Recent clinical studies have indicated that grapefruit juice (GFJ) improves insulin resistance and reduces weight gain in humans. The effect of GFJ on glucose tolerance and metformin-induced lactic acidosis in normal, non-diabetic in rats is hereby investigated. Three groups (A, B, C) of 20 male Wistar rats each, were treated with stepwise, escalated oral doses of 0, 1.0, 2.0, 3.0 (group A), and 3.0 ml/kg body weight (groups B and C) of GFJ. Group C rats additionally received 250 mg/kg body weight of metformin. All the animals were sacrificed after 14 days of treatment. Fasting blood glucose levels were significantly (P < 0.0001) lower in GFJ-treated test (2.9 +/- 0.4 mmol/L) compared with control (3.7 +/- 0.39 mmol/L) rats, but 1.5-hr plasma insulin levels were similar. GFJ alone or in combination with metformin, significantly (P < 0.05) lowered blood glucose levels compared with control animals. Blood lactic acid levels were similar in GFJ-treated test (2.81 +/- 1.4 mmol/L) and control (2.54 +/- 0.7 mmol/L) rats, respectively, but were significantly increased (P = 0.0079) in rats that were treated with either metformin alone (5.38 +/- 2.53 mmol/L) or in combination with GFJ (8.31 +/- 3.48 mmol/L). Metformin concentration in liver tissue was significantly higher (P < 0.05) in GFJ-treated (397 +/- 19 microg/g) than in control (280 +/- 15 microg/g) rats, respectively. Plasma metformin levels were comparable between the control (95 +/- 8.1 microg/ml) and GFJ-treated test (108 +/- 20 microg/ml) rats, respectively. Liver tissue metformin concentrations and plasma lactic acid levels showed significant correlation in both control (P = 0.0122; r(2) = 0.9080) and GFJ-treated test rats (P = 0.0005; r(2) = 0.9893). Although GFJ may be beneficial to diabetic patients, it may exacerbate lactic acidosis in diabetic patients taking metformin concurrently.
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