May 1, 2017 -- Conventional wisdom has long held that salty foods boost our thirst and lead us to drink more water. But can salt also lead us to eat more, as well?
“Until now, we have always focused on the effect of salt on blood pressure,” says Jens Titze, MD, associate professor of medicine and of molecular physiology and biophysics at Vanderbilt University in Nashville. “We have to expand our conceptions of salt and diet.”
Salt and Space
Titze was the lead researcher on a new study challenging the notion that salty foods make us thirsty. Instead, he found that people who eat high amounts of salt actually drink less water than those who have smaller amounts of salt in their diet. They also become hungrier. Over the long term, that boost in appetite could lead us to overeat and gain weight.
For the study, published last month, Titze and his colleagues gained access to a unique group of subjects: Ten Russian astronauts -- or cosmonauts -- preparing for the rigors of space travel to Mars. The space flight simulation, which lasted for months, provided a stable environment for the researchers to study how salt affected them.
Throughout the study, the cosmonauts' diet did not vary except in one key way: The researchers changed the amount of salt in their food. The study subjects began on a diet that included 12 grams of salt per day. That’s about twice the amount recommended by U.S. dietary guidelines. After several weeks, researchers reduced their salt to 9 grams per day. The cosmonauts ate 6 grams of salt daily during the final third of the study period.
What happened over the course of the study upended the researchers’ expectations: The cosmonauts drank more water as their salt intake dropped.
“We simply could not understand it,” says Titze.
Titze describes another surprise. The cosmonauts complained of hunger while on the high-salt diet.
“We said you can’t be hungry, you’re getting the same amount of food,” says Titze. “The only thing that’s changed is the amount of salt.”
Salt and Our Health
Sodium, the main ingredient in salt, is an essential part of our diet, and not just for flavor. It keeps our muscles and nerves working properly, and it helps our bodies maintain the proper balance of fluids.
But when sodium levels rise too high, blood pressure often goes up as well. Over time, high blood pressure can have serious, life-threatening consequences. It can lead to stroke, heart attack, kidney disease, and other health problems.
To protect against high blood pressure, U.S. dietary guidelines recommend that we get less than 2,300 milligrams of sodium per day. That’s about a teaspoon of salt. According to the American Heart Association, the ideal target for most adults is no more than 1,500 milligrams daily.
Most adults, however, get far too much. The CDC estimates that the average American adult eats 3,400 grams every day.
In the last several years, scientists have begun to investigate whether salt affects obesity. Here’s what they have found:
- In 2015, British and Chinese researchers reported that body fat increased for children and adults on high-salt diets. Eating an extra gram of salt each day increased the risk of obesity in children by 28% and in adults by 26%. The study authors said they don’t know why salt has this effect, but other studies suggest that it may change the way our bodies burn fat.
- An Australian study published last year linked high-salt diets with a 23% increased chance of obesity in schoolchildren. Those children may eat more because the salt makes the food taste good, the authors suggest. They also speculate that when they get thirsty after a salty meal, the children reach for easily available high-calorie sodas.
- Another Australian study from 2016, led by Russell Keast, PhD, tied salt to an 11% rise in the amount of food and calories that adults take in. The authors say salt improves the flavor, and that likely tempts people to eat more.
Keast, a professor of food science and head of the Centre for Advanced Sensory Science at Deakin University said in an email that he believes salt encourages people to eat more.
While these studies show a link between salt and body fat, increased eating, and obesity, they don’t show that salt makes any of those things happen. More research needs to be done to fully understand salt’s role.
Lori Roman, president of the Salt Institute, said in an email that her nonprofit trade group “continues to follow the science closely as it develops.”
She says research shows that cattle ranchers use salt to cut their animals' appetites and limit how much feed they eat.
“This longstanding research and other research on humans would lead us to question any claims that salt might increase obesity,” Roman wrote. She added that Americans eat in the “normal range” when it comes to salt.
Salt and Mice
In the space flight simulation study, the authors did not understand why the cosmonauts drank less and became hungrier on the higher-salt diet, so they turned to mice to find out. This study revealed that when mice ate a high-salt diet, their livers produced a substance called urea, which helps keep the body's water in balance. But producing urea requires lots of energy, says Titze. In other words, it requires food, specifically protein. And that need could be what caused the astronauts’ hunger.
“The fact that they didn’t drink more but wanted to eat more was interesting,” says Vijaya Surampudi, MD, assistant professor of medicine and assistant director of the Weight Management Program at UCLA. “It means that there are mechanisms at work that we don’t yet understand.”
Mark Zeidel, MD, says the study raises important new questions and may shed light on what drives our appetites.
“What this study makes clear is that we need to better understand how things like appetite and thirst are controlled,” says Zeidel, a professor of medicine at Harvard Medical School and chairman of medicine at Beth Israel Deaconess Medical Center in Boston.
“The control of appetite is very, very complex.”
Future research will tell more about Titze's findings. In the meantime, Titze offers this advice: “If you’re on a diet and trying to reduce the amount of food you eat but you always feel hungry, start thinking of salt. Perhaps reducing it may help you.”
Cut Salt in Your Diet
Lowering how much salt you eat can be tough, says Lauren Blake, a registered dietitian at Ohio State University’s Wexner Medical Center. Here are her tips:
- Focus on whole foods and prepare them at home. Processed foods and restaurant meals have lots of added salt.
- Go easy on condiments like salad dressing, ketchup and soy sauce, which are loaded with sodium.
- Cut back on salt gradually so your taste buds can adapt. If you go cold turkey, your food will taste bland and unappetizing.
- Season your food with fresh or dry herbs, like garlic and black pepper. The more flavor you add, the less salt you’ll need.
- Try your food before you grab the saltshaker. You may not need to add more.