Low blood sugar causes: what lies behind it, with and without diabetes, and what a scan can show

In this article you will find the answer to the question of what causes low blood sugar: why it happens, how you notice it and when you need a doctor. That matters, because the answer turns almost entirely on one question: whether you take diabetes medication. Low blood sugar is common in people who use insulin,⁠¹ and rare in people without diabetes.⁠² BioProfile’s metabolism scan gives you an indication of where you stand with your metabolism today. This is not a diagnosis. It is insight, to see where you stand today and to help you make better choices in your diet and lifestyle. The scan does not measure your blood sugar and does not point to a cause. Only a doctor can assess that.

What counts as low blood sugar

Blood sugar, or glucose, is measured in millimoles per litre (mmol/l). For people with type 2 diabetes, Thuisarts calls a value below 3.9 too low.⁠⁴ For adults without diabetes the threshold is lower: a plasma glucose of 3.0 or lower, while values of 3.1 to 3.9 sometimes call for further investigation, depending on the symptoms.⁠² In adults without diabetes, symptoms start on average at around 3.0.⁠⁵

Without diabetes, low blood sugar only counts when three things come together, known as Whipple’s triad: symptoms, a measured low glucose, and symptoms that go away as soon as your glucose is back to normal.⁠² For you, this means a shaky feeling is not enough to know for sure. Only a measurement taken while you have the symptoms can show it.

The signs Thuisarts shows are sweating, yawning, trembling hands, a pounding heart, dizziness, headache, tingling, confusion and sudden mood swings.⁠⁶

Causes of low blood sugar if you take diabetes medication

For people with type 2 diabetes, Thuisarts lists five causes: skipping a meal or eating too late, a lot of physical activity, too much diabetes medication or insulin, alcohol, and being ill with vomiting or diarrhoea.⁠⁴ The Thuisarts video adds taking medication at the wrong time, and names insulin injections and certain tablets as the medicines concerned.⁠⁶ Simply put, these are mostly everyday causes: a meal, a workout, a drink, a dose.

How often it happens was summarised in a review of 46 population studies covering 532,542 people with type 2 diabetes, many of which counted episodes over one year. Of the people on insulin, about half had mild or moderate episodes and about one in five had a severe one. Of the people on sulphonylureas, a group of diabetes tablets, a third had mild or moderate episodes and about one in twenty had a severe one.⁠¹ In short, insulin carries the higher risk, and certain tablets carry that risk too.

Dutch studies show the same picture. In Hoorn, 1,667 people with type 2 diabetes who use insulin were followed for a median of 1.9 years. In that time, just under half reported only mild episodes, and 7 per cent reported severe ones.⁠⁷ In a national Dutch cohort of 2,350 insulin users, just under a third reported a severe episode in the past year, and just under one in ten no longer clearly noticed the warning signs. More complex insulin regimens went hand in hand with both.⁠⁸ If you use insulin, this means low blood sugar is a known part of living with the medicine, and that it is worth talking to your GP about missing the warning signs.

Low blood sugar without diabetes

Without diabetes, low blood sugar is rare.⁠² Emergency calls for low blood sugar in people without diabetes were also rare.⁠⁵ A 2023 review names causes including alcohol, medicines, serious illness, hormone deficiencies, stomach surgery or gastric reduction surgery, insulin autoimmune syndrome, tumours and inherited metabolic disorders.⁠²

Alcohol is the best known of these. An amount of 48 grams of alcohol can lower the liver’s own glucose production by as much as 45 per cent, and low blood sugar was found in about a quarter of people with an alcohol use disorder who were admitted with alcoholic ketoacidosis.⁠⁵ Put simply: a lot of alcohol holds back the sugar your liver makes itself.

Stomach surgery is another cause, with low blood sugar after meals.⁠² After a gastric bypass or gastric sleeve, about one in three patients reported symptoms of low blood sugar.⁠⁵ When glucose was measured continuously, in 8 studies with 280 people, low blood sugar showed up in just over half of them, and at night in about one in six, with similar figures after both operations. The longer ago the operation was, the more often it was found, and the authors conclude that it happens more often than is currently assumed.⁠³ If you have had this kind of operation, this means that symptoms after a meal can still come from the operation years later. It is important to know that the study reported low readings from a continuous glucose monitor (CGM), and not necessarily hypoglycaemia with symptoms confirmed by a blood test. CGM has limitations at low values, and current guidelines do not recommend diagnosing hypoglycaemia after weight-loss stomach surgery with CGM alone.

An insulinoma, a tumour of the insulin-producing cells of the pancreas, is the most common cause of a body making too much insulin itself,⁠² but it is rare: fewer than 4 cases per million people per year. Low blood sugar caused by other tumours is even rarer, at about 1 per million.⁠⁵ These causes do exist, and that is why a measured low glucose without diabetes is something for a doctor. At the same time, they are rare.

When to get help

If you have type 2 diabetes and your reading is below 3.9, Thuisarts advises first taking 6 sugar cubes or glucose tablets, then 2 slices of bread with jam, and measuring again after 15 minutes, 1 hour and 2 hours. This applies if the person is otherwise conscious and responsive. Call your GP or the out-of-hours GP service if blood sugar stays below 3.9 after taking sugar, or if the person is not responding normally, is having trouble breathing, is confused, or keeps shaking and sweating. Call 112 if the person is unconscious.⁠⁴

If you do not have diabetes and you recognise the signs, a doctor can assess whether a measurement is needed, because only a low glucose measured at the moment of the symptoms counts.⁠²

What a metabolism scan can and cannot show

A metabolism scan gives you insight into where you stand with your metabolism today, and a baseline to compare against later. The scan does not measure glucose, and low blood sugar only counts when a low value has been measured,⁠² so the scan cannot tell you whether a dip was low blood sugar. The scan is not a diagnosis. If what you really have in mind is a metabolism that feels slow, the article on a slow metabolism explains the symptoms and what a scan measures.

In summary

This article showed that the cause of low blood sugar depends on whether you take diabetes medication. With insulin or certain tablets it is common, and the cause is an everyday one: a skipped or late meal, a lot of physical activity, too much medication, alcohol, or being ill.⁠^4,1 Without diabetes it is rare, and the cause ranges from alcohol and stomach surgery to rare tumours.⁠^2,5 It only counts when a low glucose has been measured, the signs and the moments to get help come from Thuisarts,⁠⁴ and a scan gives self-insight. The scan is not a diagnosis.