Final answer:
Limited sun exposure and a diet lacking in vitamin D-rich foods are two main causes for decreasing vitamin D levels nationally, affecting bone health due to reduced calcium and phosphorus absorption.
Step-by-step explanation:
Two factors causing the national vitamin D level to drop are limited sun exposure and the availability of vitamin D in the diet. Sunlight is crucial for the synthesis of vitamin D in the skin, but decreased sun exposure due to factors such as living at higher latitudes, having darker skin pigmentation, and lifestyle choices that reduce time spent outdoors can lead to a deficiency. Additionally, since vitamin D is not naturally abundant in many foods, except for fatty fish and fortified products, dietary intake may be insufficient without careful planning or supplementation.
Vitamin D is critical for the absorption of calcium and phosphorus, essential for maintaining strong bones. Therefore, an individual with low levels of vitamin D might experience fragile bones due to inadequate mineralization, leading to conditions like osteomalacia in adults and rickets in children.