Tuesday, September 2, 2014

Sodium, in the spotlight for next week's #NephJC, part 3

The last article in NEJM's remarkable sodium package is an extraordinary analysis attempting to estimate the number of deaths that can be attributed to excess sodium intake.

Global Sodium Consumption and Death from Cardiovascular Causes.

The authors reviewed 205 studies of dietary sodium consumption:
  • 142 studies that used 24-hour urine collections
  • 91 with estimates of dietary intake
  • 28 with both methods
These studies came from 66 countries representing 74.1% of the adult population. It is appropriate to whistle and say wow, at this point.

Inorder to translate the sodium intake into mortality the authors first needed to estimate sodium's effect on blood pressure. They employed two Cochrane meta-analysis looking at the effect of reduced sodium intake on blood pressure (Meta 1, Meta 2). They used these meta-analysis to discover sources for there own meta-analysis. They needed age and gender specific effects of sodium on blood pressure which is why they needed to do their own analysis.

After estimating the effect of sodium on blood pressure, they then used the blood pressure data to estimate cardiovascular mortality based on the work done in two large studies (Study 1, Study 2).

Estimated global sodium intake was 3.95 g per day, quite a bit lower than the 4.4 grams measured in the PURE studies. They pointed out that 99.2% of the countries surveyed had mean sodium intakes higher than the WHO level of 2 grams a day. An astounding 88% of the world had sodium intake more than 50% over the WHO recommendation.

Their meta-analysis found that systolic blood pressure fell 3.8 mm Hg for every 2.3 grams sodium intake was reduced. This translates to a more interpretable 1.6 mm Hg for every gram reduction in sodium. This is quite close to the 1.5 mm Hg found in the PURE analysis. They used 2.3 grams because that is equal to 100 mmol of sodium, for people who like to speak like a chemist.

They then ran the blood pressure data into the mortality data from blood pressure and concluded that consuming more than 2 grams of sodium a day results in 1.65 million deaths from cardiovascular disease a year. This is 9.5% of all cardiovascular deaths in the world and nearly 20% of all premature deaths.

However, despite a very through analysis this is an exercise in somewhat meaningless statistical gymnastics. The authors fail to consider the possibility that lowering the blood pressure too far could have negative consequences, (Hello ACCORD Trial. Nice to meet you.) Or the possibility that low sodium diets could be harmful.

Note that, these figures come from a 2011 prospective trial published in a little known journal called JAMA. This signal that low sodium diets may not be beneficial is not new or unknown. It was picked up by the Institute of Medicine in their summary and recommendations to avoid very low sodium diets:
However, the evidence on health outcomes is not consistent with efforts that encourage lowering of dietary sodium in the general population to 1,500 mg/day. Further research may shed more light on the association between lower—1,500 to 2,300 mg—levels of sodium and health outcomes.
This becomes even more concerning when looked at through the lens of the PURE studies in the same issue of The Journal that show average sodium intake to be associated with the lowest mortality and danger rising on either side of the sodium consumption curve.

This is a study best taken with a grain of salt. Couldn't resist.
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