Reactive hypoglycaemia: low blood sugar after eating, symptoms and management

Blood sugar (glucose) is the body's main short-term fuel. Insulin, a hormone made by the pancreas, brings glucose levels down after food. In reactive hypoglycaemia, glucose falls too low in the hours after eating. Guy's and St Thomas' NHS Foundation Trust describes it as a low blood sugar level after eating that is different from low blood sugar that happens while fasting.

The NHS describes low blood sugar (hypoglycaemia or a hypo) as usually where blood sugar is below 4mmol/L, and says it mainly affects people with diabetes who take insulin or some other diabetes medicines. It is rare in people without diabetes, which is one reason a persistent pattern of symptoms after meals deserves a proper assessment rather than being assumed to be reactive hypoglycaemia.

The mechanism is not always clear. The trust explains that in most cases it is not known why reactive hypoglycaemia happens, and that it is thought to be caused by the body making too much insulin after food or drink that is high in carbohydrate, or being oversensitive to insulin or to other hormones that help to control blood sugar.

The symptoms are those of low blood sugar in general. Guy's and St Thomas' lists dizziness or light-headedness, extreme hunger or appetite, difficulty concentrating, sweating, a rapid heartbeat, tingling lips, trembling and blurred eyesight. The NHS list for low blood sugar also includes feeling anxious or irritable, feeling tired or weak and feeling confused.

Severe low blood sugar can cause a seizure or loss of consciousness, according to the NHS. That is much more typical of people on insulin or similar medicines than of reactive hypoglycaemia, but any episode of confusion, collapse or fitting needs urgent medical attention.

Because low blood sugar is unusual in people without diabetes, doctors look for other explanations. The NHS says that in people without diabetes it can sometimes be caused by conditions including malnutrition, Addison's disease and a growth (tumour) in the pancreas. For people with diabetes, treatment with insulin or tablets such as gliclazide and glimepiride, missed or delayed meals, a lot of exercise without extra carbohydrate and alcohol, particularly without eating, are the common reasons.

It is therefore sensible to separate symptoms that happen after meals from those that happen when fasting or overnight, and to tell the GP about any medicines, alcohol use and previous stomach surgery. Taking a note of when symptoms start in relation to meals, what was eaten and the blood sugar reading, if you have a meter, makes the assessment much easier.

The NHS advises seeing a GP if you do not have diabetes and get symptoms of low blood sugar such as shaking, sweating, a fast heartbeat and feeling confused. If you have diabetes, you can also speak to your diabetes care team.

Guy's and St Thomas' says it uses an oral glucose tolerance test to diagnose reactive hypoglycaemia, usually at hospital. You have a sugary (glucose) drink and your blood sugar level is monitored to see how your body reacts to carbohydrate. The result is interpreted alongside symptoms: a low reading with matching symptoms that settle once the sugar level rises is what doctors look for. A GP may refer to an endocrinology or diabetes service if the picture is unclear or if a rarer cause is suspected.

Guy's and St Thomas' sets out a three-step approach. Have a sugary drink or snack: a small 150ml to 200ml glass of non-diet fizzy drink or fruit juice, or 15g to 20g of fast-acting carbohydrate such as 3 to 4 jelly babies, 4 to 6 glucose tablets or 1 tube of glucose gel. Test blood sugar after 10 to 15 minutes, and repeat if there is little or no change. Then prevent it dropping again by eating your next meal if it is due and contains starchy carbohydrate, or a snack with slow-release carbohydrate such as a slice of bread, 2 digestive biscuits or a glass of cows' milk.

The NHS adds that if someone with very low blood sugar becomes unconscious they should not be given food or drink, should be put in the recovery position, and given a glucagon injection if one is available and you know how to use it. It says to call 999 if the person is not responding normally or is unconscious and glucagon is not available, has not worked within 10 minutes, or they have been drinking alcohol.

Managing diet is usually the best way to prevent reactive hypoglycaemia, according to Guy's and St Thomas', and you might be referred to a dietitian for personalised advice. Its guidance includes the following.

• Eat 3 meals a day with a small snack between each, roughly every 3 hours, to avoid long gaps.

• Include vegetables, salad or fruit, wholegrain starchy foods and a protein food in each meal.

• Add a portion of unsaturated fat such as olive oil, nuts, seeds or avocado, which can slow down the digestion of carbohydrate.

• Avoid large portions of starchy carbohydrate such as rice, bread, potatoes, pasta and chapattis; have a small to medium portion with each meal and space them out.

• Choose low glycaemic index foods such as wholegrain bread, rye bread, oats, beans, lentils, most fresh fruit (but not fruit juice or dried fruit) and sweet potato.

• Limit sugary foods and drinks, and drink only a small amount of alcohol, with a meal rather than on an empty stomach, within 14 units a week.

• Caffeine can make symptoms worse, so decaffeinated tea or coffee and diet soft drinks are preferred.

• Keeping to a healthy weight and doing regular moderate-intensity exercise also helps.

For many people, regular meals and a steadier carbohydrate pattern are enough to bring episodes under control. If episodes continue frequently after following the dietary guidance, Guy's and St Thomas' advises seeing your GP or consultant.

On the NHS, a GP can arrange initial blood tests and refer to a hospital diabetes or endocrine clinic for a glucose tolerance test and dietitian advice. Some people use private clinics or home glucose monitoring, but a reading from a device should still be interpreted by a clinician, because symptoms and low readings need to be matched to confirm the diagnosis.