The treatment described by the doctor could alleviate symptoms attributable to several vitamin deficiencies. Pellagra is caused by deficiency of vitamin B3. The best sources are meat products, but green vegetables and milk are also good sources. In modern times, milk is a less desirable source for these vegetables because it is dense in calories and we tend to be overweight already. Prior to WW II and the mechanization of "everything," many people lived as subsistence level farmers and lacked good nutrition. Milk would be a good substitute for everything that they lacked.
I'm not a doctor, but I suspect that Pellagra and other vitamin deficiency related diseases would show up first in people with the lowest fat reserves and with the highest metabolic rate. A growing boy might well be the first in the family to show symptoms due to vitamin deficiency, but the others would follow soon unless the vitamins showed up in their diet.
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