Bioavailability of zinc in rats fed on tuna as a protein source in the diet

The objective of this study was to investigate the dietary zinc bioavailability in diets containing raw, cooked, steamed, and canned tuna as a protein source in the diet, and to evaluate different means of preparation, as well as various storage periods of the canned tuna, which was prepared according to industrial standards. Sterilized oil-free canned tuna was also made. The biological assays used for the study of the zinc utilization were carried out on Wistar rats fed with semi-synthetic diets for 12 days varying only in the protein source, which was casein-methionine or tuna provided as follows: raw, cooked in brine, steamed, sterilized with or without soybean oil, canned and stored for one or three years. Body weight, food intake, faecal and urinary zinc excretion, and zinc content in rat liver were monitored. No modifications were shown in food efficiency when diets including tuna were used, compared with those values from the casein-methionine diet. Thus, final body weight results were similar in all groups tested. Dietary zinc absorption was enhanced by consuming a raw white tuna diet. This beneficial effect disappeared after steaming or cooking in brine, but it was slightly recovered after sterilization with oil and storage of the canned tuna. Animals fed on the diets including the oil-free canned tuna, showed two or three times higher faecal and urinary zinc excretion than those fed on casein and canned tuna stored for 0, 1 or 3 years, reaching a negative balance. A negative effect was therefore found in both dietary and endogenous zinc utilization due to the consumption of oil-free canned tuna.