Salt is a molecule that makes your body retain water. As such, consuming excess amounts of salt can increase the amount of water in your body, leading to a modest amount of weight gain. Eating less salt will produce a modest amount of weight loss. However, eating a healthy diet -- beyond simply consuming less salt -- and exercise are more effective means of losing weight, and salt reduction has more benefit for controlling high blood pressure than it does for weight loss.
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About Sodium Chloride
Sodium chloride -- which is the main ingredient of table salt -- performs several essential functions in your body. For example, it is required to maintain the appropriate fluid balance in your circulatory system. It is also required for the appropriate function of muscles and nerves. However, too much salt can be bad for your health, leading to high blood pressure and a modest amount of weight gain. Most Americans get more than enough salt in their daily diets; a normal, healthy person should consume less than 2.3 grams of sodium per day, recommends the Institute of Medicine.
Salt and Body Weight
Salt affects the volume of fluid in your circulation because it is an "osmotically-active" molecule -- that is, it is a molecule that pulls water to it. Generally speaking, the more salt you consume, the more water you will have to consume so that you don't dehydrate your tissues. This extra water is needed to hydrate the salt, and it hangs around in your circulation -- and elsewhere -- as extra body weight.
Salt and Weight Loss
While there is a dearth of studies specifically examining the effects of salt reduction on weight loss -- most studies examine the combined effects of sodium restriction and weight loss on high blood pressure -- from the preceding discussion it should be clear that salt reduction, in theory, can cause temporary weight loss, as your body fill flush out excess fluid. However, if you want to keep off the weight you've lost by reducing your sodium intake, you will have to keep your sodium intake low indefinitely, as your body will start to retain water again once your sodium intake increases.
Sodium and Weight Gain
Sodium itself won't cause you to gain weight -- aside from a temporary water weight increase -- but many salty foods also contain lots of calories and large amounts of fat, and this will contribute to weight gain. Sodium-laden fast food, for example, can contain an entire day's worth of calories in one meal, while salty packaged meals and snack foods come loaded with calories, but generally offer little nutrition value and don't keep you full for long. Choose whole, unprocessed foods -- not only are they generally low in sodium, they're also less calorie-dense than processed food.
Other Health Benefits
Although sodium restriction likely isn't the best method of achieving weight loss, it does have at least one other very important health benefit. A low-sodium diet helps protect against high blood pressure, a condition that can lead to heart disease and stroke, and is a major contributor to early death in the United States.