic-winning muscles) we will never know.
As stated earlier, loss of the Phase 1 insulin response can occur. This usually happens when beta cells are chronically over-secreting insulin due to a chronically-high intake of sugary/starchy carbs and are unable to store any. This results in a lag in insulin response. This isn't a problem if low glycaemic load carbs are eaten and BG levels change only a little or very slowly. However, if high glycaemic load carbs are eaten, this produces a large and rapid rise in BG level. If a NFB loop with a lag in it is presented with a sudden change in input level, its output overshoots. This results in too much insulin being secreted, which eventually results in low BG levels! This is known as rebound hypoglycaemia. The solution? Stick to low glycaemic load carbs.
When no sugary/starchy carbs are being digested, BG starts to fall. Adrenaline and noradrenaline (catecholamine hormones) are secreted by the adrenal medulla into the blood and also by sympathetic neurons. Like glucagon (see below), they stimulate the mobilisation of glycogen and triacylglycerols (stored fats) by triggering the production of cyclic AMP (adenosine monophosphate). Adrenaline and noradrenaline differ from glucagon in that their glucose-producing effect is greater in muscle glycogen than in liver. They also inhibit the uptake of glucose by muscle. Instead, fatty acids released from adipose tissue are used as fuel. Adrenaline also stimulates the secretion of glucagon and inhibits the secretion of insulin. Thus, catecholamines such as adrenaline and noradrenaline increase the amount of glucose released into the blood by the liver and decrease the utilization of glucose by muscle.
Pancreatic alpha cells secrete glucagon. This hormone mobilises the conversion of liver glycogen into glucose. The liver only stores about 70g of glycogen, but when combined with water, a larger mass of glucose can be generated. Eventually, liver glycogen stores become depleted and BG level falls again. Glucagon also stimulates gluconeogenesis in the liver, which is the production of glucose from non-carbohydrate precursors, like the conversion of glucogenic amino acids, such as glutamine, into glucose. This causes slow muscle wastage unless there is sufficient protein intake to provide the necessary amino acids. When BG falls to about 3.3mmol/l, the pituitary gland kicks-in and secretes ACTH (adrenocorticotropic hormone) which stimulates the release of cortisol from the adrenal cortex. Cortisol further stimulates gluconeogenesis in the liver. When BG level falls to about 2mmol/l, the pituitary secretes GH (Growth Hormone) which has an anti-insulin effect.
Insulin has many metabolic effects in the body apart from lowering BG level. It's a very anabolic hormone and an insulin spike is usually desired post workout to maximise the uptake of glucose and amino acids by muscle cells. There's nothing wrong with the occasional short-term insulin spike. It's chronically high insulin levels that cause long-term health problems like high blood pressure, high total cholesterol with low HDL-c & clogging of arteries.