Thousands of studies have explored the effects of sodium on health: comparing sodium consumption and health across countries (ecological studies), retrospective studies comparing individuals with heart disease or stomach cancer with healthy subjects (case-control studies), long-term follow-up studies (prospective observational studies), experiments in which one group of people consumed less sodium than another group (randomized controlled trials), and laboratory studies on the effects of sodium on cells and tissues. All these efforts have resulted in a broad consensus on the connections between sodium and health.
Salt and Cardiovascular System
The vast majority of research conducted on the links between sodium intake and health has focused on high blood pressure, stroke, heart disease, and heart failure. Most studies show that blood pressure increases with the amount of sodium in the diet, and that reducing sodium lowers cardiovascular diseases and long-term premature mortality. Several meta-analyses (statistical summaries of study results) have been conducted on studies regarding salt intake and blood pressure. The largest and most recent of these was conducted by a team from the University of Naples Medical School in Italy and the University of Warwick in England (1). They pooled the results of 13 cohort studies (studies involving two different groups of individuals) that included 177,025 men and women who were followed between 42 months to 19 years. During the follow-up, over 11,000 participants experienced a heart attack or stroke, developed another form of cardiovascular disease, or died from cardiovascular disease. Higher salt intake was associated with a 23% increase in strokes and a 14% increase in heart disease.
Salt and Bones
The more salt you consume, the more calcium the body excretes through urine. If you lack calcium, the body supplements its needs by drawing from the bones. Therefore, a high-sodium diet could have an additional adverse effect, namely bone weakening, better known as osteoporosis (2). A study in postmenopausal women showed that hip bone density loss over two years was more related to urinary sodium excretion than to calcium intake (3). Other studies have shown that reducing salt intake promotes a positive calcium balance (more available calcium than needed), suggesting that reducing salt intake could slow down age-related calcium loss from bones.
Ways to Act
It is certain that the abundance of salt in our diet is a silent killer, responsible for thousands of deaths each year. The real question is what can be done to help people reduce their consumption. The answer is simple. It is necessary to implement concerted action among four groups of stakeholders with divergent interests: individuals, healthcare professionals, agri-food companies, and governments. Public willingness can have substantial effects on the salt content of foods.
In Finland, a government-led information program, a law on labeling salt and sodium quantities, and pressure on the agri-food industry have led to a 30% reduction in generally observed salt intake (which has decreased from 12 to 9 g/day).
In the United Kingdom, a partnership between a scientific action group called Consensus Action on Health and Science (CASH) and the British Department of Health led to a 20 to 30% reduction in the salt content of processed foods sold in supermarkets.
In the United States, a study conducted by the RAND Corporation estimated what would happen if average daily sodium consumption reached 2.3 g. According to this model, this change would prevent 11 million cases of high blood pressure and save $18 billion in medical costs each year (4).
Reductions of this magnitude in less developed countries, where excess salt is a real problem, could have an even greater impact.
Sources: Harvard School of Public Health. References: (1) Strazzullo P, D’Elia L, Kandala NB, Cappuccio FP. Salt intake, stroke, and cardiovascular disease: meta-analysis of prospective studies. BMJ. 2009; 339:b4567. (2) He FJ, MacGregor GA. A comprehensive review on salt and health and current experience of worldwide salt reduction programmes. J Hum Hypertens. 2009; 23:363-84. (3) Devine A, Criddle RA, Dick IM, Kerr DA, Prince RL. A longitudinal study of the effect of sodium and calcium intakes on regional bone density in postmenopausal women. Am J Clin Nutr. 1995; 62:740-5. (4)Palar K, Sturm R. Potential societal savings from reduced sodium consumption in the U.S. adult population. Am J Health Promot 2009; 24:49-57.