Studies Look at Sodium's World Health Impact

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Studies Look at Sodium's World Health Impact

News Review from Harvard Medical School

August 14, 2014

News Review From Harvard Medical School -- Studies Look at Sodium's World Health Impact

Two new studies add to the discussion about salt and its role in high blood pressure and heart disease. In one study, researchers used data from 66 countries to estimate sodium consumption by age, sex and country. They concluded that the average person consumed about 4,000 milligrams (mg) or 4 grams of sodium daily. They also put together results of more than 100 prior studies. These studies looked at the effects of sodium on blood pressure, and blood pressure on heart disease, stroke and death. Researchers also looked at causes of death in these countries. They concluded that cutting average sodium intake in half would reduce deaths by 1.65 million per year. In the other study, researchers obtained first morning urine samples from 101,945 people. They were from 17 countries. The samples were used to estimate 24-hour sodium  levels in urine. The average was nearly 5,000 mg. In the next 3.7 years, 3.3% of those in the study died or had a heart attack, stroke or heart failure. These outcomes occurred more often in people with sodium levels higher than 6,000 mg or lower than 3,000 mg. The New England Journal of Medicine published the studies August 14. HealthDay News and other media wrote about them.  

 

By Robert H. Shmerling, M.D.
Harvard Medical School

 

What Is the Doctor's Reaction?

Do you regularly add salt to your food? Do you worry about people who do? Perhaps you're on a low-sodium diet on the advice of your doctor or because of something you read in the news. 

My guess is that most people think little about sodium on a daily basis. There's a salt shaker in most kitchens, and salt is a staple of many recipes. It's a simple and essential part of our diet. Yet there is a lot of confusion about how much is too much, how much is too little and how much difference it makes.

There's even confusion about the terms, "salt" and "sodium." What we think of as table salt consists of sodium chloride. It accounts for about 90% of sodium we consume. So when we talk about sodium in our food and its impact on health and disease, we can use the terms "salt" and "sodium" interchangeably.

Guidelines suggest that the average adult consume no more than 2,300 milligrams (mg) of sodium daily. That's the amount in just one teaspoon of salt. Guidelines suggest that many people limit salt intake even more. Yet the average American currently consumes more than 3,400 mg each day. Too much sodium can cause your body to hold on to (retain) fluid. This can increase blood pressure.

Two new studies in the New England Journal of Medicine come to somewhat different conclusions about salt intake. In one, researchers conclude that eating too much sodium may contribute to millions of preventable deaths.

Researchers combined data from more than 100 prior studies regarding sodium intake in 66 countries. They estimated the impact of sodium intake on blood pressure and on deaths from heart and blood vessel disease. The researchers estimated that:

  • Worldwide, average sodium intake was nearly 4,000 mg daily
  • There would be 1.65 million fewer deaths per year worldwide if average sodium intake was closer to 2,000 milligrams daily
  • About 40% of deaths attributed to excess sodium intake occur in people younger than age 70
  • Reducing sodium intake to recommended levels would prevent about 10% of deaths related to heart and blood vessel disease

Although these are only estimates, the numbers are impressive. High blood pressure (hypertension) can be treated. It is among the most modifiable factors that increase the risk of heart and blood vessel disease. And these diseases are among the leading causes of early death worldwide. So it's hard to ignore these findings.

But another study in the same journal found that the picture may be more complicated. It suggested that there may also be risks linked with too little sodium.

The study included more than 100,000 people from 17 countries. Those with the middle range of sodium intake consumed an estimated 3,000 to 6,000 mg each day. People in this group had lower rates of death and heart attack, heart failure or stroke than those with higher or lower intake.

The American Heart Association and other experts recommend that most Americans eat less salt than the middle range in this study. So how can lower amounts of sodium be riskier than the higher amounts that most people now consume? One reason could be that people with high blood pressure or other reasons for a high risk of heart disease are usually advised to eat less salt. Their higher-than-average rates of heart disease and related deaths may then be erroneously linked to their lower salt intake. So I think it's too soon to throw out current advice to eat less salt. 

 

What Changes Can I Make Now?

Most U.S. adults fall into a group that should limit sodium intake to 1,500 mg daily, according to current guidelines. You should restrict your sodium intake to this low level if you:

  • Are older than age 50
  • Are African-American
  • Have high blood pressure
  • Have chronic kidney disease
  • Have diabetes
  • Have heart failure

Limiting sodium intake can help people in these risk groups prevent or control high blood pressure. Consuming less sodium can help avoid "fluid overload." This means that the body is unable to remove some fluid and therefore holds on to it.

Most sodium in the U.S. diet comes from added salt. So preparing or choosing low-salt foods can make a big difference in how much sodium you consume. There are many ways to reduce sodium in your diet. Consider these ideas:

  • Read labels. Choose foods marked as "low sodium," "reduced sodium" or "no salt added."
  • Avoid processed foods. Most fresh fruits and vegetables are naturally low in sodium.
  • Season your food with less salt. Choose low-salt or no-salt seasonings and spices.

The sodium content of many foods may surprise you. For example, high sodium levels are often found in:

  • Soy sauce
  • Ketchup
  • Breads
  • Cold cuts and many other meats
  • Pizza
  • Cheese
  • Snacks such as pretzels or chips

The impact of lowering dietary salt is significant -- and fast. When salt intake is reduced, blood pressure can begin to fall within a few days.

 

What Can I Expect Looking to the Future?

These new studies are likely to fuel debate about advice to eat less salt. Already, media outlets are presenting opposing opinions from heart disease experts.

I think the scientific evidence showing the hazards of high salt intake is difficult to ignore. I believe that in the future salt consumption will fall in the United States and in other places where it is high. To understand just how much salt is too much, and too little, we need long-term clinical trials. These studies would compare people placed on diets containing varying amounts of sodium. It's likely that the ideal amount will vary for different groups of people.

Craving salty foods is learned. And it can be "un-learned." It takes time to get used to foods that are less salty -- but for many, I think it's worth the effort.

Last updated August 14, 2014


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