Lycopene is one of the major carotenoids in Western diets and is found almost exclusively in tomatoes and tomato products. It accounts for about 50% of carotenoids in human serum. Among the common dietary carotenoids lycopene has the highest singlet oxygen quenching capacity in vitro. Other outstanding features are its high concentration in testes, adrenal gland and prostate. In contrast to other carotenoids its serum values are not regularly reduced by smoking or alcohol consumption but by increasing age. Remarkable inverse relationships between lycopene intake or serum values and risk have been observed in particular for cancers of the prostate, pancreas and to a certain extent of the stomach. In some of the studies lycopene was the only carotenoid associated with risk reduction. Its role in cancer risk reduction still needs to be clarified. Patients with HIV infection, inflammatory diseases and hyperlipidemia with and without lipid lowering treatment may have depleted lycopene serum concentrations. Before embarking on large-scale human trials the distribution of lycopene and its biological functions need to be further evaluated.

The contribution of vitamin C (ascorbic acid) to the prevention of iron deficiency anemia by promoting the absorption of dietary non-heme iron-especially in persons with low iron stores--is well established. But the question has been raised whether high-dose intakes of vitamin C might unduly enhance the absorption of dietary iron in persons with high iron stores or in patients with iron overload, possibly increasing the potential risk of iron toxicity. Extensive studies have shown that overall the uptake and storage of iron in humans is efficiently controlled by a network of regulatory mechanisms. Even high vitamin C intakes do not cause iron imbalance in healthy persons and probably in persons who are heterozygous for hemochromatosis. The uptake, renal tubular reabsorption and storage of vitamin C itself are also strictly limited after high-dose intake so that no excessive plasma and tissue concentrations of vitamin C are produced. The effect of high-dose vitamin C on iron absorption in patients with iron overload due to homozygous hemochromatosis has not been studied. Of special importance is the early identification of hemochromatosis patients, which is assisted by the newly developed PCR test for hereditary hemochromatosis. Specific treatment consists of regular phlebotomy and possibly iron-chelating therapy. These patients should moreover avoid any possibility of facilitated absorption of iron and need to limit their intake of iron. Patients with beta-thalassemia major and sickle cell anemia who suffer from iron overload due to regular blood transfusions or excessive destruction of red blood cells need specialized medical treatment with iron chelators and should also control their intake of iron. The serum of patients with pathological iron overload can contain non-transferrin-bound iron inducing lipid peroxidation with subsequent consumption of antioxidants such as vitamin E and vitamin C. The role of iron in coronary heart disease and cancer is controversial. Early suggestions that moderately elevated iron stores are associated with an increased risk of CHD have not been confirmed by later studies. In vitro, ascorbic acid can act as a prooxidant in the presence of transition metals such as iron or copper, but in the living organism its major functions are as an antioxidant. High intakes of vitamin C have thus not been found to increase oxidative damage in humans. Accordingly, the risk of CHD or cancer is not elevated. On the contrary, most studies have shown that diets rich in vitamin C are inversely related to the incidence of these diseases.

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Even though a certain part of oxalate in the urine derives from metabolized ascorbic acid (AA), the intake of high doses of vitamin C does not increase the risk of calcium oxalate kidney stones due to physiological regulatory factor: gastrointestinal absorption as well as renal tubular reabsorption of AA are saturable processes, and the metabolic transformation of AA to oxalate is limited as well. Older assays for urinary oxalate favored in vitro conversion of AA to oxalate during storage and processing of the samples. Recurrent stone formers and patients with renal failure who have a defect in AA or oxalate metabolism should restrict daily vitamin C intakes to -100 mg. But in the large-scale Harvard Prospective Health Professional Follow-Up Study, those groups in the highest quintile of vitamin C intake (> 1,500 mg/day) had a lower risk of kidney stones than the groups in the lowest quintiles.

There is widespread belief among athletes that special nutritional practices--in particular high-protein diets--will enhance their achievements in competition. Supplementation with vitamins, especially vitamin C, is equally popular. But because genetic predisposition, hard physical training and psychological factors play a most important role in determining performance, and because any potential difference in achievement will be small, it is almost impossible to obtain scientific evidence of a beneficial effect of a particular nutrient. There have been many investigations during the past four decades of the potential effect of high-dose vitamin C supplementation on physical performance. The variables used have included maximum oxygen uptake, blood lactic acid levels, and heart rate after exercise, and in some cases performance was assessed in competitive events. The results have been equivocal: Most studies could not demonstrate an effect. On the other hand, a suboptimal vitamin C status results in an impaired working capacity which can be normalized by restoring vitamin C body pools. Athletes, who follow irrational, unhealthy eating patterns often not including vitamin-C-containing fruit and vegetables, are in need of nutrition education.