Hypoglycaemia: Warning Signs and What to Do

Hypoglycaemia, or low blood glucose, can turn a normal training session into an emergency within minutes. It mostly affects people treated with insulin or certain diabetes tablets, but athletes, people on very low-calorie diets and those misusing insulin for bodybuilding are also at risk. Our editorial team explains how to spot it early and how to respond.

What counts as hypoglycaemia

Blood glucose is normally kept within a fairly narrow range by insulin, glucagon, adrenaline and other hormones. In the UK, glucose is measured in millimoles per litre (mmol/L). For people with diabetes, Diabetes UK uses the simple message "four is the floor": a reading below 4.0 mmol/L should be treated as a hypo.

International guidance classifies severity in levels. Level 1 is below 3.9 mmol/L, level 2 is below 3.0 mmol/L and is clinically significant, and level 3 is defined not by a number but by the need for help from another person because of confusion or loss of consciousness.

In people without diabetes, true hypoglycaemia is uncommon. When it does occur, doctors look for Whipple's triad: symptoms consistent with a hypo, a low measured glucose at the time, and relief of symptoms when glucose is raised. Feeling shaky after a missed meal does not automatically mean a disorder.

For context, a typical fasting value in someone without diabetes is roughly 4–6 mmol/L. A value slightly below 4 mmol/L after a long run or an overnight fast, without symptoms, is often normal in people who are not taking glucose-lowering medicines.

Recognising the signs

Symptoms come in two groups. Autonomic symptoms are caused by the body's alarm response, mainly adrenaline. Neuroglycopenic symptoms reflect the brain running short of glucose and tend to appear as levels fall further.

• Early (autonomic): sweating, trembling, pounding heart, hunger, anxiety, tingling lips, pale skin.
• Later (neuroglycopenic): confusion, difficulty concentrating, blurred vision, slurred speech, unusual behaviour, irritability, poor coordination.
• Severe: seizures, loss of consciousness.

During exercise these signs are easy to miss. Sweating, a fast heart rate and fatigue are expected parts of hard training, so a hypo may be mistaken for simple exhaustion. Unexpected clumsiness, confusion or a sudden drop in performance are important clues.

Some people with long-standing diabetes develop "hypoglycaemia unawareness", where early warning symptoms are blunted. Frequent hypos, alcohol and certain medicines such as beta-blockers can make this worse, which is why continuous glucose monitors are increasingly used in the UK.

Night-time hypos deserve special mention. Signs include nightmares, sweat-soaked sheets, waking with a headache or feeling unusually tired in the morning. Anyone on insulin who notices these after evening training should discuss it with their diabetes team.

Immediate action: the 15–20 g rule

If someone is conscious and able to swallow, UK guidance recommends 15–20 g of fast-acting carbohydrate straight away. Examples include about 150–200 ml of pure fruit juice, five to six glucose tablets, or a small glass of a full-sugar soft drink. Chocolate and biscuits act too slowly because of their fat content.

Recheck glucose after 10–15 minutes. If it is still below 4.0 mmol/L, repeat the fast-acting carbohydrate. Up to three cycles are usually advised before seeking urgent medical help.

Once glucose is back above 4.0 mmol/L, follow up with a longer-acting carbohydrate, such as a sandwich, a piece of fruit or the next meal if it is due. This reduces the chance of a second fall, especially after prolonged exercise.

If the person is drowsy, cannot swallow safely, is having a seizure or is unconscious, do not put food or drink in their mouth. Place them in the recovery position and call 999. If they carry glucagon and someone trained is present, it can be given while waiting for the ambulance.

Exercise-related hypos

Muscles take up glucose rapidly during exercise, and they keep doing so for many hours afterwards while glycogen stores are rebuilt. In people using insulin or sulfonylureas, this can cause a hypo during training, shortly after it or even overnight.

Long aerobic sessions carry the highest risk; short sprints and heavy resistance work sometimes raise glucose temporarily because of the adrenaline response. The 2017 consensus statement on exercise in type 1 diabetes by Riddell and colleagues describes how to adjust carbohydrate and insulin, always with a diabetes team.

Healthy athletes without diabetes rarely become truly hypoglycaemic, but it can happen during very long endurance events with poor fuelling, or with severe energy restriction before competitions. Shakiness and light-headedness in these settings deserve attention, not bravado.

Alcohol increases risk because it blocks the liver from releasing glucose. A night out after a long training session, especially with little food, is a classic set-up for a hypo the next morning.

Insulin misuse in sport: a potentially fatal risk

Insulin is sometimes misused by bodybuilders for its anabolic and glycogen-storing effects. It is a prescription-only medicine and is prohibited at all times by the World Anti-Doping Agency. Case reports, including one in the British Journal of Sports Medicine, describe severe hypoglycaemia in non-diabetic users.

This practice can kill. Severe hypoglycaemia can cause seizures, permanent brain damage, heart rhythm disturbances and death, sometimes during sleep when nobody notices the warning signs. Because the dose-response is narrow and absorption varies, even experienced users have been found unconscious.

Our editorial team does not provide any guidance on insulin use outside medical treatment. Anyone who has used insulin without a prescription and experiences confusion, sweating or drowsiness should be treated as a medical emergency, and the people around them should know to call 999.

Other substances can also lower glucose: sulfonylurea tablets, some growth-promoting peptides and very large doses of certain supplements combined with fasting. Unexplained hypos always need medical investigation.

Conclusions of the editorial team

Hypoglycaemia is defined by low glucose plus symptoms. In UK diabetes care, any reading below 4.0 mmol/L should be treated. Early signs such as sweating and shaking can be mistaken for normal exercise fatigue.

Treatment is simple when the person is conscious: 15–20 g of fast-acting carbohydrate, a recheck after 10–15 minutes and then a longer-acting snack. When the person cannot swallow safely, call 999 and do not give anything by mouth.

Insulin misuse outside medical care is extremely dangerous and can be fatal. Insulin is not a supplement and should only ever be used as prescribed.

Readers may also find useful our editorial guides on fuelling long endurance sessions, alcohol and recovery, and how continuous glucose monitors are used by athletes.

References

1. International Hypoglycaemia Study Group. Glucose concentrations of less than 3.0 mmol/L (54 mg/dL) should be reported in clinical trials: a joint position statement of the American Diabetes Association and the European Association for the Study of Diabetes. Diabetes Care. 2017;40(1):155–157.
2. Seaquist ER, Anderson J, Childs B, et al. Hypoglycemia and diabetes: a report of a workgroup of the American Diabetes Association and the Endocrine Society. Diabetes Care. 2013;36(5):1384–1395.
3. Cryer PE, Axelrod L, Grossman AB, et al. Evaluation and management of adult hypoglycemic disorders: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2009;94(3):709–728.
4. Riddell MC, Gallen IW, Smart CE, et al. Exercise management in type 1 diabetes: a consensus statement. Lancet Diabetes Endocrinol. 2017;5(5):377–390.
5. Evans PJ, Lynch RM. Insulin as a drug of abuse in body building. Br J Sports Med. 2003;37(4):356–357.
6. Cryer PE. Hypoglycemia in Diabetes: Pathophysiology, Prevalence, and Prevention. 3rd ed. Alexandria (VA): American Diabetes Association; 2016.