Saturday, January 21, 2012

Should we all cut back on salt—regardless of our blood pressure level?



Most of the sodium in our diet comes from salt, or sodium chloride. Sodium has many important functions in the body, including:
  • transmitting nerve impulses
  • contracting and relaxing muscle fibers
  • maintaining proper fluid balance.
But Americans get much more than they need—3,400 mg of sodium per day, on average. (See the chart below for recommended intake.)The kidneys regulate the body's sodium level by getting rid of any excess. But if there's too much sodium in the bloodstream, the kidneys can't keep up. Excess sodium in the blood pulls out water from the cells. As this fluid increases, so does blood volume. That means more work for the heart, increased pressure in the blood vessels, and often, eventually, stiffened vessel walls, chronic high blood pressure, and an increased risk of heart attack or stroke.
Current recommended intakes of sodium for healthy adults by age group
Group Adequate intake (AI) of sodium* Salt equivalent Upper limit (UL) of sodium intake**
Ages 19-50 1.5 g/1,500 mg 3,800 mg, or 23 teaspoon (tsp.) 2.3 g/2,300 mg (equivalent to 5.8 g/5,800 mg, or 1 tsp., salt)
Ages 51-70 1.3 g/1,300 mg 3,200 mg, or ~½ tsp. Less than 2.3 g, but a precise amount has not been determined
Ages 71 and over 1.2 g/1,200 mg 2,900 mg, or ½ tsp.
*The average amount needed to replace sodium lost daily through sweat while providing enough other essential nutrients.
**UL may be higher for people who lose large amounts of sodium in sweat, such as athletes and workers exposed to extreme heat.
Source: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, National Academies Press (2004).
Some people are especially sensitive to sodium—their blood pressure rises and falls directly with their sodium intake. That puts them at increased risk for cardiovascular disease, even if they don't have high blood pressure. It also means that they particularly benefit from restricting sodium intake.
Those most prone to salt sensitivity include:
  • the elderly
  • African Americans
  • people with hypertension
  • people with diabetes
  • people with chronic kidney disease.
What about the rest of us?
As many as one in three adults in the United States has high blood pressure, also called hypertension. Many more have "prehypertension," which means that while they don't have high blood pressure yet, they're likely to develop it. Many studies have shown that blood pressure is directly related to dietary sodium, so it makes sense for at-risk individuals to cut back. But what about the rest of us?
The Centers for Disease Control and Prevention (CDC) says that limiting sodium intake should be just about everyone's concern. In a 2009 study, CDC researchers concluded that 70% of American adults should aim for a sodium intake of no more than 1,500 mg per day. People who fell into this group of about 145 million people included:
  • everyone over age 40
  • all African Americans
  • people with hypertension.
Some public health experts believe the 1,500-mg-per-day cap should be extended to everyone. Others say that proposal ignores other factors influencing blood pressure. Almost everyone agrees that we couldn't reach the 1,500-mg limit without reducing the amount of salt in processed and prepared foods—the main source of dietary sodium.
Sources of sodium
Most of the sodium we eat comes from restaurant meals and processed foods, including canned vegetables and soups, pasta sauces, frozen entrees, luncheon meats, and snack foods.
Sodium is also found in many condiments besides ordinary table salt—including soy sauce, Worcestershire sauce, salad dressings, ketchup, seasoned salts, pickles, and olives. Baking soda, baking powder, and monosodium glutamate (MSG) also contain sodium.
What do the studies show?
Many studies have investigated links between sodium intake, blood pressure, and cardiovascular disease. Some of the most compelling evidence has come from the Dietary Approaches to Stop Hypertension (DASH) trials.
The first DASH trial showed that a diet rich in the following could lower blood pressure:
  • fruits
  • vegetables
  • low-fat dairy products
  • whole grains
  • beans
  • nuts
  • fish
  • lean meats
  • poultry.
In a follow-up trial, this diet was compared with one closely resembling the average American diet, and both diets were further divided into three sodium levels: high (3,500 mg/day), moderate (2,400 mg/day), and low (1,500 mg/day). More than 400 volunteers followed their assigned diets for 12 weeks, changing their sodium intake every four weeks.
Across the board, less sodium intake led to lower blood pressure. The DASH diet with sodium restricted to just 1,500 mg per day worked best for all participants. And for people with hypertension, it was almost as effective as medication.
The researchers concluded that we could all benefit from reducing our sodium intake. But the study did little to quell controversy over the issue. Critics charged that it was too brief to justify a general recommendation, and they warned of health risks from insufficient sodium in the diet. Proponents say that's unlikely, because most human beings don't even need as much as 1,500 mg a day for good health. Of course, this 12-week study couldn't predict the impact of reduced sodium intake on the risk of cardiovascular disease down the road.
A 2007 follow-up study to the Trials of Hypertension Prevention (TOHP) provided a longer-term perspective. The original TOHP study involved two randomized trials of lifestyle interventions conducted in the late 1980s and early 1990s. A team led by Harvard researchers tracked down the original TOHP participants and found that those who had permanently lowered their sodium intake to between 2,000 and 2,600 mg per day and continued to watch their salt intake had almost 30% fewer cardiovascular events, including death, in the following 10 to 15 years.
The TOHP trials didn't require drastic dietary changes. Instead, the volunteers learned how to look out for hidden salt and avoid it. Those who were able to reduce their salt intake by one-third to one-half teaspoon per day reaped the cardiovascular benefits.
One size doesn't fit all
The controversy about universal salt restriction will probably continue. As in most health matters, one size doesn't fit all. How salt affects your blood pressure and health depends on many things, including your:
  • genes
  • age
  • race
  • medical conditions.
What to do
If you're under age 50, your blood pressure is in the healthy range (under 120/80 mm Hg), and your health is good, you have little reason to worry about your dietary sodium intake, at least for now. Still, try to limit it to no more than 2,300 mg per day.
The risk for high blood pressure rises with age, so you'll do yourself a favor if you wean your taste buds from a yen for salt. Research has shown that people who slowly reduce their intake find that they eventually prefer less salt.
If you're older, obese, African American, or have diabetes, you may be salt-sensitive. Most experts agree people in these categories should cut back. Keep your sodium intake to less than 1,500 mg a day.
If you have hypertension, prehypertension, kidney disease, or heart failure, keep your sodium intake to less than 1,500 mg a day.
If you're hypertensive or prehypertensive or just want a healthy eating plan, consider following one of the three diets that were tested in the OmniHeart trial. The first is the DASH-like diet explained in this PDF published by the National Heart, Lung, and Blood Institute. The second was high in unsaturated fats, and the third was high in protein. (See www.omniheart.org for specifics.)
Results showed that all three diets lowered blood pressure, improved cholesterol levels, and reduced the risk of heart disease. (The high-unsaturated-fat and high-protein diets improved cholesterol levels and blood pressure even more than the DASH diet, which was higher in carbohydrates.)
The health benefits of these eating plans can't be attributed to any single ingredient—the magic is probably in the mix—but one reason they work is that they're rich in potassium. Potassium has been shown to be beneficial to blood pressure.
Whatever diet you follow, make sure it includes plenty of fruits and vegetables, which contain little or no sodium and are an important source of potassium. Potassium-rich choices include bananas, orange juice, cantaloupe, spinach, avocado, and sweet potato.

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