Dodgy assertions from CASH's head medic

The salt-haters have been praising the reduction in dietary salt for an important role in the 42% fewer stroke fatalities and 40% drop in those dying from coronary heart disease:

The researchers, who include Britain's leading campaigner against added salt in food, claim that diminishing levels of salt was "an important contributor" to falls in blood pressure over the eight-year period. "As a result, the decrease in salt intake would have played an important role in the reduction of stroke and ischaemic heart disease mortality during this period," say the authors.

"Would have played"? That's a funny way of saying "was shown at a 95% confidence level to have played"... Co-author Graham MacGregor is the chair of CASH; his daytime job is Professor of cardiovascular medicine at the Wolfson Institute of Preventive Medicine at Queen Mary. So surely we can expect a rigorous and impartial analysis of the data from him.

If I'd been looking to prove or disprove this assertion, I'd have looked at stroke and heart disease rates in a range of patients over this time frame, where I had some objective measure of salt in their diet (urine samples), and looked to see whether patients with lower salt levels (in a group of patients with otherwise similar exercise, age, gender, racial stats) were correlated with lower stroke and heart disease rates. Is this what they did?

Patrick Wolfe, professor of statistics at University College London, took issue with the authors for assuming that the improved blood pressure seen in the 2003-2011 was largely the result of reduced salt intake. "Plausibility of assumption does not equal evidence," he said.

Oh. Apparently not, then. That's a piss-poor basis for the claims CASH (and international co-conspirator WASH) have been touting around about salt reduction. As commentor ID4968047 notes this reduction in strokes and heart disease could equally have come from the reduction in smoking in the past 10 years - the obligation is on Prof. MacGregor to show otherwise. Looking at CASH's writeup of the paper (the link to the paper isn't available yet, looks like) they say:

Confounding factors that were looked at include age, gender, ethnicity, education, incomes, alcohol consumption, fruit and vegetable intake and BMI.

Exercise and smoking are not mentioned. Nor do they reference the increase in statin use - and indeed Aseem Malhotra from Action on Sugar claims that statins are harmful and don't reduce mortality which is interesting as they seem to be a prime competitor to CASH/Action on Sugar's crusades against sugar and salt. Malhotra's claims got panned for lack of evidence by Prof. Rory Collins from Oxford.

It seems that others in the medical stats community have doubts too:

David Spiegelhalter, professor of the public understanding of risk at Cambridge university, cited the researchers' admission that the fall over that time in systolic blood pressure would be expected to reduce strokes by just 11% and heart attacks by 6%, small amounts of the total falls. [my emphasis] Reduced blood pressure did not represent the authors' claimed "substantial contribution" to the reduced death rates.

This is not to say that Graham MacGregor is obviously wrong in his claims. They might be true but it is a real reach to claim that this study supports them. And if this is the best he can do, I'd suggest the Marcela Trust / OMC Investments crowd who are backing CASH find someone with a better stats background to organise their crusade against salt and sugar.

Update: just managed to dig up the link to the full text in BMJ Open. From a quick look the focus was on linking salt reduction with BP reduction but not explicitly with stroke/CVD reduction.

The authors themselves admit:

It is likely that several factors, that is, the fall in BP, total cholesterol and smoking prevalence, the reduction in salt intake and the increase in the consumption of fruit and vegetables, along with improvements in the treatments of BP, cholesterol and CVD, contributed to the decrease in stroke and IHD mortality.

They have a stab at isolating the effect of salt by casting tea leaves:

it was estimated that a 2.7 mm Hg reduction in systolic BP that occurred with salt reduction would be predicted to reduce stroke by approximately 11% and IHD by 6%.

but even then the 2.7mmm Hg reduction figure they quote is the net over 8 years including factors such as decrease in smoking and increase in statins, so to attribute it to just salt reduction is "optimistic". They appeal to studies in Japan and Finland in the late 60's / early 70's but the huge gaps in time, diet and environment between now and then render the comparison unconvincing. If that's the best argument they've got to offer, I'd hate to see the ones that didn't get selected for use in the paper.

The conclusions are what kill the paper for me:

The reduction in salt intake is likely to be an important contributor to the falls in BP in England from 2003 to 2011. As a result, the decrease in salt intake would have played an important role in the reduction in stroke and IHD mortality during this period. [my emphasis]

That's a terribly weak conclusion even to my relatively untrained eyes. If they could state this more strongly, they would. Instead, they reserve their strength for polemic:

... the mean salt intake in England (8.1 g/day in 2011) was still 35% higher than the recommended level of 6 g/day, and 70% of the adult population (80% men and 58% women) had a daily salt intake above the recommended level.[14] Therefore, continuing and much greater efforts are needed to achieve further reductions in salt intake to prevent the maximum number of stroke and IHD deaths.

Reference 14 doesn't justify the 6g/day level, it's just a measurement of sodium levels. The authors don't make any reference I can see to why the recommended level should be 6g/day and not (say) 10g/day or 3g/day. If you're appealing to magic figures in your conclusion it doesn't give great confidence in the rest of your article.