The debate rages on over whether dietary salt (NaCl) increases the risk of cardiovascular events, with no clear answer in sight. Yet few people are paying attention to another, more insidious effect of salt: it may increase our calorie intake, and eventually, the size of our waistlines.
Humans are born with specific hard-wired food motivations, which guide us to food properties that kept our ancestors alive and fertile in times past. We have an instinctive attraction to sweetness because, in the world of our ancestors, it indicated ripe fruit or honey-- both important sources of calories and other nutrients. Most of the other food properties we're instinctively drawn to, such as starch, fat, and glutamate, signify high-calorie foods.
Yet one of our hard-wired food motivations stands out from the rest: our attraction to salt. Since salt is calorie-free, salt appetite is one of the few instinctive food drives that doesn't relate directly to acquiring calories. Interestingly, salt is the only essential micronutrient (vitamin/mineral) we can taste at the concentrations normally found in food. Not only our brains, but also our tongues, are hard-wired to seek salt above all other micronutrients.
Sodium and chloride are essential micronutrients for all animals, but not all animals find dietary salt appealing. For example, rats don't exhibit a salt appetite unless they've been deprived of salt intermittently (1). Yet humans are so drawn to salt that when there are no constraints on our behavior, we eat many times more of it than our hunter-gatherer ancestors did (presumably, as judged by modern and historical hunter-gatherers).
Although salt itself doesn't contain calories, it is a powerful reward factor. If you don't believe that, try eating food that contains no added salt for a day. This is just another way of saying that the human brain instinctively values salt. And when the brain places a high instinctive value on food, it tends to drive us to eat more of it. We know that applies to other food reward factors, but does it apply to salt as well?
To test the hypothesis, Dieuwerke Bolhuis and colleagues recruited 48 volunteers and fed them a standardized breakfast, followed by an experimental lunch of elbow macaroni and sauce. Volunteers were randomized to four different versions of the experimental lunch, and each person ate each version on a different day (2):
1) low-fat (0.02% fat, wt:wt)/low-salt (0.06% NaCl, wt:wt)
2) low-fat/high-salt (0.5% NaCl, wt:wt)
3) high-fat (34% fat, wt:/wt)/low-salt
Volunteers were allowed to eat as much as they wanted of the experimental lunch, and calorie intake was recorded.
As expected, volunteers served the calorie-dense high-fat pasta consumed more calories-- in fact, a full 60 percent more. This is consistent with previous findings that people tend to overeat foods that are calorie-dense.
Yet salt also increased calorie intake, by a smaller but still meaningful 11 percent. This held true in both the low-fat and the high-fat context.
This straightforward study adds to the evidence that food reward factors can increase calorie intake. Yet it also extends the evidence, showing that even non-caloric reward factors can indirectly increase calorie intake when they're added to caloric foods, by increasing the overall reward value of the meal.
The 11 percent difference in calorie intake may not seem like much, but keep in mind that 11 percent is approximately the difference in calorie intake between a lean person and an overweight person.
I don't know whether this effect would persist over weeks, months, and years-- which is what really matters for body weight. Hopefully, future research will address this. Yet judging by the ability of other types of food reward restriction to cause long-term weight loss, including low-carbohydrate, low-fat, and vegan diets, it seems plausible.
Despite countless studies and massive funding, we still don't have a very clear view of the role of dietary salt in human health. About the only thing researchers agree on is that very high intakes are probably harmful. The fact that so much controversy remains after so much research suggests to me that salt intake probably isn't a major determinant of health, at least cardiovascular health, which has been the primary focus. But in the absence of clear evidence, I tend to fall back on the evolutionary view, which suggests that it may not be a good idea to eat a quantity of salt that far exceeds what our ancestors would have eaten for nearly all of our evolutionary history.