Socioeconomic Inequalities in Dietary Sodium Intake: Upstream Versus Downstream Interventions

Noncommunicable disease (NCD) prevention strategies now prioritize four major risk factors: tobacco use, alcohol use, lack of physical activity, and, crucially, poor diet.¹ Dietary sodium intake in high-income countries is on average double the World Health Organization’s maximum recommended intake of two grams per day. Excess sodium intake increases the risk of high blood pressure, cardiovascular disease, and gastric cancer. Might higher sodium intake also partly explain disease inequalities, that is, the increased high blood pressure and greatly increased cardiovascular disease burden typically observed in deprived groups?

ULTRA-PROCESSED “JUNK” FOOD

In this issue of AJPH, de Mestral et al. (p. 563) report on a meta-analysis and systematic review of population-based studies seeking to quantify the social patterning of sodium intake among healthy adults in high-income countries. The review included 51 studies that assessed 171 associations between socioeconomic status and sodium intake. Among urine-based estimates, 67% demonstrated higher sodium intakes in people of low socioeconomic status; 30% showed neutral results, and only 3% bucked the expected trend. A similar but less consistent relationship was seen in studies in which (less satisfactory) diet-based estimates of sodium intake were used.

In their meta-analysis, de Mestral et al. used a random-effects model that estimated a 14% standardized mean relative difference between groups of low and high socioeconomic status. This difference equated to a sodium intake that was approximately 500 milligrams per day higher among people of low socioeconomic status than among those of high socioeconomic status.

The authors concluded that these findings

[confirm] the current evidence on socioeconomic disparities in diet, which may influence the disproportionate noncommunicable disease burden among disadvantaged socioeconomic groups.^(p563)

The included studies came mainly from the United States, Europe, and Australasia. The primary source of dietary sodium in these countries is clear and consistent: ultra-processed “junk” food high in sodium (and often also sugar and saturated fats). This systematic review therefore adds further evidence helping to explain why Westernized diets are so consistently associated not only with a higher risk of disease and premature death from a wide range of NCDs but also with an unequal disease burden.

UPSTREAM STRATEGIES

Substantial population reductions in sodium intake are therefore urgently needed. However, debate continues about the most effective approaches. We recently systematically reviewed the evidence on possible interventions to inform future prevention programs.² We included 70 studies in our systematic review (49 empirical studies and 21 modeling studies). “Downstream” interventions such as individual approaches and worksite or community interventions generally produced weak results.

The largest population-wide reductions in sodium consumption were achieved in Finland, Japan, and the United Kingdom via comprehensive “upstream” strategies involving multicomponent population-wide policies. These comprehensive strategies typically included reformulation of processed foods, food labeling, and media campaigns. Not surprisingly, mandatory reformulation appeared consistently more effective than voluntary reformulation or public–private partnerships.³

This “effectiveness hierarchy” emphasizing powerful upstream policy approaches surely deserves greater attention in future NCD prevention strategies, with correspondingly less emphasis on weaker downstream, individually based interventions. Argentina and South Africa are among the few but growing number of countries that implement mandatory sodium reformulation policies.

DOWNSTREAM STRATEGIES

De Mestral et al. also note that downstream, individually focused interventions tend to widen dietary inequalities between affluent and deprived groups.^4,5 Kypridemos et al. recently attempted to quantify this effect.⁶ They estimated the potential effectiveness and equity of the United Kingdom’s sodium reduction strategy, which included sustained political pressure on industry to achieve target reductions in every food group, media campaigns, and food labeling, via a validated policy model called IMPACT_NCD. The model suggested that although the strategy may have prevented or postponed approximately 57 000 cases of cardiovascular disease and gastric cancer in the English population (approximately 55 million individuals) since 2003, it has failed to reduce and may have increased existing socioeconomic inequalities in those diseases.

By contrast, addition of mandatory sodium reformulation to the existing strategy could further prevent or postpone approximately 20 000 cardiovascular disease and gastric cancer cases by 2030. Crucially, mandatory reformulation would also achieve larger disease reductions in the most deprived groups, thus narrowing inequalities.⁶

COST, ACCESS, AND KNOWLEDGE

Dietary choice is thus not only behavioral; it is also economic. For deprived individuals, energy-dense junk food is generally cheaper, more available, and more convenient than healthier options. Conversely, in more affluent groups, compliance with dietary guidelines (including avoiding sodium) may be facilitated by better access to diverse resources, including money, time, and cooking skills. Thus, preventive interventions may not benefit all population subgroups equally.

Moreover, evidence remains sparse concerning the potential health equity impact of dietary interventions with respect to promoting health. Differing responses to universally delivered interventions might result in differences in intervention effectiveness.

McGill et al. systematically reviewed 36 studies on interventions designed to reduce inequalities in healthy eating in an effort to determine whether intervention effects differed according to socioeconomic status.⁵ They categorized 47 interventions via an adapted “4 P” (price, place, promotion, product—expanded to include person) framework. Most of the price (e.g., taxes, subsidies, economic incentives) and place (e.g., vending machines in schools) interventions reviewed showed evidence of reduced inequalities. Conversely, inequalities were widened in a third of the person-based interventions (e.g., involving health education or dietary counseling). Very few of the studies focused on product (e.g., decreasing salt content) or promotion (e.g., implementing mass media public information campaigns) interventions.⁵

“NANNY STATES” CAN BE HEALTHIER

Upstream policy interventions designed to reduce sodium intake (or tobacco use, alcohol use, or exposure to other environmental hazards) may thus be more powerful, more equitable, and more cost-effective (or even cost-saving) than downstream interventions.² Yet, regulation and taxation are also more politically difficult and may be criticized as promoting a “nanny state”; however, the nanny state, rather than being interfering and intrusive, is in fact a powerful factor for good.⁷ Indeed, in today’s society, the very foundations of a safe and healthy environment for our children and families, including clean drinking water, unpolluted air, car seatbelts, safe airplanes, immunizations, and smoke-free environments, are underpinned by regulation and legislation.⁷

In summary, the de Mestral et al. systematic review provides further evidence that achieving the World Health Organization’s target of individual sodium intakes below two grams per day could potentially offer great societal benefits, particularly to those who are most disadvantaged.