With an increase in focus on consuming carbohydrates in endurance athletes, an important question raised is whether or not the increase in carbohydrate intake can lead to insulin resistance or even type-2 diabetes. Let’s start with the basics:
Glucose enters your bloodstream in two main ways. First, your gut will break down carbohydrates from your food and turn them into glucose, releasing it into your bloodstream for your cells to use as energy. Second, the body stores glucose (called glycogen) in muscle and the liver. In cases of fasting or exercise, the body will turn glycogen into glucose and release it into the bloodstream. Glucose leaves the bloodstream through a hormone called insulin. You can think of insulin as a key that opens the locked doors of cells so that glucose can go into those cells and provide energy.
Having elevated resting blood glucose levels is called diabetes. There are two types of diabetes. Type-1, which is usually something you’re born with, is caused by the pancreas not making enough insulin. Type-2 diabetes is more lifestyle related, and happens when your body doesn’t respond normally to insulin. Think of this as if the key doesn’t fit in the locks of the doors very well. One of the benefits of exercise (particularly endurance exercise) is that it increases insulin sensitivity. It makes the locks work with the key very well. This is one of the reasons why exercise is often prescribed in the fight against type-2 diabetes.
There are several ways of measuring blood glucose levels to assess diabetes risk. The classic way has been to simply measure your immediate resting blood glucose level by analyzing your Fasting Plasma Glucose (or FPG). As the name implies, you fast, which decreases the short-term influence of recent glucose from food, and then measure the amount of glucose in your blood. If your FPG is high, it means that your cells aren’t letting glucose in, meaning the insulin isn’t working (or, in the case of type-1 diabetes, it means you don’t have any insulin). Recently, Hemoglobin A1c (HbA1c) is used more frequently to test blood glucose levels because instead of a snapshot in time, like FPG, HbA1c is thought of as a 60- to 90-day average of your blood glucose.
The big issue that I’m writing about today is HbA1c in athletes. The use of FPG and HbA1c are common and useful ways of measuring type-2 diabetes risk, but they aren’t perfect. .
Studies (linked below) have shown that HbA1c is actually elevated in athletes. At first, it can be tempting to see this as “it’s because of all the carbs we eat, and it’s bad.” And while yes, probably the first part is partially true (the carbs), the evidence suggests that the second part (that it’s bad) isn’t true.
Seeing a high HbA1c in athletes doesn’t at first make sense, because we know that exercise increases insulin sensitivity, and HbA1c is typically thought of as a measure of insulin sensitivity. What’s doubly interesting is that when these same athletes were measured for many other markers of insulin sensitivity, all the other markers came back as better in athletes than in sedentary people, with HbA1c being the only marker that was worse in athletes than in sedentary people.
The reason for this is that HbA1c isn’t a measure of average resting glucose over 60- to 90- days, it’s an average of the glucose over the entire time period, both resting and exercise. This means that even if your resting levels are quite low (which it is in athletes), HbA1c can show high because highly trained athletes are able to release and use a LOT more glucose during exercise than the average person, in addition to having larger amounts of glycogen (stored glucose).
Let’s break that down. During exercise, we burn a lot of glucose. The more fit you are, the more glucose you burn per unit of time (to go faster, you need to burn more fuel). That glucose comes from both blood sugar and from breaking down glycogen. Since your body has adapted to burn more glucose, it’s also adapted to RELEASE more glucose into the bloodstream, resulting in short-term elevated blood glucose levels in order to fuel the exercise. Because HbA1c is an average of the blood glucose levels over time, this short-term increase in glucose flux during exercise makes it look like the athlete is walking around with elevated resting levels (the dangerous part), when, because of the increase in insulin sensitivity and glucose storage, they’re actually walking around with LOWER levels of blood sugar outside of exercise.
In short, don’t trust an isolated high HbA1c reading if you’re an athlete. I wouldn’t ignore it, but make sure you’re getting some other testing too, like FBP or HOMA-IR, as a high HbA1c might not be indicative of an actual problem.
Citations:
https://www.cdc.gov/diabetes/basics/type2.html
https://www.heart.org/en/health-topics/diabetes/diabetes-complications-and-risks
https://www.webmd.com/diabetes/glycated-hemoglobin-test-hba1c
Glycemic Control in Athletes: Lippi
Irisin: A Potential Link between Physical Exercise and Metabolism—An Observational Study in Differently Trained Subjects, from Elite Athletes to Sedentary People: Benidini
