Acute effects of fructose and glucose ingestion with and without caffeine in young and old humans.

Aging is associated with a decline in energy expenditure (EE), glucose intolerance, and a reduction in body nitrogen content. In addition, a reduction in the thermic response to glucose but not to fructose or protein has been reported in the elderly. The present study was conducted to further examine nutrient-induced thermogenesis and the effects of specific sugars on amino acid metabolism in relation to age. After 3 days on a weight-maintaining, 250-g carbohydrate diet, 16 healthy non-obese men and women in two age groups (18 to 29 and 66 to 80 years) consumed on 4 different days 500 mL of either a 75-g fructose or 75-g glucose solution, with or without 300 mg caffeine or vitamin C as a placebo. Blood substrate and hormone levels and EE, using indirect calorimetry, were measured at timed intervals for 3 hours after consumption of the drinks. There was no difference in the carbohydrate-induced increase in EE in either young or old even after adjustments for body weight and fat-free mass (FFM). An approximately 20-fold increase in serum caffeine levels increased EE in both groups (P < .003), but had minimal effects on substrate and hormone responses. In contrast to glucose, fructose induced a marked elevation in plasma alanine from combined basal levels of 301 +/- 24 to approximately 500 +/- 18 mumol/L (mean +/- SEM) in both groups (P < .001). However, both fructose and glucose ingestion resulted in a similar decline in branched-chain and aromatic amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)