Postprandial glucose levels are the levels of blood glucose after meals. In Western urban environments, the main contributors to elevated postprandial glucose are foods rich in refined carbohydrates and sugars. While postprandial glucose levels may vary somewhat erratically, they are particularly elevated in the morning after breakfast. The main reason for this is that breakfast, in Western urban environments, is typically very high in refined carbohydrates and sugars.
HbA1c, or glycated hemoglobin, is a measure of average blood glucose over a period of a few months. Blood glucose glycates (i.e., sticks to) hemoglobin, a protein found in red blood cells. Red blood cells are relatively long-lived, lasting approximately 3 months. Thus HbA1c (given in percentages) is a good indicator of average blood glucose levels, if you don’t suffer from anemia or a few other blood abnormalities.
Based on HbA1c, one can then estimate his or her average blood glucose level for the previous 3 months or so before the test, using one of the following equations, depending on whether the measurement is in mg/dl or mmol/l.
Average blood glucose (mg/dl) = 28.7 × HbA1c − 46.7
Average blood glucose (mmol/l) = 1.59 × HbA1c − 2.59
Elevated blood glucose levels cause damage in the body primarily through glycation, which leads to the formation of advanced glycation endproducts (AGEs). Given this, HbA1c can be seen as a proxy for the level of damage done by elevated blood glucose levels to various body tissues. This damage occurs over time; often after many years of high blood glucose levels. It includes kidney damage, neurological damage, cardiovascular damage, and damage to the retina.
Most regular blood exams focus on fasting blood glucose as a measure of glucose metabolism status. Many medical practitioners have as a target a fasting blood glucose level of 125 mg/dl (7 mmol/l) or less, and largely disregard postprandial glucose levels or HbA1c in their management of glucose metabolism. Leiter and colleagues (2005; full reference at the end of this post) showed that this focus on fasting blood glucose is a mistake. They are not alone; many others made this point, including some very knowledgeable bloggers who focus on diabetes (see “Interesting links” section of this blog). Leiter and colleagues (2005) also provided some interesting graphs and figures, including eye-opening correlations between various variables and arterial stiffness. The figure below (click to enlarge) shows the contribution of postprandial glucose to HbA1c.
Note that the lower the HbA1c is in the figure (horizontal axis), the higher is the postprandial glucose contribution to HbA1c. And, the lower the HbA1c, the closer the individuals are to what one could consider having a perfectly normal HbA1c level (around 5 percent). That is, only for individuals whose HbA1c levels are very high, fasting blood glucose levels are relatively reliable measures of the tissue damage done be elevated blood glucose levels.
The table below (click to enlarge) shows P values associated with the impact of various variables (listed on the leftmost column) on arterial stiffness. This measure, arterial stiffness, is strongly associated with an increased risk of cardiovascular events. Look at the middle column showing P values adjusted for age and height. The lower the P value, the more a variable affects arterial stiffness. The variable with the lowest P value by far is 2-hour postprandial blood glucose; the blood glucose levels measured 2 hours after meals.
Fasting glucose levels were reported to be statistically insignificant because of the P = 0.049, in terms of their effect on arterial stiffness, but this P value is actually significant, although barely, at the 0.05 level (95 percent confidence). Interestingly, the following measures are not even on the radar screen, as far as arterial stiffness is concerned: systolic blood pressure, LDL cholesterol, HDL cholesterol, triglycerides, and fasting insulin levels.
What about the lipid hypothesis, and the “bad” LDL cholesterol!? This study is telling us that these are not very relevant for arterial stiffness when we control for the effect of blood glucose measures. Not even fasting insulin levels matters much! Wait, not even HDL!!! A high HDL has been definitely shown to be protective, but when we look at the relative magnitude of various effects, the story is a bit different. A high HDL’s protective effect exists, but it is dwarfed by the negative effect of high blood glucose levels, especially after meals, in the context of cardiovascular disease.
What all this points at is what we could call a postprandial glucose hypothesis: Lower your postprandial glucose levels, and live a longer, healthier life! And, by the way, if your postprandial glucose levels are under control, lipids do not matter much! Or maybe your lipids will fall into place, without any need for statin drugs, after your postprandial glucose levels are under control. One way or another, the outcome will be a positive one. That is what the data from this study is telling us.
How do you lower your postprandial glucose levels?
A good way to start is to remove foods rich in refined carbohydrates and sugars from your diet. Almost all of these are foods engineered by humans with the goal of being addictive; they usually come in boxes and brightly colored plastic wraps. They are not hard to miss. They are typically in the central aisles of supermarkets. The sooner you remove them from your diet, the better. The more completely you do this, the better.
Note that the evidence discussed in this post is in connection with blood glucose levels, not glucose metabolism per se. If you have impaired glucose metabolism (e.g., diabetes type 2), you can still avoid a lot of problems if you effectively control your blood glucose levels. You may have to be a bit more aggressive, adding low carbohydrate dieting (as in the Atkins or Optimal diets) to the removal of refined carbohydrates and sugars from your diet; the latter is in many ways similar to adopting a Paleolithic diet. You may have to take some drugs, such as Metformin (a.k.a. Glucophage). But you are certainly not doomed if you are diabetic.
Leiter, L.A., Ceriello, A., Davidson, J.A., Hanefeld, M., Monnier, L., Owens, D.R., Tajima, N., & Tuomilehto, J. (2005). Postprandial glucose regulation: New data and new implications. Clinical Therapeutics, 27(2), S42-S56.