Blood sugar after eating: what is normal and what the target is

It is the question I get most often now that glucose meters are sold in every pharmacy: "I tested after eating and got 168 — is that bad?". The short answer is "it depends whether you have diabetes", and the long one deserves this article, because there are three things almost everybody gets wrong.

The reference values

That 180 is a general, individualisable target: it is not a sacred line identical for everyone. A young person recently diagnosed may be given a stricter target by their doctor, and an older person with other conditions a more relaxed one. Yours is set by whoever manages your treatment.

The three mistakes almost everybody makes

1. Counting the time from when you finish eating

This is the most frequent one and the most confusing. The two hours are counted from the first mouthful, not from when you put your fork down.

It seems like a detail and it is not: if you take forty minutes to eat and start counting when you finish, you are testing at minute 160 instead of 120. By then the peak has already come down and your number looks better than it was. You are fooling yourself with the best intentions in the world.

2. Believing a high reading diagnoses diabetes

It does not. Glucose after a normal meal is not a diagnostic test, for a very simple reason: nobody knows what you ate or how much. A plate of rice and a chicken salad are not comparable, so the number cannot be read against any official table.

The test that does diagnose is the oral glucose tolerance test: you are given 75 grams of dissolved glucose, exactly the same for everyone, and measured at two hours. There the cut-offs do exist:

And there is one exception that is enough on its own: a random glucose of 200 or more, measured in a lab blood test, accompanied by typical symptoms — heavy thirst, passing a lot of urine, unintended weight loss — is enough for a diagnosis. If your meter shows it and you have those symptoms, see your doctor that same day.

3. Looking for a "prediabetes range" after eating

There is not one. You can search all you like: there is no official diagnostic range for prediabetes based on post-meal glucose after an ordinary meal. Prediabetes is defined by fasting glucose, by HbA1c or by the tolerance test, not by what your meter shows after dinner.

If your post-meal glucose worries you, the right conversation is not searching for a table online: it is asking for an HbA1c and a fasting glucose.

So what is testing after meals for?

For something very valuable, even if it is not diagnosis: learning how each meal affects you specifically.

Two people with the same HbA1c can respond very differently to the same plate of pasta. Measuring yourself is the only way to know which one you are. And that information does change decisions:

• Compare the same meal done two ways. The same rice, one day on its own and another preceded by salad and chicken. The device will show you the difference.
• Check whether the walk works. Test at two hours on a day you sit on the sofa and on a day you walk fifteen minutes. It is the most convincing demonstration you will ever see.
• Spot the silent culprits. The "natural" juice at breakfast, the afternoon beer, the white bread at dinner. Seeing it as a number convinces more than any talk from me.

How to lower the peak without giving up food

Four changes, ordered by what they give for what they cost:

1. The order of the plate. Vegetables and protein first, carbohydrate last. Same plate, same money, same time: only the order of the fork changes.
2. Ten minutes walking afterwards. Muscle takes the glucose away exactly when there is most of it circulating.
3. Carbohydrate whole, not blended. The apple instead of the juice, the whole potato instead of mash, real wholemeal bread instead of white.
4. Fat and protein alongside. Olive oil, nuts, egg, fish. They slow stomach emptying and flatten the rise.

None of the four forces you to give anything up. It is exactly the approach I use in the programme, and what I see in my practice is that this is why people stick with it.

When to seek help

Talk to your doctor — not to me and not to the internet — if you are consistently above 200 mg/dL after meals, if you notice heavy thirst, constant need to urinate or unintended weight loss, or if you take insulin or sulfonylureas and get hypoglycaemia.

And if your meter reads above 250–300 mg/dL and you also feel unwell (vomiting, stomach pain, fast breathing, drowsiness or confusion), do not wait for the appointment: go to A&E or call 112.

And if what you want is to understand your numbers and stop working blind, that part is my job.