"If vitamin D does lower blood pressure in African-Americans, it can have a significant public health impact," [nephrologist John] Forman says. [NPR]
No. It might have widespread impact on individuals' health. But that's not what "public health" should mean.
The meaning of “public health” has sprawled out lazily over the decades. Once, it referred to the project of securing health benefits that were public: clean water, improved sanitation, and the control of epidemics through treatment, quarantine, and immunization. Public health officials worked to drain swamps that might breed mosquitoes and thus spread malaria. They strove to ensure that water supplies were not contaminated with cholera, typhoid, or other diseases. The U.S. Public Health Service began as the Marine Hospital Service, and one of its primary functions was ensuring that sailors didn’t expose domestic populations to new and virulent illnesses from overseas.
Those were legitimate public health issues because they involved consumption of a collective good (air or water) and/or the communication of disease to parties who had not consented to put themselves at risk. It is difficult for individuals to protect themselves against illnesses found in air, water, or food. A breeding ground for disease-carrying insects poses a risk to entire communities.
But a health problem is not a public health problem just because it's a widespread health problem. Obesity, riding a motorcycle without a helmet, and getting too little Vitamin D may be bad for an individual. But the individual who engages in these activities isn't endangering me. It may well be a good idea for African-Americans -- and others -- to get more Vitamin D in order to reduce high blood pressure. But that's health advice for individuals, not a public health issue.