Caffeine and insulin response in obese subjects

Nutrition Research Newsletter, August, 2004

Obesity is associated with impaired glucose tolerance (IGT) and with type 2 diabetes. Both IGT and type 2 diabetes include hyperinsulinemia and insulin resistance. Researchers have associated the metabolic characteristics of these problems--elevated basal insulin, elevated insulin response to glucose intake and a decrease in glucose uptake in the peripheral tissues--with visceral adiposity. Lifestyle interventions, including caloric restriction and exercise, can improve insulin and glucose control in obese individuals.

The influence of caffeine on glucose tolerance and insulin sensitivity is currently unknown. In lean subjects, caffeine has been shown to result in a greater insulin response and no decrease in the glucose response. Skeletal muscle is the major tissue that becomes insulin-insensitive following caffeine ingestion, but the mechanism is unclear.

Investigators examined the acute effects of caffeine ingestion on glucose and insulin homeostasis in obese subjects before and after a nutrition and exercise intervention. Subjects included young, obese, sedentary individuals with no history of disease, including diabetes mellitus. During a 2-week baseline period, a nutritionist analyzed the subjects' usual diets. Measurements of waist and hip circumference, height, and body weight were made, and percentage body fat was determined using dual energy X-ray absorptiometry. Maximal oxygen consumption (V[O.sub.2] max) was determined while on a treadmill. Two oral glucose tolerance tests (OGTTs) were conducted.

The nutrition and exercise intervention lasted 12 weeks. Daring this period, the subjects were provided with an individualized target of a daily energy decrement of 300 kcal, with 20% to 30% of their energy intake from fat. They were instructed to refrain from caffeine in any from during this period. At the conclusion of the 12-week intervention, the subjects underwent dual energy X-ray absorptiometry, V[O.sub.2] max testing and two OGTTs (caffeine compared with placebo).

The intervention resulted in decreases (P d" 0.05) in body weight (8.5 [ or -] 1.5 kg), percentage body fat (2.8 [ or -] 0.7%), and fasting glucose, insulin, and proinsulin concentrations; and increases in the insulin sensitivity index for the placebo OGTT (P d" 0.05). Caffeine caused a greater (P d" 0.05) OGTT insulin response and a lower (P d" 0.05) ISI both before and after weight loss. The proinsulin-insulin ratio indicated that neither weight loss nor caffeine affected the nature of the beta cell secretion of insulin.

While a nutrition and exercise intervention improved insulin-glucose homeostasis in obese men, caffeine ingestion impaired the insulin-glucose homeostasis. It appears that caffeine ingestion contributes to insulin resistance.

H. Petrie, S. Chown, L. Belfie, et al. Caffeine ingestion increases the insulin response to an oral-glucose-tolerance test in obese men before and after weight loss. Am J Clin Nutr;80:22-28 (July, 2004). [Correspondence: TEG, Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada NIG 2W1. E-mail: terrygra@uoguelph.ca].