Control of blood glucose by insulin and glucagon

The hormones insulin and glucagon help keep blood glucose concentration stable. They have opposing effects on the liver: insulin promotes the storage of glucose, while glucagon promotes its release into the blood.

When blood glucose rises

When blood glucose concentration rises, the pancreas releases insulin. Insulin causes the liver to take up glucose from the blood and convert it into glycogen, a storage form of glucose. This lowers the blood glucose concentration.

When blood glucose falls

When blood glucose concentration falls, the pancreas releases glucagon. Glucagon causes the liver to break down stored glycogen into glucose and release the glucose into the blood. This raises the blood glucose concentration.

Each response counters the original change: insulin acts when blood glucose rises, while glucagon acts when it falls.

Insulin control of blood glucose

The pancreas monitors the concentration of glucose in the blood. When blood glucose concentration rises too high, the pancreas produces and releases insulin.

Insulin lowers blood glucose concentration in two ways:

• It causes body cells to take up more glucose from the blood.
• It promotes the conversion of excess glucose into glycogen, which is stored in liver and muscle cells.

As these actions lower blood glucose concentration towards normal, the pancreas releases less insulin.

Type 1 diabetes

In Type 1 diabetes, the pancreas produces insufficient insulin. Insulin is a hormone that helps control blood glucose concentration by causing body cells to take up glucose from the blood and promoting its storage.

Without enough insulin, these responses are reduced. Blood glucose concentration is therefore not controlled effectively and can remain too high.

Treatment with insulin

Type 1 diabetes is normally treated with insulin injections. These supply the missing hormone, helping cells take up glucose and promoting glucose storage. This lowers blood glucose concentration.

Type 1 and Type 2 diabetes

The primary problem in Type 1 diabetes is insufficient insulin production. In Type 2 diabetes, the primary problem is that body cells become less responsive to insulin.

Type 2 diabetes

In Type 2 diabetes, body cells respond less effectively to insulin, a hormone produced by the pancreas. Insulin normally helps cells take up glucose from the blood. When cells respond less effectively, blood glucose is not controlled properly and can remain too high.

Managing Type 2 diabetes

A carbohydrate-controlled diet helps manage the amount of glucose entering the blood from food. Regular exercise also helps because active muscles take up glucose from the blood. Both can therefore help control blood glucose.

Unlike Type 1 diabetes, where the main problem is insufficient insulin production, the primary problem in Type 2 diabetes is the reduced response of body cells to insulin.

Interpreting insulin-response graphs

An insulin-response graph shows how blood glucose concentration changes, often over time. It may also show when insulin was administered, allowing the change afterwards to be examined.

Reading blood glucose values

First, check what each axis represents and note its scale and units. Time is commonly shown on the horizontal axis and blood glucose concentration on the vertical axis, but always check the labels.

To read a concentration at a particular time, find the time on its axis, move to the relevant curve, then read across to the concentration axis. Use the scale to judge values between marked intervals.

To find the change between two readings, subtract the starting concentration from the later concentration. A negative result means the concentration fell; a positive result means it rose. The change has the same units as the concentration readings.

Comparing insulin responses

If the graph marks when insulin was administered, compare concentrations before and after that time. A later fall is consistent with a response to insulin, but the graph alone may not establish that insulin was the only cause of the change.

When comparing curves for people with and without diabetes, look at:

• Concentration: Which curve is higher at the same time?
• Direction and size of change: Does each concentration rise or fall, and by how much over the chosen interval?
• Rate of change: How much does each concentration change over the same length of time? A steeper curve on the same graph indicates a more rapid change.

For example, suppose insulin is given at $$0$$ minutes. Over the next $$30$$ minutes, one person's concentration falls from $$9$$ to $$6\ \text{mmol/L}$$, while another's falls from $$7$$ to $$5\ \text{mmol/L}$$. The first person's concentration falls by $$3\ \text{mmol/L}$$ and the second's by $$2\ \text{mmol/L}$$. Because these changes occur over the same time interval, the first person's average fall is faster. Their concentration is nevertheless higher at both readings.

Base conclusions on the curves shown. Do not assume the size, direction or speed of a response solely from whether a person has diabetes.

Pancreatic glucagon release

When blood glucose concentration falls too low, the pancreas produces and releases the hormone glucagon into the blood. Glucagon travels to the liver, where it causes stored glycogen to be broken down into glucose. The liver releases this glucose into the blood, raising blood glucose concentration.

As blood glucose concentration rises towards normal, the stimulus for glucagon release decreases, so the pancreas releases less glucagon. This is negative feedback: the response reduces the change that triggered it.

Obesity and Type 2 diabetes risk

Obesity is a risk factor for Type 2 diabetes: having obesity increases a person’s chance of developing the condition, but does not make it certain.

Insulin is a hormone that helps control blood glucose by causing body cells to take up glucose from the blood. Excess body fat can contribute to cells responding less effectively to insulin. As a result, blood glucose becomes harder to control, increasing the risk of Type 2 diabetes.

A risk factor is not the same as a guaranteed cause. Not everyone with obesity develops Type 2 diabetes, and not everyone with Type 2 diabetes has obesity.