SODIUM and SALT , BLOOD PRESSURE, and MAJOR DISEASE
Abstract: Sodium from salt has a far lesser effect on blood pressure and major disease than might be inferred from the massive publicity about its potential harm. This finding is based on the averaged results of more than 100 research studies. The added sodium from the 1000 mg/day of a high salt food will increase the blood pressure of a person with normal blood pressures of 125/75 to 125.6/75.4; and the pressures of a hypertensive person of 160/100 only to 161.8/101. Reducing lifetime salt intake by a fairly substantial 1000 mg/day will add only about 100 Well-Days to the longevity of a hypertensive person, and less to those having low and moderate blood pressures.
Background: Few subjects on health have been studied and discussed more than the effect of sodium or salt on blood pressure and health. Curiously, actual research now discloses that salt intake is a relatively minor health factor compared to most other risks now included in Life Ahead.
The classic chemical engineering model of salt and blood pressure is: Salt that is measured as sodium combines with water to increase the volume of blood being circulated by the heart. This places an added load on the heart that raises blood pressure somewhat. Higher blood pressures in turn increase the risk of CV diseases.
When individuals are young and/or have normal low blood pressures the arteries are wide open and easily can handle the increased blood flow from added salt intake. The effect of salt on blood pressure then can be very small, and any accompanying increase in risk of CV disease is trivial. But when arteries narrow and stiffen from atherosclerotic deposits as usually develops with age, the pressure needed on the heart to circulate the added blood volume from salt increases. Now added salt can increase blood pressures by enough to increase risk of the CV diseases.
The Big Argument: Medline now identifies more than 20,000 studies citing sodium and blood pressure. A majority of those testing its effect on people appear to have found “No statistically significant effect.” Understandably this has led to strong disagreements about its importance among researchers. A major project taken since 1984 to “Finalize” the effect of salt on blood pressure by massive research in many countries around the world was called “Intersalt.” The participants still disagree - some near violently - about the results.
A Balanced View that Emerges is: Salt does affect blood pressure somewhat but its effect is of significance mostly for those that have elevated blood pressure. But the effect usually is small, and the effect of salt on those having low blood pressures is near trivial. The only possible way to identify the true effect of salt on blood pressure is to derive results from averages of many different studies. Individual studies simply do not have anywhere near the resolving power to identify the small effects involved.
The Research Results: A meaningful average result is obtained from the very large Meta or summary analyses of Gradaul, NA, JAMA 1998, 279:1383:
56 trials of reducing sodium an average of 3700 mg/day on individuals with normal blood pressures: Reduction of 1.2 mm (limits 0.6-1.8) in systolic and 0.26 mm (limits -0.3-0.9) in diastolic pressures.
58 trials of reducing sodium an average 2700 mg/day on hypertensive persons: Reduction of systolic blood pressures by 3.9 mm (limits 3.0-4.8); diastolic blood pressures by 1.9 mm (limits 1.2-2.5)
Note that the amount of sodium considered in these comparisons is very large – and near equal to the entire salt consumption of many US people today. The amount of sodium in a daily portion of rather high salt food would range from 300 to 1000 mg, and this would change blood pressures only from one tenth to a third of the above quite small amounts.
These results are confirmed by thee more Meta Analyses or summary studies of most available published research. Their average result for similar amounts of sodium is: A blood pressure reduction for about 3000 mg/day is 1.5 / 0.7 for normal blood pressures and 4.4 / 2.0 for hypertensives. The present research from more than a hundred different studies is now well into the overkill in amount. Thus these net average result are unlikely to be altered much by further research. More about diet and blood pressure is included in the Life Ahead Library summary of Blood Pressure.
The Life Ahead Formula: The formula developed for use in Life Ahead integrates further this summary of research data as:
Difference in Systolic Blood pressure = (BPS - 100) * 0.000031 * Sodiumdiff , and
Difference in Diastolic Blood pressure= (BPD - 60) * 0.000025 * Sodiumdiff
BPS and BPD are present blood pressures. Sodiumdiff is the difference in sodium of interest in mg/day. This formula reflects that at very low blood pressures of 100 / 60, the lowest values accepted, there will be a no effect of sodium on blood pressure. The effect of sodium will increase directly with increase in blood pressure above this level. These formulas produce results consistent with the average of research to date from the above Meta analyses. Values from the formula for increases in Systolic / Diastolic pressures from indicated amounts of salt in a food are, as example:
Added Sodium of Food Effect of Sodium on Blood Pressure
at Blood Pressure 120 / 75 140 / 90 160 / 100
200 mg/day Typical Food 0.13 / 0.1 0.24 / 0.15 0.6 / 0.4
500 mg/day High Salt Food 0.3 / 0.2 0.6 / 0.4 0.9 / 0.5
1000 mg/day Highest Salt Food 0.6 / 0.4 1.0 / 0.7 1.8 / 1.0
These increases in blood pressure for daily portions of various foods are very small. Even the salt in a daily portion of a highest salt food as a junk food at a daily 1000 mg of sodium would raise blood pressure only 1.0 / 0.7 mm for a person with mild hypertension at 140 / 90, and 1.8 / 1.0 mm for a hypertensive person at 160 / 100 blood pressure. Keep in mind that these figures are consistent with average results from more than a hundred world-wide studies obtained mostly during the past two decades of time, and are values of accuracy far beyond the resolving power of any single study.
The effect of blood pressure on CVD is well established. Thus translating the effect of sodium on blood pressure into risk of heart disease and stroke, we obtain these values: For 1000 mg per day added salt and a 140 / 90 blood pressure: Risk of heart disease is increased 2.8%, risk of stroke increased 4.0%. For a person with 160 / 100 blood pressure, increase in heart disease 3.8%; increase in stroke 5.3%. Risks would be much smaller and near insignificant for foods with much lesser sodium content than this very high value, and/or for those with normal blood pressures.
Although the benefits of salt restriction will be small vs. most other factors in Life Ahead, there still is a reason for those with high blood pressures to keep salt intake at moderate levels. For those at the highest above blood pressure level a reduction of 1000 mg/day in sodium intake every day for the rest of life should add about 100 Well-Days to longevity. Eating of an occasional food that includes high salt would compute to near zero effect. Also with today’s efficient blood pressure medications most people today should be able to avoid the sustained high blood pressures that elevate the effect of salt.
The Direct Effect of Salt on CVD: Curiously, despite the vast amount of research spent on salt vs. blood pressure, little research seems to have been done studying more directly the effect of salt on disease. It clearly is possible that salt could affect disease by mechanisms other than just its effect on blood pressure. Just three useful studies were found published. A first, (J. He, JAMA 1999;282:2027) cited a 0.69 risk factor on heart disease and 0.53 risk factor on stroke for a reduction of 2300 mg/day of sodium for those overweight, but no effect on those of normal weight. With no reasonable explanation of how such a difference could occur as a result of body weight, the association here with sodium only for overweight people could be suspect. Those overweight probably ate more than those of normal weight, and especially of junk and other high fat foods that include more saturated fats and low fiber in addition to salt. The accurate separation of effects of salt from effects of other constituents of foods eaten is near impossible to accomplish in usual epidemiological studies. .
A second study (J. Tuomilehto, Lancet 2001, 357:848) did find a substantial benefit of restricting sodium by a similar 2300 mg: A risk ratio of 0.66 (0.5-0.88) for Coronary disease and 0.69 (0.54-0.88) on all CVD. But the effect was significant only on men, not on women. The third study, (Alderman, Lancet 1998,351:781) cited sodium to be inversely associated with both coronary disease and overall mortality. In other words, more sodium was associated with lower disease.
In summary, results of these direct studies provide no useful picture of a direct effect of sodium on disease. At this time we have only the effects derived from blood pressure as significantly established. It is tempting to conclude that much more research is needed to study directly the effect of salt on disease. But such research entails serious problems. It may be impossible to separate out the effect of salt from the effects of other food nutrients associated with it. Thus results of any observation research study on this will be suspect.
Another interesting study (Am J Clin Nutr, 2006, 83:1289) was done on about 2,000 Elderly Taiwanese men in a veteran's retirement home. One group ate normal salt, the other at about half of the normal salt plus an equivalent amount of potassium for about 2 1/2 years. . This produced both a substantial reduction in sodium and a large increase in potassium. Potassium is now verified for reducing risk of stroke, and this suggests it should also be capable of reducing heart disease. The group fed the sodium-potassium mixture had a 41% lower risk of cardiovascular disease mortality (risk =0.59, 5-95% limits 0.35-0.95) than did the group fed only sodium. Confirmation of this result would be welcomed. But this could be done only in a facility that had complete control of user diets.
Life Ahead computes the effect of sodium from its effect on blood pressure from the above formula, and then from the effect blood pressure modified by this effect of sodium on CVD diseases. Risks are computed from this combination at each age of life. But to be conservative with recognition of the potential errors in the available analyses, the effect of sodium on blood pressure is increased by 1.5 times or 50% over that measured in the 114 studies. This will compute a 3.2 mm increase for the change from 1500 to 3300 measured in the DASH research vs. the actual 3.0 mm obtained. Even with this conservatism, the effect of salt on outcomes of major disease usually small for those with usual blood pressure.
A Conclusion: Many people today have been so influenced by the barrage of health advice on salt as a cause of high blood pressure that they view salt as a virtual poison. Research now shows that this is incorrect. Rather, salt intake is a very small health factor for most people, and one of modest concern for those that are hypertensive. Most salt intake comes from processed foods, soups, and junk foods. Today’s massive research shows that the benefit on blood pressure from restricting some sprinkling of salt on foods to enhance their flavor would tiny – and this should be true even for those that are hypertensive.
Sodium is only one of 23 nutrients in most foods that are verified to affect health. Foods that are high in sodium may have offsetting benefits in other nutrients. You can obtain an estimate of the overall health value of any food from all of its included nutrients using Life Ahead. Soups, for example can include near 1000 mg of sodium for an 8 ounce portion. Life Ahead computes some values for a 50 year-old person eating one portion of some soups as part of an average US diet. Taking some different soups every day for rest of life produced computed values ranging from a loss of 130 Well-Days to a benefit of 80 Well-Days. These are small differences on likely outcome; Other constituents of the soups appear to have a much larger effect on overall health than just their content of salt and the likely contribution of all soup nutrients in a soup to health can be displayed in Life Ahead Model #3.