To prevent osteoporosis, elderly people should focus on a diet rich in calcium and vitamin D. Calcium can be found in dairy products like milk, cheese, and yogurt, as well as in tofu, almonds, and leafy green vegetables. Vitamin D helps with calcium absorption and can be obtained from sunlight and foods like salmon, tuna, and egg yolks. It's also important to reduce intake of high-salt, high-sugar, and high-caffeine foods, as these can affect calcium absorption. Drinking plenty of water and maintaining regular physical activity are also crucial.