Many people know too much salt in their diet is a bad thing. Not nearly as many know exactly why.

“They’re surprised at the degree to which it can affect them,” said Dr. Cheryl Laffer, a professor of medicine at Vanderbilt University in Nashville, Tennessee. “And at the amount of salt that there is in the American diet.” According to the Centers for Disease Control and Prevention, about 90% of Americans
age 2 and older eat too much sodium. Most of it is in the form of salt, also known as sodium chloride.

Here are six things salt does to the body.

Let’s start with the heart. With the circulatory system, salt’s effects are “a very simple plumbing problem,” said Dr. Fernando Elijovich, a professor
of medicine at Vanderbilt University. The heart is the pump and blood vessels are the pipes, he said. Blood pressure goes up if you increase how much blood
has to move through the pipes. Blood pressure also rises if you shrink those pipes. Salt does both. When there’s excess salt in your system, the heart pumps more blood in a given time, boosting blood pressure. And over time, salt narrows the vessels themselves, which is the most common “plumbing” feature of high
blood pressure.

The harm can come quickly. And over time. Within 30 minutes of eating excess salt, your blood vessels’ ability to dilate is impaired, Elijovich said. The damage from persistent high blood pressure shows up down the road, in the form of heart attacks, strokes and other problems.

The good news, Laffer said, is the benefits of cutting back on excess salt also show up quickly. If you significantly reduce how much salt you eat, your blood pressure goes down within hours or days. And keeping it low can make a significant long-term difference.

“In the U.K., they actually had a nationwide effort to reduce salt in commercial foods,” she said. “Within a couple of years, they had reduced the numbers of heart attacks and other bad outcomes. And that was pretty striking.”

It’s a whole-body issue. Beyond the heart, excess salt can strain the kidneys. Part of their function is to excrete salt, Laffer said. “But the kidneys, in hypertension, may not excrete salt appropriately. They may hold onto it.” That can lead to problems ranging from swollen ankles to fluid buildup around the heart and lungs. Salt also can threaten the brain by damaging blood vessels and raising blood pressure, which is a major risk factor for stroke. It also might change the behavior of the brain stem, which helps regulate salt balance and blood pressure.