When writing the revised edition of Eat Right For Your Type, I alluded to the difficulties of doing nutritional and dietary research, and used salt as a prime example. It has been suggested that a statistically significant, well-powered study on the health effects of salt would have to enroll 28,000 patients for at least five years. Now, the recent media abounds in news about the exoneration of salt as a cause of many modern ailments. 
One problem with these types of simplistic association studies is the fact that people are different. For example salt sensitivity and hypertension are linked to several common variations in our DNA (SNPs) in three genes (SLC4A5, GRK4 and DRD2. In these people, media advice about the newfound safety of salt may actually be harmful.
SLC4A5 is involved in intracellular pH regulation and which it accomplishes by moving sodium bicarbonate between the outside world and inside of the cell. GRK4 regulates the triggering and firing of special receptors on the surface of the cell and has linked to both genetic and acquired hypertension. The DRD2 gene encodes one of the receptors for the neurotransmitter dopamine. Having the ‘AA’ genotype at the rs7571842 SNP on SLC4A5 significantly increases your sensitivity to salt with regard to blood pressure. Lesser effects were observed for rs2960306(TT) in GRK4 and rs6276 (TT) in DRD2.
So back and forth we go.
However, the story of sugar may be more interesting than the tale of salt: At least as a backstory of just how screwed up and jaundiced nutrition research can be.
As reported by the New York Times and other outlets, information has come to pass that a trade group called the Sugar Research Foundation, known today as the Sugar Association, paid three Harvard scientists the equivalent of about $50,000 in today’s dollars to publish a 1967 review of research on sugar, fat and heart disease. The studies used in the review were handpicked by the sugar group, and the article, which was published in the prestigious New England Journal of Medicine, minimized the link between sugar and heart health and cast aspersions on the role of saturated fat.
One of the scientists who was paid by the sugar industry was D. Mark Hegsted, who went on to become the head of nutrition at the United States Department of Agriculture, and where in 1977 he helped draft the forerunner to the federal government’s dietary guidelines.
Dr. Hegsted used his research to influence the government’s dietary recommendations, which emphasized saturated fat as a driver of heart disease while largely characterizing sugar as empty calories linked to tooth decay. Today, the saturated fat warnings remain a cornerstone of the government’s dietary guidelines, though in recent years the American Heart Association, the World Health Organization and other health authorities have also begun to warn that too much added sugar may increase cardiovascular disease risk.
This suggests that five decades of research into the role of nutrition and heart disease, including many of today’s dietary recommendations, may have been largely shaped by the sugar industry.
Salt photo: By Lexlex – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=68106171