Too much focus on low-quality science?

The controversy concerning dietary sodium results primarily from low-quality studies and their commercial marketing and promotion.1^–5 Low-quality studies do not adequately assess sodium intake, they use extreme variation in dietary sodium and they measure outcomes over a duration of a few days. They do not address known confounding factors for the outcomes being tested nor do they control for blood pressure (the main mechanism of sodium-induced harm) and they are conducted in populations with diseases where reverse causality is likely. Such poor-quality studies are often promoted and occasionally conducted by consultants of the Salt Institute (an umbrella organization of the salt industry). The publication of such studies, leveraged by the private sector, has created a false aura of scientific controversy around dietary salt.1^–5

Although the call for a large randomized controlled trial on dietary sodium is not new, it has limited feasibility in Western countries where the food supply contains so much sodium.6 In Africa, where some populations still have low sodium intake, it was deemed unethical to increase dietary sodium in a trial setting. In China, where sodium added during cooking is a major source of dietary sodium, a large randomized controlled trial with a salt substitute is underway, but results will be confounded by very high baseline sodium intake and the need to use a salt substitute with potassium (a beneficial nutrient). In other countries, extensive dietary advice and support, when used alone, has proven ineffective at substantially lowering dietary sodium over the long haul.7 Hence, a large trial based on advice alone is unlikely to lower sodium intake, let alone show changes in outcomes.

It is important to also consider the World Health Organization (WHO) forum and technical meeting, “Reducing salt intake in populations,” discussed by MacLeod and Cairns.8 The WHO forum was developed around controversial new evidence from the PURE study, which categorized an individual’s long-term sodium intake based on a single “spot” (fasted first morning) urine sample.9^,10 This method is widely recognized as inadequate to assess a person’s usual sodium intake, would not meet the minimum study quality criteria of blood-pressure studies for inclusion in the WHO evidence review11^,12 and is therefore unlikely to have a bearing on dietary sodium recommendations.13^,14

It is concerning that the PURE validation study for using spot urine samples was fraught with methodologic issues that could inflate the perceived utility of such samples.5^,9 The content and topics of the WHO forum were structured around several presenters with known conflicts of interest and close industry involvement. Although a “balance” of scientists representing public health and scientific organization views were later invited to the WHO forum, they were not asked to contribute to MacLeod and Cairns’ article.8

The conclusion that dietary sodium reduction is controversial was announced in WHO advertising before the forum program was even finalized, resulting in the withdrawal of several invited speakers. Organizers did not respond to a call for public disclosure on the distribution and use of industry funds raised. Commercial sponsorship by the food and beverage industry of food-policy meetings has been viewed as a public health threat.15

That a small group of dissident scientists, most of whom have conducted weak and flawed research, and a few scientists with long histories of working with food and salt industries disagree is not a surprise. That MacLeod and Cairns’8 article caters to this small conflicted group is a concern as it provides undeserved credence and endangers public health.