Diabetes numbers AFTER eating - what is ok and what is not?

When you have type 2 diabetes, your ability to maintain normal blood sugar (glucose) levels is reduced – either because you do not produce enough insulin to let glucose into cells or because cells no longer respond to its effects (insulin resistance). As a result, glucose cannot leave the blood stream to enter muscle or fat cells where it is burned as fuel or converted into energy stores for future use, and instead builds up in the circulation.

Diabetes is diagnosed when you have persistently raised blood glucose levels (hyperglycemia) so that, after a period of fasting your blood glucose level is 7.0 mmol/L (126mg/dl) or more. Fasting tests are used because, after eating, your circulating glucose levels can rise quite high especially if you eat carbohydrate-rich foods. The way that a food affects your blood glucose levels after eating is known as its glycemic index.

Scientists now know that the rise in blood glucose that occurs an hour or two after eating – known as postprandial hyperglycemia – is particularly harmful. These rapid sugar spikes attack the lining of arteries and small blood vessels to hasten hardening and furring up of the arteries (atherosclerosis). As a result, postprandial hyperglycemia is linked with an increased risk of diabetes complications such as heart attack and stroke, as well as damage to the eyes (which can lead to blindness), kidneys (which can lead to kidney failure) and peripheral circulation (which can lead to leg ulcers and erectile dysfunction for example).

Even if you have good diabetes control and your hemoglobin A1c (HbA1c) and fasting glucose levels are at target levels, you can still have postprandial hyperglycemia. In one study involving 3,284 people with type 2 diabetes, a post-prandial blood glucose value of greater than 8.89 mmol/l (160mg/dl) was found at least once in 84% of those taking part. This study suggested that factors which increased the likelihood of having high glucose levels after eating were older age, a longer duration of diabetes, having raised blood fat levels, high blood pressure and not being obese. Taking sulfonylurea medications (eg glibenclamide, glipizide, glimepiride) was also associated with greater glucose swings after meals. 

So what is an acceptable blood glucose level after eating? When diagnosing impaired glucose tolerance you may be asked to undergo a 2-hour oral glucose tolerance test if the result of your fasting blood test is borderline (for example, between 6.1 mmol/L – 6.9 mmol/L) to see what happens to your blood glucose level after consuming a known amount of glucose.  

Oral Glucose Tolerance Test

An OGTT is normally carried out first thing in the morning after you’ve not eaten for between 8 to 14 hours) – this ensures your blood glucose levels are not affected by a recent meal although you may drink water. First, a base-line fasting blood sugar level is taken, then you are asked to drink a solution containing 75g glucose dissolved in 250-300ml water, which must be consumed within 5 minutes. You then remain reasonably inactive for two hours, without smoking, and a further blood sample is taken exactly two hours after the glucose drink to measure your blood glucose level. 

In someone without diabetes, blood sugar levels normally go no higher than 7.8 mmol/L (140 mg/dL) two hours after having the drink. 

If someone’s two hour blood glucose level is between 7.8 mmol/L (140 mg/dL) and 11.1 mmol/L (199.8mg/dL), they have impaired glucose tolerance and are at increased risk of developing diabetes in the future although diet and lifestyle changes can help to avoid this.

Diabetes is diagnosed if your blood glucose level is 11.1 mmol/L (199.8 mg/dL) or higher two hours after drinking the glucose solution.

These blood sugar cut-off points are based on a large body of research which shows that damage to the body starts to happen in the early stages of type 2 diabetes, even when fasting glucose and HbA1c are within normal ranges. Diabetes studies have even shown that 1- or 2-hour levels during a glucose tolerance test were better predictors of cardiovascular risk (of heart attack or stroke) than either fasting glucose or HbA1c levels.

This ‘silent’ postprandial hyperglycemia is a causes macrovascular complications such as myocardial infarction or stroke as well as microvascular complications at these levels so that damage due to hyperglycemia has already begun. It’s therefore important to reduce blood spikes after eating as much as possible.

A good general goal is to maintain a blood sugar level of less than 10 mmol/L (180 mg/dL) in the one to two hours after a meal. Your doctor can tell you the best number for you to aim for.

How postprandial hyperglycemia damages the circulation

Glucose is a highly reactive molecule that attacks and attaches itself to proteins in the circulation. For example, glucose can react with the red blood pigment, hemoglobin, to form glycosylated (or glycated) hemoglobin which is why levels of HbA1c are used to assess how good your glucose control was over the previous 6 to 12 weeks. The normal range for HbA1c is 4% - 6%. For someone with type 2 diabetes and good glucose control, the level of HbA1c should ideally be less than 6.5%. Minimising your postprandial glucose levels will help to improve HbA1c – important because every percentage point above normal increases the chance of developing complications such as eye or kidney problems by as much as 20%. 

By attacking blood vessel walls, postprandial hyperglycemia triggers inflammation and greatly increases the production of chemicals known as advanced glycation end products or AGEs – an appropriate acronym as these chemicals are linked with premature ageing. These AGEs and oxidised fats (lipid peroxidation products) in turn affect the production of nitric oxide (NO) which is needed for blood vessels to dilate. As a result, arteries are less able to dilate when needed which contributes to a rise in blood pressure.   

Overcoming morning spikes in glucose levels

Some people find their glucose levels are high in the morning even before eating and they rise even higher after breakfast. Having a slightly high blood sugar level in the morning is known as the dawn phenomenon and this may be pronounced in as many as one in two people with diabetes. 

The dawn phenomenon is due to our natural biorhythms in which production of insulin hormone is suppressed during sleep, and levels of other hormones (growth hormone, glucagon and cortisol) rise which stimulates the liver to make new glucose. For people without prediabetes (metabolic syndrome) or diabetes, the rise in glucose is minimal as insulin is quickly released to ensure glucose levels stay within normal limits. For those with diabetes, insulin production is either reduced or cells no longer respond to it properly, so that blood glucose levels stay higher than normal. Also, in some people with type 2 diabetes, the liver doesn’t respond to signals telling it there is plenty of sugar within the circulation and it continues to churn out new glucose.  

If your glucose levels are raised in the morning it is still important to eat a healthy breakfast – don’t skip this important meal. Select foods that have minimal impact on blood glucose levels, and talk to your doctor to see if your treatment needs adjusting. 

How to reduce postprandial hyperglycemia

  • Eat little and often – six small meals a day is better than having three large ones
  • Select foods with a lower glycemic index such as those found in the Mediterranean diet beans, wholegrains, vegetables, fruit, lean meat, fish, nuts, seeds, olive oil, eggs and dairy products
  • Avoid white potatoes, white bread, white rice, sweetened breakfast cereals, biscuits, cakes and other foods containing white flour and/or sugar
  • Drink a glass of water with each meal to help you feel full and eat less overall
  • Stay active for at least 20 minutes after eating – go for a walk or at least avoid sitting or lying down after a meal