Does Dietary Salt Promote Obesity?

 

Evidence is accumulating that links an increasing consumption of dietary salt, especially within the context of a high-fat modern diet with increased calorie intake and weight gain (1).

Dr. Bolhuis and colleagues investigated the effects of both dietary fat and salt content of meals on ad libitum calorie intake. They also examined the effects of fat taste sensitivity on satiation responses to dietary salt. These researchers demonstrated that increasing both dietary salt and/or fat intake promoted the passive overconsumption of total calories in human subjects. The results of their research is shown in Figure 1 below.

Figure 1: The effects of fat and salt content of meals on total ad libitum energy intake; Figure shows the average kcal per meal (Bolhuis DP. J Nutr. 2016).
Figure 1: The effects of fat and salt content of meals on total ad libitum energy intake; Figure shows the average kcal per meal (Bolhuis DP. J Nutr. 2016).

Dr. Dieuwerke Bolhuis and colleagues concluded that their results showed that increasing the dietary salt content of a meal promotes the passive overconsumption of energy in human subjects. Increasing the fat content of a meal increased ad libitum calorie intake more than increasing the salt content, both increased dietary salt and fat led to greater energy intake. They also showed increasing dietary salt content in a fatty meal may override fat-mediated satiation in individuals who are more sensitive to the taste of fat and so less prone to passively over consume high-fat meals. Clearly it is the combination of high-fat calorie dense meals also high in salt that is most likely to promote overeating in the short term (2). If diets higher in salt and fat promote overeating, this may help explain how the typical modern diet is promoting weight gain, insulin resistance, and type 2 DM. A recent study in mice showed that overeating a calorie dense diet reduced the production of a satiety hormone (urogaunylin) normally released by the small intestine. Without this satiety hormone the mice became even more prone to overeating and weight gain and this was true of mice of a normal weight as well as obese mice (3). So if diets higher in salt and fat promote overeating, this latest data in mice suggests that overeating would beget more overeating.

The prevalence of obesity and particularly type DM have been increasing dramatically in recent decades in several Asian countries as they adopt a more typical modern calorie dense more Western-style diet. Traditional Asian diets, while high in salt, were not associated with a high prevalence of obesity or type 2 DM. However, as Asian populations move out of rural areas and into cities and adopt a more typical high-fat Western diet, they have been experiencing a marked increase in the prevalence of overweight people and an even more marked increase in the prevalence of type 2 DM. India, China, and South Korea have all seen a marked increase in average BMI and type 2 DM, especially in their urban populations. There seems little doubt that the typical modern diet high in fat and refined carbohydrates is somehow promotes excessive calorie intake, insulin resistance, obesity, and type 2 DM. However, because the salt content of traditional Asian diets has long been high there was little reason to blame the weight gain on dietary salt intake.

How Salt Promotes Overeating

The mechanism by which increasing the salt content of meals promotes increased calorie intake and weight gain was evaluated in a study by Dr. Young Zhang and colleagues. Their subjects were a group of 38 rural non-obese, healthy Chinese subjects 25 to 50 years old. They measured the level of ghrelin in the blood of their subjects after a 3 day baseline period where the subjects consumed their normal diets. Ghrelin is known as the “hunger hormone” as elevated levels have been shown to increase both hunger and ad libitum energy intake. Dr. Zhang and colleagues found their subjects had an average sodium intake on their usual baseline diet of 4043mg. They were then fed a similar isoenergetic diet with either a low (2272mg) or high (6164mg) sodium content for two 7 day periods. Not surprisingly these normotensive subjects saw their BP increase on the high-salt diet and drop on the low-salt diet. A 24 hour urinary sodium measurement confirmed compliance with both the low-salt and high-salt experimental diets. As seen in Figure 2, fasting ghrelin levels averaged 192.4pg/ml on their normal salt baseline diet. Ghrelin levels dropped modestly but significantly to 172.9pg/ml on the low-salt diet. However, ghrelin levels then nearly doubled up to 320.7pg/ml on the high-salt diet.

Figure 2. The effect of low-salt and high-salt intake on fasting ghrelin in all subjects.
Figure 2. The effect of low-salt and high-salt intake on fasting ghrelin in all subjects.

Dr. Zhang and colleagues concluded: “Our data indicate that a high-salt diet elevates fasting ghrelin levels in healthy human subjects, which may be a novel underlying mechanism of obesity” (4). Other research has suggested increasing dietary salt may promote obesity by enhancing the pleasure of eating. Increased salt intake promotes increased thirst and there is strong evidence linking an increased consumption of beverage calories to weight gain.

Bottom Line: More research is needed to further elucidate how added dietary salt may be contributing to overeating, weight gain, insulin resistance, and type 2 DM. However, given the overwhelming evidence linking increased salt intake with the development of hypertension and cardiovascular disease, there is no reason not to make it clear that most people would be far better off limiting their sodium intake to no more than about 1500mg/day, which is what the American Heart Association now recommends for all Americans (5). And if added dietary salt is another component of the typical modern Western diet promoting overeating and weight gain, then we will have yet another reason for the FDA to reconsider classifying added dietary salt as "generally recognized as safe".

By James J. Kenney, PhD, FACN

References:

  1. Lee, M, Kim MK, Park H, et. al. Gender-based differences on the association between salt-sensitive genes and obesity in Korean children aged between 8 and 9 years. PloS ONE 2015:10:e0120111 and Yoon YS, Oh SW. Sodium density and obesity; The Korean National Health and Nutrition examination Survey 2007-2010, Eur J Clin Nutr. 2013;67:141-6.
  2. Bolhuis DP, Costano A, Newman LP, Keast RSJ. Salt promotes passive overconsumption of dietary fat. J Nutr. 2016;146:838-45. Or http://jn.nutrition.org/content/146/4/838.abstract.
  3. http://www.nature.com/nutd/journal/v6/n5/full/nutd201618a.html
  4. Zhang Y, Li F, Liu F-Q, et al. Elevation of fasting ghrelin in healthy human subjects consuming a high-salt diet: A novel mechanism of obesity? Nutrient available at: http://www.mdpi.com/2072-6643/8/6/323
  5. http://www.heart.org/HEARTORG/Conditions/HighBloodPressure/PreventionTreatmentofHighBloodPressure/Shaking-the-Salt-Habit_UCM_303241_Article.jsp#.V3A9X3lTE8E
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