Dietary habits are related to many problems including food, energy, environment and health. Thus, the researches in dietary habits must be conducted in the paradigm of complex system. But, it is important to accumulate the experimental observations by the reductionistic approarch. In this review, I focus on the new approach to the materials and methods.   First, I examined the function of oleuropein, a major polyphenol in olive leaf using the rats fed on high cholesterol diets. As the result, the values of total bile acids and total lipids in the feces decreased significantly in the rat fed on the olive leaf powder and oleuropein. The values of total cholesterol in the feces and triglyceride in the liver decreased significantly in the group fed on the olive leaf extract. These results indicate that the oleuropein facilitates the lipid absorption and the bile acid reabsorption in the ileum.   Second, in order to reveal the relationship between renal reabsorptional systems of vitamin B₁₂ (B12) and vitamin A, I analized the effect of B12 deficient status on the localization of megalin, the receptor of both vitamins, and retinol binding protein (RBP). As the result, in B12 deficient rats, megalin was accumulated on the membrane in the proximal tubules and a few tubules containing RBP were identified, which suggested inhibition of megalin-mediated endocytosis and renal vitamin A uptake.   Third, I produced D-histidine specific antibody to trace biokinetics. Histochemical approach may be a useful tool to understand the nutrition and dietary habits.