Final answer:
The cause for Americans' lower vitamin D levels can be attributed to limited availability in natural foods, reduced sunlight exposure, and differences in skin type. Supplements can be prescribed to address deficiencies.
Step-by-step explanation:
The cause for Americans' vitamin D levels to be lower than desired is primarily due to the limited availability of vitamin D in natural foods and reduced exposure to sunlight. Vitamin D is not naturally found in many foods, except for fatty fish and fortified products like milk and cereal. The body produces vitamin D when the skin is exposed to sunlight, but factors such as dark skin complexion and living in northern latitudes with weaker sunlight can contribute to deficiencies.
In cases of deficiency, a doctor can prescribe vitamin D supplements to help improve levels. It's important to note that individuals with darker skin may require more time in the sun compared to those with lighter skin to produce the same amount of vitamin D.