Framingham is the small city that researchers will not leave alone. We are now into the Framingham Offspring Study, examining relationships between some dietary factors and blood pressure control. The study population included 2632 individuals 30-64 years of age who were not on blood pressure medication at the start of the 16 years of follow-up. In the first 5 years of the study, all kept a 6-day diet diary which was used to estimate daily intake of sodium, potassium, magnesium, and calcium. All analysis corrected for age, gender, smoking, activity level and the other usual suspects.
Results were similar for systolic and diastolic blood pressures, so I will just show systolic here:
How can that be? We have advised healthy individuals to lower salt intake forever to avoid hypertension, yet these folks make that look like exactly the wrong advice. Could there be a confounding diet variable? Let's throw potassium into the mix:
So low sodium, with or without high potassium intake, produced higher blood pressures than higher sodium plus high potassium intake. Similar results were found for intake of calcium and magnesium, other ions in the diet that seem to ameliorate hypertension.
There are some caveats here, like with every research study. First, even though this is a large, longitudinal study, these are people who are second or third generation research subjects. Being in cardiovascular research likely makes them more health conscious, so their level of sodium intake may not reflect the general US population. Also, these are healthy, non-hypertensive people. Findings might be quite different in people with high blood pressure or salt-sensitivity. They also have no biomarkers of intake; all diet data is from a 6-day diet diary in the initial years of the study. How eating patterns may have changed over time is just not known. Citizens of Framingham get the same health news as the rest of us; did many start green juicing or go paleo?
This study does bring into question reducing sodium intake for the general population. Those of us with hypertension probably need it, but it may not be important for the majority of people. Intake of other minerals may be far more important for general health (check this post out for more on the interrelationships between Na and K transporters in the kidney).
The bottom line? We should all eat a varied diet, avoiding junk food and focusing on fruits, veggies, and dairy with lean protein sources. That DASH-type diet has been shown to reduce weight and blood pressure.