The truth about salt

Mayor Bloomberg is a nanny. Or at least a “food nanny” according to The New York Post. The Post has lambasted Bloomberg for started a campaign to lower salt consumption among New Yorkers. The newspaper also produced this reflux-inducing photo of Bloomberg as a reincarnation of Mrs. Doubtfire:

Restaurant owners have contributed to further criticism of the mayor, joining a new coalition named “My Food, My Choice” to oppose the regulation of salt levels in food. Their claim: that consumers can choose how to regulate their own salt intake (it’s a matter of personal freedom!), and moreover, that—according to quoted medical experts—there’s no strong evidence for any public health benefits from sodium reduction.

In this week’s blog, we revisit the data on salt and health—and ask ourselves whether Mayor Bloomberg looks hot enough in a dress to take away our saltshakers.

“Choosing” salt

Do most consumers have the “freedom” to avoid salt? That is, can we easily cut down on our daily salt intake?

It turns out that the average American consumer doesn’t actually add much (if any) salt to their diet. Saltshakers are really a vestige of the 1950’s. Today’s American consumer ingests most of her salt through prepared foods. What’s the key source? Pizza slices? TV dinners? Restaurants? Angelina?

Actually, the answer is bread. Just one slice of bread contains up to half a gram of salt, and most people fulfill at least half of their daily value of sodium with just one sandwich. The average American eats more bread that meat. That consumer could choose to substitute bread for vegetables, lets say (making for a sloppy peanut-butter meal between two slices of tomato). But the problem is cost: fresh produce like a single roma tomato costs about fifteen times more than processed foods like two slices of bread, and disposable income to buy food has decreased dramatically. Absolute wages for working Americans have has dropped by 6% since 1972 without accounting for the additional impact of inflation, even though the work time and productivity of the typical worker has increased by 114% since that time.

What’s even more concerning is that salt has properties that appear addictive. Back in the 1990’s, the salt industry admitted that a major justification for the use of salt in processed food is that “salt makes unpalatable food edible.” New evidence suggests that high salt diets tend to up-regulate receptors that make low-salt foods seem unpalatable until receptors have an opportunity to down-regulate over the course of about one month of eating low-salt foods. It’s as if we can conceptualize salt in processed foods the same way we think of nicotine in tobacco products.

So perhaps “choosing” a low salt diet is not easy for the cash-strapped, or for those accustomed to the flavor of a high-salt diet. But if we follow mayor Bloomberg to reduce the salt content of food, will the effort make any difference to health outcomes?

The data on salt and health

What do we really know about the link between salt and health? While there’s a variety of data about salt’s relationship to cancer, osteoporosis, and asthma, the majority of studies to date have focused on salt’s relationship to blood pressure, which in turn affects the risk of strokes and heart attacks. When we looked for the data, we found three common themes concerning the impact of salt on health:

[1] Salt is critically related to blood pressure elevation: There have been three kinds of evidence that support this claim. “Natural experiments” are the oldest, and perhaps the most interesting. Two rural villages in Nigeria were studied decades ago because both were inhabited by the same tribe of subsistence farmers and consumed essentially the same diet. The only exception was that one of the villages had access to salt from a nearby lake; blood pressure and sodium intake were significantly higher in that village. Subsistence farmers in rural Kenya were similarly studied; they had consumed a low salt diet before they migrated to a city, where salt intake increased to levels seen in Western countries, and the group’s blood pressure rose to significantly higher levels than in a control group who did not migrate. A more carefully-controlled six-month double blind study in newborn babies also revealed that those with a modestly restricted salt intake had a significantly lower blood pressure than those with a normal sodium intake. The babies were followed for 15 years, and there remained a significant difference in blood pressure among them after adjusting for social and demographic variables. In a review of data from published reports of 24 different communities (47,000 people) around the world, a difference in sodium intake of 100 mmol/24 h was associated with an average difference in systolic blood pressure that ranged from 5 mm Hg at age 15-19 years to 10 mm Hg at age 60-69. These studies in humans have been further investigated in highly-controlled animal studies. Increasing salt intake among chimps to the levels consumed by Americans increased the chimps’ blood pressure levels by an average of 30 mmHg over the course of a year, in the absence of any other environmental changes. When the salt intake was reduced in the chimpanzees’ diet, blood pressure fell to previous levels.

[2] Blood pressure is strongly related to cardiovascular outcomes: Elevated blood pressure has been found to be a major risk factor for stroke and ischemic heart disease. Studies of individuals at high risk of cardiovascular disease have demonstrated large mortality reductions from reducing blood pressure even in the “normal” (average) blood pressure range.

[3] Salt intake is observed to directly relate to the likelihood of cardiovascular death: A random sample of the Finnish adult population showed that salt intake is related to increased cardiovascular mortality and total mortality. Salt intake was a strong statistical determinant of coronary artery disease, cardiovascular deaths and total mortality. The results from the Dietary Approaches to Stop Hypertension (DASH)-Sodium Trial showed that the most substantial benefit in reducing systolic blood pressure was gained from reducing sodium intake from 2.3 g to 1.5 g per day. Attaining this lower level of intake on a population basis would require that sodium in processed and restaurant foods be lowered an average of ~80%. A review of data from 68 crossover trials and 10 randomized controlled trials of dietary salt reduction had similar results: in people aged 50-59 years, a reduction in daily sodium intake of about 3 g of salt lowered systolic blood pressure by an average of 5 mm Hg, and by 7 mm Hg in those with high blood pressure (170 mm Hg). (Some studies, often written by those funded by the salt industry, have tried to contest this finding, but have been deeply flawed by mis-sampling and small sample sizes.)

How useful is this seemingly-small change in blood pressure? It’s estimated that this level of salt reduction by Western populations would reduce strokes by 22% and ischemic heart disease by 16%. The effect would be larger than what could be achieved by treating all hypertensive people with medications; the reduction would lower blood pressure by at least twice as much as medical treatments in the British population, for example. When the results of these trials are projected onto the US population by mathematical models, it’s predicted that reducing salt by 3 gm/day would result in as many as 66,000 fewer strokes and 99,000 fewer heart attacks, saving up to $24 billion in healthcare costs annually. Such an intervention would be cost-saving even if only a modest 1 gm/day reduction were achieved gradually over the decade from 2010 to 2019, and would be more cost-effective than treating all hypertensive individuals with medications.

Paths for intervention

The data on salt’s heart impact have been compelling enough for at least 32 countries to take on major salt reduction initiatives. Of these 32, at least 28 include consumer education efforts and efforts to work with industry, 10 include introducing new food labels (such as “traffic signals” with a red light indicating high-salt foods), and 2 include mandatory food-content regulations to provide a maximum salt level in various foods.

To date, the EU has aimed for a 16% reduction in salt levels of processed foods among Europeans over the next 4 years. The UK and New York City have, meanwhile, set individual targets for about 85 different food categories.

Among the efforts to date, five countries have demonstrated an impact on salt consumption, food salt levels, or consumer awareness from their salt reduction campaigns. Finland started its campaign for salt reduction as far back as 1978, setting population salt intake targets, monitoring salt intake through surveys of the population’s urine salt levels (what an effort!), and promoting mass media education. The Finnish food industry replaced salt en masse with a sodium substitute and issued warning labels on food products. By 2002, Finland had achieved a 3g reduction in its average population salt intake (from 12 to 9g/person/day) and a 60% decline in coronary heart disease and stroke deaths.

The Finnish experience has not been replicated with as much success in other countries that have undertaken salt reduction, mostly because the other countries are still in earlier phases of their campaigns. The UK’s Food Standards Agency started its campaign on salt in 2003, and by 2008, had achieved almost a 1g/person/day reduction in daily salt intake, which is thought to save 6000 lives per year. Salt levels have been reduced 25% to 45% in food products. Ireland has since followed suit, publicly praising food companies that participate in salt reduction while also publicly listing those who decline to participate. More subtle techniques have been used in France and Japan; the former has focused on salts in breads, and reduced average salt intake from 8.1 to 7.7 g/person/day. Japan has reduced strokes by 80% as a result of reducing salt intake, but unfortunately the lack of sustained government commitment in Japan has led to recent rises in salt consumption.

What these country-wide programs reveal is a striking mortality benefit of salt regulation. And to answer critics, there is no evidence that the salt reduction has resulted in food-borne illness, iodine deficiency, higher calorie consumption as a result of less-satisfying meals, or unsafe substitutions with other additives. That’s likely because the salt content in most processed foods is so high that the slight reductions called for in public health regulations have not significant modified food safety.

Advocacy options

In the US, the Food and Drug Administration launched a petition to revise the regulatory status of salt (currently in the ‘Generally Recognized as Safe’, or GRAS, status) and establish food labeling requirements. Members of the US branch of the non-profit group World Action on Salt (WASH) have submitted comments in response to the Food and Drink Association hearing regarding salt’s classification, hoping that salt will be more extensively regulated, and processed foods more clearly labeled for consumers. Voluntary  targets have already been set in the US for salt reduction in processed foods; the Institute of Medicine, a governing body of experts, has recommended that these targets be made mandatory.

The future of advocacy on salt may need to ensure that such targets are taken up by industry and made mandatory if not taken up voluntarily. But to be aware of progress on this front, we’ll also need knowledge: a clear effort to track the data on food content, and specify how the salt content of individual products and overall food types is being modified in the effort to control salt. In his nannying efforts, Mayor Bloomberg has established such targets for both processed foods and restaurants; perhaps other mayors should follow his Mary Poppins style to keep track of each spoonful of sodium.