In his words “To one whose work has lain in India, and who for more than twenty years has been engaged in a study of the relation of faulty food to disease, the belief that such food is of paramount importance in the causation of disease amounts to certainty.”

McCarrison was a young medical doctor when he was stationed by the British administration in what is today northern Pakistan, at the beginning of the 20’s century. He had a neck for research and noticed soon after his arrival that the Hunza tribe are incredibly healthy in comparison to other populations in his region. He attributed the health to their diet and thus began a long career in nutritional research.

After WWI the British realized that they did not know what to feed their Indian soldiers, so they established a nutritional research center in Madras headed by McCarrison. The results of his 18 years of research are summarized in two Cantor lectures that McCarrison presented in England in 1936 after he was promoted to major-general and received a Sir title from King George V for his scientific achievements. via Miki Ben-Dor

"Add to this that the average Bengali or Madrassi uses relatively little milk or milk-products, that by religion he is often a non-meat-eater, that his consumption of protein, whether of vegetable or of animal origin, is, in general, very low, that fresh vegetable and fruit enter into his dietary but sparingly, and we have not far to seek for the poor physique that, in general, characterizes him. In short, it may be said that according as the quality of the diet diminishes with respect to proteins, fats, minerals and vitamins, so do physical efficiency and health; a rule which applies with equal force to the European as to the Indian."

The Big Fat Surprise by Nina Teicholz also discusses this, page 14 
"In the early 1900s, for instance, Sir Robert McCarrison, the Britist government's director of nutrition research in the Indian Medical Service and perhaps the most influential nutritionist of the first half of the twentieth century, wrote that he was "deeply impressed by the health and vigour of certain races there. The Sikhs and the Hunzas," notably, suffered from "none of the major diseases of Western nations such as cancer, peptic ulcer, appendicitis, and dental decay." These Indians in the north were generally long-lived and had "good physique[s]," and their vibrant health stood "in marked contrast" to the high morbidity of other groups in the southern part of India who ate mainly white rice with minimal dairy or meat. McCarrison believed he could rule out causes other than nutrition for these differences, because he found that he could reproduce a similar degree of ill-health when feeding experimental rats a diet low in milk and meat. The healthy people McCarrison observed ate some meat but mostly "an abundance" of milk and milk products such as butter and cheese, which meant that the fat content of their diet was mainly saturated."