Exercise Intensity and Diabetes: HIIT Me?

by Sam Knox

Current standards for the treatment of Type 2 diabetes call for at least 150 minutes per week of moderate-intensity physical activity in addition to at least two sessions per week of resistance training.

High-intensity interval training (HIIT) is an alternative to continuous aerobic exercise that alternates brief periods of all-out effort with periods of rest.  The length of both the sprint and rest periods can vary, but the most common protocol is based on the “Wingate Test”, which is essentially a 30-second sprint otherwise used to measure anaerobic capacity.  A typical interval-training session might consist of 30-second sprints followed by 4-minute rest periods repeated 4-6 times.

A recent study involving Type 2 diabetics offers evidence that low-volume HIIT can provide the same long-term benefits as prolonged, moderate-intensity aerobic exercise at a considerable savings in time.

Don’t sign-up for that spinning class just yet, though.

The emphasis on prolonged, moderate-intensity activities is not without merit. The greatest risk for diabetics engaged in moderate exercise is a gradual decline in blood sugar that can result in hypoglycemia.  Vigorous exercise, on the other hand, causes transient hyperglycemia in diabetics and non-diabetics alike.

In healthy individuals, blood sugar rises to ~135 mg/dL after 15 minutes of vigorous exercise, still within the safe range, and returns to fasting levels within one hour.  In Type 1 diabetics, the same intensity and duration of exercise produced blood sugars that exceeded 150 mg/dL in spite of continuous insulin infusion and returned to fasting levels in two hours only by doubling the amount of insulin infused.

There is very little information available about the effect on blood sugar in Type 2 diabetics during and immediately after vigorous exercise.  Another study that measured the effects of HIIT claims that “glucose homeostasis” was maintained, but even if it could be shown that blood sugar remained within safe limits on average, it doesn’t necessarily follow that an individual would achieve the same result.

So, while it’s too soon to conclude that high-intensity interval training can safely replace prolonged moderate-intensity exercise for Type 2 diabetics, the known effects on blood sugar of vigorous exercise raise the possibility that carefully controlled high-intensity intervals might reduce the risk of hypoglycemia.  In one study, for example, interval training reduced the decline in blood sugar in a small group of Type 1 diabetics, and another study with a similar group showed that a 10 second sprint at the end of an exercise session could help maintain blood sugar post-exercise.

Notes:

“Current standards….”:  ADA Standards of Medical Care in Diabetes – 2012

“…moderate-intensity…”:  64-76% MHR, 46-63% VO2max (ACSM)

“A recent study…”:  Low-volume high-intensity interval training reduces hyperglycemia and increases muscle mitochondrial capacity in patients with type 2 diabetes.

“Vigorous exercise…”:  77-93% MHR, 64-86% VO2max (ACSM)

“…hyperglycemia…”:  The smart money, apparently, is on “catecholamines” (adrenalin, noradrenalin) as the culprits.  See “Intense Exercise Has Unique Effects…” below.

“In healthy individuals…”:  Intense Exercise Has Unique Effects on Both Insulin Release and Its Roles in Glucoregulation, Implications for Diabetes

“…still within the safe range.”:  Less than 140 mg/dL.  ADA Standards of Medical Care in Diabetes – 2012, American Association of Clinical Endocrinologists

“Another study…”:  The effect of intense exercise on postprandial glucose homeostasis in type II diabetic patients.

“In one study…”:  The decline in blood glucose levels is less with intermittent high-intensity compared with moderate exercise in individuals with type 1 diabetes.

“…and another study…”:  The 10-s maximal sprint:  a novel approach to counter an exercise-mediated fall in glycemia in individuals with type 1 diabetes.