Glycemic and insulin response to popular protein supplements

Nutrition Research Newsletter, Jan, 2005

Carbohydrate, the primary fuel source for the body, is necessary to support prolonged physical activity. Intake of carbohydrate during exercise enhances endurance exercise performance, however, research evaluating pre-exercise carbohydrate feedings has shown that elevated insulin levels in response to the carbohydrate causes declines in blood glucose and inhibits lipid mobilization during subsequent exercise in some populations. Additionally, protein consumption has also been shown to elicit increases in plasma insulin concentrations.

In an attempt to support or improve exercise performance, many individuals choose to supplement their diets with nutritional bars and drinks. However, a protein nutritional bar or drink containing high amounts of protein and little carbohydrate would not provide fuel for exercise. Additionally, the lack or carbohydrate and the potential insulin response may impair exercise performance. Currently only limited data are available regarding the glycemic and insulinemic responses to common servings of many food supplements, such as protein nutritional bars and drinks. Therefore, researchers have valuated the glucose and insulin response to a 50-g glucose feeding, a high-glycemic-index food (white bagel) and a known low-glycemic-index food (peanuts), and a solid (protein bar) and a liquid protein (protein drink) supplement.

Twelve healthy, nondiabeteic male subjects were recruited from a university population. On the first visit to the laboratory, the subject's height, weight, and body composition were measured. Body composition was measured using air-displacement plethysmography. The subjects reported to the laboratory on five separate occasions in a 12 hr fasting state. The subjects were instructed to keep a detailed 24 hr dietary record and to repeat this diet before each of the successive experimental trials. A resting blood sample was drawn. After the first blood sample, the subject consumed the prescribed feeding. Subsequent blood samples were drawn at 10, 20, 30, 40, 50, 60, 75, 0, and 120 minutes. Blood samples were analyzed for glucose and insulin concentrations. Total energy content for glucose in the glucose drink, white bagel, peanuts, protein bar, and protein drink were 200, 240, 240, 250, and 22 kcal, respectively.

The area under the curve for glucose was greater in the glucose drink group vs. all treatment groups except the white bagel group) P < .05). At 20 to 40 minutes, plasma glucose was elevated in the glucose drink group vs. the peanuts group, the protein bar group, and the protein drink group (P < .05). The glycemic response was greater in the glucose drink group vs. the white bagel group at 30 min (8.1 [ or -] 0.5 vs. 6.5 [ or -] 0.3 mmol/L, respectively) (P < .05). The area under the curve for insulin was lower in the peanuts group vs. all treatment groups (P < .05). Insulin concentrations peaked at 40 min in the glucose drink group (285.5 [ or -] 18.3 pmol) and was similar in all but the peanuts group (130.5 [ or -] 14.3 pmol) (P < .05).

This study shows that a common serving of a commercially available protein supplement resulted in a marked insulin response with no glycemic response because of the lack of carbohydrate content. Consumers need to be educated that the way these products are marketed could be misleading.

A. Parcell, M. Drummond, D. Christopherson, et al. Glycemic and insulinemic responses to protein supplements. J Am Diet Assoc; 104:1800-1804 (December, 2004). [Correspondence: Allen C. Parcell, PhD, Brigham Young University, Human Performance Research Center, 120-E Richards Building, Provo, UT 84602. E-mail: allen_parcell@byu.edu].