Final answer:
The doctor advises Dominique to do more outdoor activities to increase his body's synthesis of vitamin D through sunlight exposure, which is crucial for calcium absorption and maintaining strong bones.
Step-by-step explanation:
The doctor suggests Dominique engages in more outdoor activities because exposure to sunlight is crucial for the synthesis of vitamin D in the skin. Vitamin D is essential for the proper absorption and regulation of calcium, which is vital for maintaining strong bones and teeth. Without sufficient vitamin D, one may experience bone softening diseases, such as rickets in children or osteomalacia in adults, due to inadequate calcium levels. Since vitamin D synthesis occurs when UV radiation from sunlight converts a cholesterol derivative in the skin into cholecalciferol, which is then transformed in the liver and kidneys into the active form of vitamin D, outdoor activities increase the opportunity for this synthesis, particularly in people living in colder climates or those with darker skin who may require more sunlight exposure to produce the same amount of vitamin D. Furthermore, activated vitamin D helps not only in the absorption of calcium from the digestive tract but also in its reuptake in the kidneys, maintaining normal serum concentrations essential for bone health.