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Most of the metabolic effects of fructose are due to its rapid utilization by the liver and it by-passing the phosphofructokinase regulatory step in glycolysis, leading to far reaching consequences to carbohydrate and lipid metabolism. These consequences include immediate hepatic increases in pyruvate and lactate production, activation of pyruvate dehydrogenase, and a shift in balance from oxidation to esterification of nonesterified fatty acids, resulting in increased secretion of very-low-density-lipoprotein (VLDL). These effects are augmented by long-term absorption of fructose, which causes enzyme adaptations that increase lipogenesis and VLDL secretion, leading to triglyceridemia, decreased glucose tolerance, and hyperinsulinemia. Acute loading of the liver with fructose causes sequestration of inorganic phosphate in fructose-1-phosphate and diminished ATP synthesis. Consequently, the inhibition by ATP of the enzymes of adenine nucleotide degradation is removed and uric acid formation accelerates with consequent hyperuricemia. These effects are of particular significance to potentially hypertriglyceridemic or hyperuricemic individuals.

About 10-15% of all urinary stones are uric acid stones. High uric acid excretion, low urinary volume, and pH value have been suggested to be the most important risk factors for the formation of uric acid stones. In the present study, we examined the effect of 3 diets with different purine content on the risk of uric acid stone formation. Ten healthy males received 3 different standard diets for 5 d each. Diet 1 corresponded to the usual German dietary habits of healthy men aged 19-35 y. Characteristic, therefore, is high intake of energy, alcohol, and purine, excessive protein intake, and very low fluid intake (1.5 L/d). On the contrary, diet 2 (mixed diet) and diet 3 (lactoovovegetarian diet) were established according to the dietary recommendations of the German Society of Nutrition. Diet 2 and diet 3 were isoenergetic with equal amounts of the main nutrients and a fluid intake of 2.5 L/d. In the course of the test period, 24-h urine samples were collected. Urinary supersaturation with uric acid was estimated by the computer program EQUIL. The risk of uric acid stone formation was highest on diet 1 due to the low urinary volume, low pH, and the high urinary uric acid excretion. On diet 2, relative supersaturation with uric acid was significantly reduced by 85%, consequent to the decrease of uric acid excretion and concentration and the increase of urinary pH value. The intake of diet 3 led to a further significant decline in the risk of uric acid stone formation by 55% compared with diet 2. These results indicate that intake of a balanced lactoovovegetarian diet with low animal protein and purine content and high fluid intake leads to the lowest risk of uric acid stone formation.