Blood sugar

Blood sugar is the concentration of glucose in the blood – your body keeps it within a narrow range because the brain and muscles constantly need fresh supplies. The brain alone uses around 120 grams of glucose a day, about a fifth of your total energy requirement.

It is measured in milligrams per decilitre (mg/dl) or millimoles per litre (mmol/l); the conversion factor is 18. According to the German Diabetes Society (DDG), fasting values – that is, after at least eight hours without food – below 100 mg/dl (5.6 mmol/l), measured in a laboratory, are considered normal. Between 100 and 125 mg/dl is referred to as impaired fasting glucose (prediabetes); 126 mg/dl or above on two different days means diabetes. After a meal, the level rises temporarily and should be back below 140 mg/dl after two hours.

Two hormones from the pancreas maintain the balance: insulin lowers blood sugar by channelling glucose into the cells, while glucagon releases sugar from the liver's stores when needed. How quickly your blood sugar rises after a meal depends on what's in it: a glass of juice sends it up quickly, a slice of wholegrain bread with cheese much more slowly, because fibre, protein and fat slow down absorption.

In everyday life, this means: if you combine carbohydrates with fibre and protein and go for a short walk after eating, you noticeably smooth out your blood sugar curve. That helps against cravings and the afternoon slump, because working muscles take up glucose even without insulin.

Shakiness, sweating, sudden ravenous hunger and difficulty concentrating can point to low blood sugar (hypoglycaemia, below 70 mg/dl) – rare in healthy people, but important to know about for anyone taking diabetes medication. Conversely, intense thirst, frequent urination and tiredness are classic signs of persistently high blood sugar. In that case, it's worth having a blood test at your GP's.