As we have discussed in the last three articles in my series on salt, the evidence for universal salt reduction is weak and often conflicted. Across different cultures, dietary salt intake is at best weakly correlated with blood pressure or cardiovascular risks, and associated with poorer health outcomes at either extreme of salt intake, both low and high. As a general recommendation, it seems that salt restriction for most people may be both unnecessary and possibly harmful in the long run.
While most people have no reason to restrict salt to the levels recommended by various health organizations, there are a few health conditions in which lower salt consumption may be necessary, based on clinical and population data. Generally, these are people with serious health problems, particularly suboptimal kidney function, and the data supporting salt restriction in these individuals is somewhat controversial.
Salt intake with impaired renal function
For those who have high blood pressure, there is evidence that some hypertensive individuals have inherited salt sensitivity, thought to be caused primarily by impaired sodium transport in the kidney. (1) Our understanding of the salt-sensitivity mechanisms is still underdeveloped, but we do know that certain individuals are far more sensitive to fluctuations in dietary salt. Those individuals with this trait will have a significant blood pressure response to a high salt intake, and would likely benefit from reducing their intake of salt. However, it is thought that potassium intake can greatly impact these effects, and may even eliminate salt sensitivity symptoms. (2, 3) In fact, salt sensitivity is dose-dependently suppressed when dietary potassium is increased within its normal range, so these individuals may benefit more from including ample potassium rather than limiting sodium.
Though the evidence is mixed, patients with chronic renal disease may have better outcomes consuming a lower amount of salt. (4, 5) Those with impaired kidney function typically have reduced glomerular filtration rates and may have more difficulty excreting high levels of sodium. It’s possible that increased dietary salt exposure is toxic to the kidneys when sodium filtration is impaired, and may lead to unsafe levels of proteinuria. These patients need to be cautious about the amount of salt in their diet, though this is a highly individual situation, and largely depends on the type and severity of kidney disease.
High sodium intake may cause excess calcium excretion
Additionally, those who are prone to kidney stones may need to reduce their salt intake, as high sodium excretion also leads to a higher level of calcium excretion in the urine. (6) Again, evidence on this topic is mixed, but it has been demonstrated that excess sodium intake is associated with increased urinary excretion of sodium and calcium, and subjects who consumed the highest levels of sodium tended to have the greatest urinary calcium excretion. Higher calcium excretion may lead to kidney stone formation, particularly if fluid intake is inadequate.
Because of this increased calcium excretion with higher sodium intake, those with osteoporosis may benefit from a lower salt intake as well. (7) Increased losses of calcium in the urine, particularly in the context of low dietary calcium, could be problematic for those at risk for low bone density. However, a high salt intake is not believed to cause osteoporosis, and the potential osteoporotic effects of a high salt intake can be offset by an adequate intake of calcium and potassium.
Of course, it’s important to remember that the majority of these studies have been conducted on subjects consuming the standard American diet of sodium-laden processed food with a heavy emphasis on grains and a deficit of many important vitamins and minerals that we know play significant roles in hypertension, cardiovascular disease, and kidney health. If these sodium studies were conducted in a population consuming a nutrient dense Paleo-type diet, it’s possible the negative effects associated with a high sodium intake would be negligible. As we’ve seen, many of the cultures eating the highest levels of salt have less incidence of cardiovascular disease, kidney disease, and osteoporosis than Americans do. There is evidence that adequate consumption of other minerals may be far more important in blood pressure regulation and other related health outcomes.
Other minerals important for controlling blood pressure
There has been much research into the other dietary minerals that may play a role in blood pressure. The evidence has been mixed on whether certain minerals, particularly supplemental minerals, reduce blood pressure or risk for cardiovascular disease. However, epidemiological and anthropological data suggest that a diet high in certain minerals, such as potassium, magnesium, and calcium, may be beneficial in reducing high blood pressure.
Potassium is likely far more important than sodium intake in the control of blood pressure, as well as reducing the risk of hypertension, kidney stones and osteoporosis. (8) It is believed that human biological machinery evolved to process dietary potassium in amounts many times those of sodium, as Paleolithic man consumed an estimated 10500 mg of potassium each day, compared to a current US intake of 2500 mg. (9) Therefore, the sodium-potassium ratio of the modern diet is hugely mismatched to our genetically determined renal processing machinery. Additionally, the cardioprotective effects of a relatively high potassium intake have been hypothesized as a basis for low CVD rates in populations consuming primitive diets, where hypertension has been shown to affect only 1% of the population. (10)
Research suggests that increased intake of potassium, found in fruits and vegetables, may be more effective than, and possibly synergistic with, moderately restricting dietary NaCl in reducing not only the renal excretion of calcium, but also the level of blood pressure, the expression of hypertension, and the development of osteoporosis and kidney stones. (11) Therefore, a diet high in potassium-rich plant foods is crucial to preventing the negative outcomes typically associated with a high salt intake.
Magnesium has also been studied for its potential effects on blood pressure, which are poorly understood. Epidemiological studies have typically shown an inverse relationship between dietary magnesium intake and blood pressure, however data from clinical studies have been less convincing of magnesium’s role in treating hypertension. (12) Despite this conflicting evidence, some studies have shown that intracellular magnesium deficiency affects insulin resistance, alters vascular tone leading to hypertension, and induces pro-inflammatory changes and endothelial dysfunction, ultimately increasing the risk for CVD. (13) Therefore, a diet high in magnesium is likely beneficial for anyone at risk for hypertension or heart disease.
Calcium is another major mineral thought to play a role in blood pressure management. A high dietary intake of calcium, but not calcium supplementation, has been associated with both a decrease in blood pressure and the risk of developing hypertension. (14) In fact, calcium supplementation has been associated with a 30% increased risk of heart attack, and is potentially dangerous for those at risk for heart disease. (15) For those looking to protect themselves against hypertension and subsequent cardiovascular disease, a calcium-rich diet should suffice, with no supplementation required or recommended. (And of course, remember to keep vitamin K2 intake adequate as well!)
Take home message? Use your own judgment!
Ultimately, the amount of salt required for good health is based on individual needs, health status, and genetic predisposition to salt sensitivity. The evidence for salt restriction, even for those with cardiovascular or renal disease, is mixed and often times inconclusive. It’s important to remember that the data regarding sodium intake has been from populations typically eating a standard American diet, and it’s unknown whether salt intake would demonstrate any detrimental effects in a population eating a potassium, magnesium, and calcium rich whole foods Paleo diet. These are important points to consider when deciding how much salt to include in your own diet.
For my final article on salt, I will discuss the types of salt I recommend, and how much salt is ideal for most people.
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