A system stressing a life balance between yin and yang qualities: yin foods grow above ground and usually have a high water content, while yang foods tend to be roots, stems and seeds grown in colder, wet environments. Grains are closest to a neutrality between yin and yang and are therefore the most important component of the macrobiotic diet. Utensils used to prepare foods are carefully chosen, avoiding copper or aluminum. Lengthy cooking is usually required. Malnutrition is a potential problem.