Thanks—this is super interesting and I agree that hypertension is a promising cause area—and that taxes on unhealthy foods may be a promising intervention.
The estimate of the degree to which an average 1 mg reduction in sodium consumption reduces incidence of high systolic blood pressure in a single country and hence the global disease burden of hypertension, relies on an extremely long and complicated chain of calculations – there is hence a high degree of uncertainty here.
I agree with your assessment that this is an area of uncertainty. In particular, I think we need to be careful about assuming linear effects on BP and burden of disease based on a very small change in daily sodium intake. You’ve thought deeply about this problem, so would be keen to hear your thoughts on what I’ve sketched out below.
You estimated that a tax would reduce sodium consumption by 67 mg per day; a meta-analyses suggests that reducing dietary salt by a mean of 4.4 g per day leads to a mean reduction of systolic BP of 4.18 mm Hg. There was a bigger drop (5.39 mm Hg) in hypertensive people. [Note: my best guess is that your estimate of sodium reduction is for molecular sodium whereas these numbers are for salt—NaCl - 4.4 g of NaCl would be about 1.7 g of Na.] Based on this, I would say it’s very generous to assume any more than 0.5 mm Hg average reduction in systolic BP from 67 mg less sodium per day. 0.5 mm Hg is well within measurement error. This may be a case where a very small effect multiplied by a huge number of people still has a huge effect, but I think more evidence would be helpful. In particular, can we find evidence that:
the relationship between sodium intake and BP is linear and holds true for milligram quantities of change? BP is a homeostatic system where I don’t think we can always assume linear effects.
very small reductions in BP over time lead to meaningful changes in health outcomes?
Maybe deriving additional estimates of the health impacts of −67 mg/day salt intake, using different reference classes, could reduce this uncertainty.
Agreed that the linear vs non-linear issue is a major outstanding issue. My rough sense now is that you capture more of the benefits at higher levels of blood pressure (less so at lower levels/for healthier people) - so diminishing returns—but in general I don’t have a good answer and it’s something I’ll have to get a better sense of via expert interviews.
Thanks—this is super interesting and I agree that hypertension is a promising cause area—and that taxes on unhealthy foods may be a promising intervention.
I agree with your assessment that this is an area of uncertainty. In particular, I think we need to be careful about assuming linear effects on BP and burden of disease based on a very small change in daily sodium intake. You’ve thought deeply about this problem, so would be keen to hear your thoughts on what I’ve sketched out below.
You estimated that a tax would reduce sodium consumption by 67 mg per day; a meta-analyses suggests that reducing dietary salt by a mean of 4.4 g per day leads to a mean reduction of systolic BP of 4.18 mm Hg. There was a bigger drop (5.39 mm Hg) in hypertensive people. [Note: my best guess is that your estimate of sodium reduction is for molecular sodium whereas these numbers are for salt—NaCl - 4.4 g of NaCl would be about 1.7 g of Na.] Based on this, I would say it’s very generous to assume any more than 0.5 mm Hg average reduction in systolic BP from 67 mg less sodium per day. 0.5 mm Hg is well within measurement error. This may be a case where a very small effect multiplied by a huge number of people still has a huge effect, but I think more evidence would be helpful. In particular, can we find evidence that:
the relationship between sodium intake and BP is linear and holds true for milligram quantities of change? BP is a homeostatic system where I don’t think we can always assume linear effects.
very small reductions in BP over time lead to meaningful changes in health outcomes?
Maybe deriving additional estimates of the health impacts of −67 mg/day salt intake, using different reference classes, could reduce this uncertainty.
Agreed that the linear vs non-linear issue is a major outstanding issue. My rough sense now is that you capture more of the benefits at higher levels of blood pressure (less so at lower levels/for healthier people) - so diminishing returns—but in general I don’t have a good answer and it’s something I’ll have to get a better sense of via expert interviews.