By Sheri Colberg, PhD
Insulin action is affected by many different factors, but arguably the one that may have the biggest impact in the case of diabetes and management of hyperglycemia is the amount of glycogen that is already stored in skeletal muscles. The body can only store excess carbohydrates in the voluntary muscles and the liver, and the capacity of both is extremely limited (i.e., about 400-500 grams in muscle, depending on the amount of muscle mass, and about a quarter of that amount in the liver). If someone is physically inactive and eats a carbohydrate-rich diet, there are few places to store the excess intake of carbohydrates except as fat in various tissues around the body, including adipose, muscle, liver, and more. An improved ability to oxidize fat helps restore metabolic balance.
The question remains, then, whether it is best to reduce muscular glycogen content and enhance fat oxidation using manipulations of diet, exercise, or both. The answer is not fully clear at this time. By way of example, a recent article by Sartor et al. in the European Journal of Applied Physiology (1) examined whether a carbohydrate (CHO)-reduced diet combined with high-intensity interval training (HIIT) enhances the beneficial effects of the diet alone on insulin sensitivity and fat oxidation in obese individuals. In that study, 19 obese subjects underwent 14 days of eating a CHO-reduced and energy-restricted diet. About half of them (10) combined the diet with HIIT, which consisted of four minute bouts done at 90% of each subject’s VO2peak up to 10 times, 3 times a week. The main findings of this study were that two weeks of a CHO-reduced diet improved insulin action (measured via oral glucose tolerance) and fat oxidation, but that energy-balanced HIIT did not have any additional effect beyond increasing aerobic fitness and preserving lean body mass.
Changing the composition of the diet has also been investigated as an alternative to simply lowering carbohydrate intake. In a study published in the American Journal of Clinical Nutrition in 2009 by Solomon et al. (2), the effects of combining a 7-day low-glycemic index (low-GI) diet and exercise training intervention on insulin sensitivity were examined in older, obese adults. The exercise was more moderate than in the Sartor et al. study, in this case consisting of 60 minutes per day undertaken at 80-85% maximum heart rate, and it was done in combination with either a low-GI (averaging ~40 on the GI scale) or high-GI (~80 GI) diet isocaloric to individual energy requirements. Insulin sensitivity and hepatic glucose output were assessed with an insulin clamp technique in that study. Their major findings were that either GI diet equally improved insulin action, insulin suppression of hepatic glucose production, and post-absorptive fat oxidation, suggesting that the metabolic improvements after short-term exercise training in these subjects reflect increased physical activity and are not affected by a low-GI diet.
Finally, to round out this discussion, let’s take a look at a study by Shah and colleagues (3) that investigated the effects of diet and diet in conjunction with exercise on intrahepatic fat (IHF) content in obese, older adults. It is well known that both obesity and aging increase IHF content, which leads to nonalcoholic fatty liver disease and insulin resistance. In that study, the researchers evaluated 18 obese, older (65 years and older) adults who undertook six months of dieting or diet + aerobic exercise. The liver fat content decreased a similar amount in both groups, accompanied by comparable improvements in insulin sensitivity. Diet with or without exercise in that study resulted in the same outcome, although the addition of exercise improved physical function and other obesity- and aging-related metabolic abnormalities.
What is the best advice for your patients then? First and foremost, it appears that in the short term, having them cut back on their total carbohydrate intake (regardless of the GI) can improve their insulin action. Exercise is known to have a short-term effect on insulin action, lasting no more than 2-72 hours, so physical activity must be done regularly to have a similar, positive impact on insulin action and fat oxidation. Having them do a combination of both likely bestows the best all-around benefit. Surprised? I’m not!
(1) Sartor F, de Morree HM, Matschke V, Marcora SM, Milousis A, Thom JM, Kubis HP. High-intensity exercise and carbohydrate-reduced energy-restricted diet in obese individuals. Eur J Appl Physiol. 2010 Jul 14 [Epub ahead of print]
(2) Solomon TP, Haus JM, Kelly KR, Cook MD, Riccardi M, Rocco M, Kashyap SR, Barkoukis H, Kirwan JP. Randomized trial on the effects of a 7-d low-glycemic diet and exercise intervention on insulin resistance in older obese humans. Am J Clin Nutr. 2009;90(5):1222-9.
(3) Shah K, Stufflebam A, Hilton TN, Sinacore DR, Klein S, Villareal DT. Diet and exercise interventions reduce intrahepatic fat content and improve insulin sensitivity in obese older adults. Obesity (Silver Spring). 2009;17(12):2162-8.