The case for dietary potassium sodium imbalance as a cause of hypertension is overwhelming. Sir Austin Bradford Hill, a leading epidemiologist, laid out the criteria to distinguish causation from mere association epidemiologically in the 1960s. He summarized the criteria in an address to the Royal Society of Medicine (1).
Hill developed his criteria for determining causation for a disease when just an association could exist rather than true causation. The potassium to sodium ratio fulfills all his criteria. It fulfills even the strongest support for the causation hypothesis – experimental evidence that is overwhelming. In fact, the ratio fulfills all his criteria overwhelmingly.
Criteria To Show The Major Cause Of Hypertension
Hill's first criterion is strength. By this he means an enormous increase in disease incidence 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 differences in the dietary potassium to sodium ratio, there is an incidence of hypertension 28 times as much, or greater, in populations with a ratio greater than five versus those with a ratio less than one. For example, the Tarahumara, Bushmen and multiple other indigenous groups have hypertension in less than one percent of their population, as discussed in prior posts. The ratio of potassium to sodium in the diet of these indigenous groups is greater than 5. In contrast, 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 different persons, in different places, circumstances and times make the same observation repeatedly. Once again, the evidence is overwhelming. Studies in the 1950s and 1960s satisfied the different time, circumstances, and places requirement with studies by multiple researchers' investigations of many different indigenous peoples' diets.
Also consistent are studies of modern populations. An example of this are the Japanese and the Fins, discussed previously. They lowered their incidence of hypertension as their dietary ratio increased. Furthermore, multiple researchers have studied multiple peoples as their ratio decreased. They consistently found that hypertension became more prevalent.
Hill's 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. All of these diseases involve the potassium sodium ratio by known biological mechanisms. Consequently, 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 this 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. Specifically, the imbalance first occurs during childhood with the typical Western diet. The hypertension then appears during adulthood.
Biological gradient is the fifth criterion. The biological gradient for hypertension and the potassium sodium ratio is clear. 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. Groups between these two ratios 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. Accordingly, the work of Guyton, Hall and Young, discussed in prior posts, has advanced biological knowledge to predict the ratios of potassium to sodium that will correspond with the likelihood of developing hypertension. Other work since then has only enhanced the plausibility.
The seventh criterion is coherence. This means that the hypothesis has no conflict with the natural history and biology of the disease. For hypertension, the dietary ratio hypothesis has no conflict with the natural history and biology of hypertension. The movement of indigenous peoples into the cities, as discussed previously, has shown the natural history of the disease. The blood pressure rises as the new urbanite's dietary ratio falls. Consequently, this natural history of hypertension is coherent with the change in dietary potassium sodium ratio.
The eighth criterion is the strongest. As Hill states, “Here the strongest support for the causation hypothesis may be revealed.” The eighth criterion is experimental evidence. This strong support comes from many research papers, most notably those of Drs Guyton, Hall and Young. Their research shows the complex interactions within the cardiovascular system.
It predicts what changes in one part of the system yield what results. As a result, their prediction is that a diet with a ratio above 5 results in almost never having an imbalance of sodium and potassium. Also, they predict that a ratio less than 0.6 almost always results in an imbalance of sodium and potassium. Consequently, these studies give a clear picture of the science. Their experiments explain 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. It 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. It is overwhelmingly evident 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.