The case for dietary potassium sodium imbalance as a cause of hypertension is overwhelming. Sir Austin Hill, a leading epidemiologist, laid out the criteria to distinguish causation from mere association in the 1960s. He summarized the criteria in an address to the Royal Society of Medicine (1).
Hill's criteria were developed for determining causation for a disease when just an association could exist rather than true causation. With the potassium to sodium ratio there is fulfillment of all his criteria, including the strongest support for the causation hypothesis – experimental evidence that is overwhelming. In fact, all his criteria are fulfilled overwhelmingly.
Criteria To Show The Major Cause Of Hypertension
Hill's first criterion is strength. By this he means an enormous increase in disease associated with the factor felt to be responsible for the disease. His examples are the scrotal cancer of chimney sweeps, who have an incidence 200 times that of non chimney sweeps, and heavy smokers who have an incidence of lung cancer 20 to 30 times as much as non smokers.
In the case of hypertension caused by the differential of the potassium to sodium ratio, there is an incidence of hypertension 28 times as much or greater in populations with a ratio less than one versus those with a ratio above 5. The Tarahumara, Bushmen and multiple other indigenous groups have hypertension in less than one percent of their population, as discussed here. The ratio of potassium to sodium in their diet is greater than 5. Modern societies with ratios of 0.6 or less have an incidence of hypertension of greater than 28%.
The second criterion is consistency. Consistency means that the observation is made repeatedly by different persons, in different places, circumstances and times. Once again the evidence is overwhelming. The different time, circumstances and places requirement is satisfied by the studies in the 1950s and 1960s of the many indigenous peoples' diet, and of the Japanese and Fins discussed here and here. Multiple researchers have studied multiple peoples as discussed here, and found the same results.
His third criterion is specificity, by which he means the association is limited to a specific type disease. Although there are several diseases that result from changes in the ratio, they are tied together as a single disease since all involve potassium and sodium by known biological mechanisms. The diseases should be considered manifestations of one disease, potassium-sodium imbalance. This criterion tends to be more important when the science underlying the disease cannot explain the disease. In the case of hypertension, the underlying science explains the disease very well.
The fourth criterion is temporality by which Hill means determining that the causative factor occurs before the resulting disease. In the case of hypertension, the potassium-sodium imbalance precedes the hypertension and leads to it over a number of years.
Biological gradient is the fifth criterion. The biological gradient for hypertension and the potassium sodium ratio is clear. The groups with a ratio of more than 5 have hypertension less than one percent of the time. The groups with a ratio of 0.6 or less have an incidence of hypertension of 28% or more. The groups between have intermediate percentages of hypertension corresponding to the potassium sodium ratio.
The sixth criterion is plausibility, meaning the factor as causation is consistent with present day biological knowledge. The work of Guyton, Hall and Young, discussed here, has advanced biological knowledge to predict the ratios of potassium to sodium that will correspond with the likelihood of developing hypertension.
The seventh criterion is coherence. This means that there is no conflict with the natural history and biology of the disease. For hypertension there is no conflict with the natural history and biology of hypertension. The movement of indigenous peoples into the cities, as discussed here, has shown the natural history of the disease which is coherent with the behavior in the Western world.
The eighth criterion is the strongest. As Hill states, “Here the strongest support for the causation hypothesis may be revealed.” The strong support is supplied by many research papers, most notably those of Drs Guyton and Young. Their research shows the complex interactions within the cardiovascular system, and predicts what changes in one part of the system yield what results. Their prediction is that a diet with a ratio above 5 results in almost never having an imbalance of sodium and potassium. They also predict that a ratio less than 0.6 almost always results in an imbalance of sodium and potassium. These studies give a clear picture of the science explaining why the population based studies have the results they have.
The ninth criterion is analogy. Hill states this means that if another known disease is caused by a factor similar to the present one, the subject factor will be more acceptable. I feel this criterion should be dropped. Analogy is more a study of linguistics than science and has little to do with proof. It is only good for forming hypotheses which must then be tested.
The past several posts have been going through a few of the many studies showing the importance of the potassium to sodium ratio. Here we have tied together all those studies to show how overwhelming the evidence is that most modern hypertension is caused by the poor potassium to sodium ratio of the modern diet. The combination of high potassium foods and avoidance of sodium in the diet is the solution for the majority of people with hypertension.
1. The environment and disease: association or causation? Hill AB. Proc R Soc Med. 1965 May;58:295-300.