One out of three adults have hypertension, but only half of them have their blood pressure under control. Another one out of three have prehypertension. Hypertension raises the risk of strokes, heart attacks, kidney disease, dementia, and more.
If there were a diet that could cure high blood pressure in four weeks, you’d think everyone would know about it.
There is…and they don’t. As it turns out, it’s the diet recommended by the American Heart Association and the American College of Cardiology.
Here’s the backstory on the healthiest diet that’s hidden in plain sight.
Frank M. Sacks is professor of cardiovascular disease prevention at the Harvard T.H. Chan School of Public Health and professor of medicine at the Harvard Medical School. He is also a senior physician at Brigham and Women’s Hospital in Boston and a member of Nutrition Action’s Scientific Advisory Board. Sacks has published more than 220 papers, including several landmark trials on diet and risk of disease.
He spoke to Nutrition Action’s Bonnie Liebman from Boston.
Click on a question to jump to the answer or keep reading for the entire interview
HOW DIET CAUSES HIGH BLOOD PRESSURE
CLICK TO ENLARGE “HOW DIET CAUSES HIGH BLOOD PRESSURE”
…AND HOW TO REVERSE IT
CLICK TO ENLARGE “DASH DIET TO LOWER HIGH BLOOD PRESSURE”
Act Now to download your FREE copy of Heart and Disease: Fighting Inflammation
without cost or obligation!
Q: Can a DASH diet really cure high blood pressure in four weeks?
A: In many people, yes. The diet we tested in the 1997 Dietary Approaches to Stop Hypertension study lowers blood pressure as much as drug treatment. That was true regardless of age, sex, weight, or race, though we didn’t test people older than age 76.
The DASH diet is actually superior to drug treatment if you also cut salt, as we did in a 2001 study called DASH-Sodium. Back to Top
Q: How did the DASH study start?
A: Before DASH, we had looked at whether individual nutrients like potassium, calcium, magnesium, fiber, and plant protein could lower blood pressure, and we didn’t find much of an effect. The idea with DASH was to look at the diets of vegetarians and other people around the world with low blood pressure and create a similar diet that would be acceptable to Americans.
The foods we found in populations with low blood pressure were primarily fruits, vegetables, fish, nuts, and whole grains. And they ate very little red meat, sugar-containing beverages, and desserts. Back to Top
Q: The DASH diet isn’t just rich in fruits and vegetables?
A: No. The DASH study pitted two diets against a typical American diet. One diet only had extra fruits and vegetables, and it did lower blood pressure. However, the full DASH diet lowered it even more. The full DASH diet had more fruits and vegetables and low-fat dairy foods, fewer sweets, and less saturated fat and cholesterol than a typical American diet. It was rich in potassium, magnesium, calcium, fiber, and protein.
Back to Top
Q: Why is a DASH diet so good at lowering blood pressure?
A: We’re not sure. We know that a high potassium intake promotes sodium excretion. Since most people are not eating very-low-sodium diets, that’s an important mechanism.
And Italian researchers have found that a Mediterranean diet— which is similar to the DASH diet—made blood vessels produce more nitric oxide, which causes them to relax. The DASH diet also decreases resistance in the small blood vessels.
Back to Top
Q: What kind of resistance?
A: Our very small blood vessels—ones that are almost small enough to be capillaries—have muscular walls so they can contract and expand. Blood pressure goes up partly because those blood vessels don’t expand, or relax, enough. So it takes more pressure to push blood through them.
The DASH diet makes the tiny vessels more compliant—in other words, they relax a little bit more. That lowers blood pressure.
Back to Top
Q: Does resistance in large blood vessels also raise blood pressure?
A: Yes. The big arteries also relax to accommodate the flow of blood. But as people get older, their arteries get stiffer, and that increases systolic blood pressure. Systolic is the pressure on blood vessel walls during heart beats. Diastolic is the pressure between beats.
A DASH diet or a low-sodium diet allows stiff arteries to expand. It restores the normal functioning of arteries even if they are stiff with age. Back to Top
Q: A DASH diet and a low-salt diet each lowers blood pressure independently?
A: Yes. After the original DASH study, we tested the DASH diet and a typical American diet with three levels of salt. With or without the DASH diet, blood pressure dropped when people went from a typical 3,300 mg of sodium a day to 2,400 mg a day to 1,500 mg a day. Back to Top
Q: Why does losing excess weight lower blood pressure?
A: Increased body fat makes large arteries stiffer. It also increases activity in the sympathetic nervous system—the part that secretes adrenaline. The adrenaline constricts blood vessels, including those that go to the kidneys, so they don’t excrete as much sodium as they should. Back to Top
Q: Can too little salt raise the risk of heart attacks and strokes, as a study reported last year?
A: Not at all. That study is full of methodological flaws that are common in studies of sodium and disease. In 2013, the American Heart Association, which recommends cutting sodium to 1,500 milligrams a day, issued a science advisory to describe the potential problems in these sorts of studies. Back to Top
Q: Like what?
A: It was an observational study, meaning that people weren’t randomly assigned to different sodium levels. And that can lead to many problems. The worst one is reverse causation, meaning that the disease causes the low sodium intake, not the other way around. Back to Top
A: People who are sicker or are debilitated eat less food in general, including sodium, and often they are advised to eat less sodium because they have diseases like heart failure or hypertension. So that could bias the results.
Another study, which was smaller but didn’t have those flaws, showed very clearly that higher sodium intakes were linked to a higher risk of cardiovascular disease. There was no increased risk of heart attacks or strokes at lower sodium intakes. Back to Top
Q: Did you expect the DASH diet to do more than lower blood pressure?
A: The DASH diet was designed strictly for lowering blood pressure, but because it was low in saturated fat and cholesterol, we thought it would also lower LDL, or bad, cholesterol. And it did. Back to Top
Q: Most people don’t realize that a DASH diet is the key diet advice for preventing heart attacks and strokes.
A: That’s true. I was a member of the task force that issued the 2013 Guideline on Lifestyle Management to Reduce Cardiovascular Risk from the American Heart Association and the American College of Cardiology.
The big emphasis in those guidelines is on a dietary pattern that emphasizes vegetables, fruits, and whole grains. It includes low-fat dairy products, fish, poultry, legumes, nuts, and non-tropical oils like olive, soy, and canola. And it has very little sweets, sugar-sweetened beverages, and red meat.
That’s the DASH diet. The task force also found strong evidence that reducing saturated fat and sodium cuts the risk of heart attacks and strokes. Back to Top
Q: Why did you follow the DASH study with the OmniHeart study?
A: The original DASH diet was high in carbs. From the beginning, I wanted to study a Mediterranean-like version that was lower, but not low, in carbs.
We wanted to see if we could lower triglycerides or raise HDL, or good, cholesterol—or whether we could lower blood pressure even more—if we reduced carbs and replaced them with unsaturated fat or with more protein, especially plant protein from beans and nuts. That turned out to be true. Back to Top
Q: Were all three diets—higher-carb, higher-protein, and higher-fat—good?
A: Oh yes. Compared to what the participants were usually eating, all three of the OmniHeart diets substantially lowered blood pressure and lowered LDL cholesterol. So we’re talking about mildly improving the greater beneficial effects of the DASH diet. Back to Top
Q: And we know that lowering LDL cholesterol lowers the risk of heart attacks?
A: Yes. The evidence that high LDL causes cardiovascular disease is very, very strong. It’s been strong for decades, but it has gotten even stronger now that a new trial found that ezetimibe, a drug sold as Zetia, lowers the risk of heart attacks. Back to Top
A: Ezetimibe and statins lower LDL by different mechanisms. So that strengthens the evidence that what matters is lowering LDL, not other things that statins or ezetimibe do.
What’s more, studies show clearly that the genes that are associated with mildly reduced LDL are also associated with reduced heart disease. And it just goes on. So the evidence that LDL causes heart attacks is very powerful. Back to Top
Q: How good is the evidence that HDL cholesterol protects the heart?
A: It’s got problems. The HDL hypothesis is that it protects against atherosclerosis by taking cholesterol from cells and delivering it to the liver for excretion. But we have these drugs that raised HDL and didn’t prevent heart attacks. Back to Top
Q: Could that be because the drugs raised the wrong type of HDL?
A: That’s a white hot topic right now. For example, in my lab we’re looking at ApoC-III, a small inflammatory protein on the surface of some HDL particles. ApoC-III, because it is a pro-inflammatory protein, may confer harmful properties on the HDL particles that carry it. It turns good cholesterol into bad! Luckily, most HDL in most people lacks ApoC-III. Back to Top
Q: Does it matter how much cholesterol HDL can carry away from arteries?
A: Yes. A new study in the New England Journal of Medicine measured how well HDL was able to remove cholesterol from cholesterol-loaded cells in artery walls. People whose HDL could remove more had a lower risk of heart attack and stroke over the next nine years. So that is helping rehabilitate HDL as an artery protector. Back to Top
Q: What was the point of OmniCarb, your newest study?
A: Different kinds of fats have been studied very, very well. But different types of carbohydrate—they seem to be more of a tricky question.
Also, since our studies on the DASH diet in the 1990s, I have continually been asked if it would make a difference if the carbs in the DASH diet had a low glycemic index rather than a high one. Back to Top
Q: What is the glycemic index?
A: It’s a measure of how much a carbohydrate-containing food raises blood sugar. But it’s tricky to use.
Take pasta. If you undercook it so it’s chewy, it will raise blood sugar less than if you cook it until it’s soft. Or let’s take 100 percent whole wheat bread. A dense European whole wheat bread would have a lower glycemic index than a fluffy whole wheat bread made of finely milled flour.
Replacing higher-glycemic foods (like bananas) with lower-glycemic foods (like apples) didn’t improve insulin, cholesterol, or anything else.
And bananas will raise blood sugar more than apples, though that’s not the only difference between those foods. Bananas are high in potassium. Apples have good phytochemicals. Both have fiber. Back to Top
Q: Why did you look at high versus low glycemic index in both higher-carb and lower-carb diets?
A: In the OmniHeart study, we found that a lower-carb diet was better than a higher-carb diet. We wanted to see if we could make a high-carbohydrate diet better by using low-glycemic-index foods. Back to Top
Q: Did it work?
A: No. Choosing low-glycemic-index foods didn’t improve blood pressure, cholesterol, or triglycerides, or make insulin more effective. In fact, it actually made insulin less effective and increased LDL cholesterol. We don’t know why. It surprised us. Back to Top
Q: Does that mean that there are no good carbs or bad carbs?
A: Some reporters have asked, “So can we just eat potato chips and white bread?” I said absolutely not, because glycemic index is not the only attribute of these foods.
When you see Skittles or french fries or cupcakes or whatever, you can’t say, “Oh, glycemic index doesn’t matter, so I can eat any of this stuff.” No.
If foods are high in refined carbs, like sugar or white flour, or if they’re low in fiber, vitamins, or minerals, they’re less healthy.
We didn’t study junk food. The type of food does matter. It’s just that glycemic index may not. Back to Top
Q: Did you look at glycemic index and weight loss?
A: No. Our study was designed to measure risk factors for heart disease and diabetes, not weight loss. But we did look at studies on glycemic index and weight loss. A couple of them showed a benefit, while others didn’t. So there isn’t much evidence in favor of low glycemic index helping weight loss. Back to Top
Q: Didn’t dieters regain less weight on a low-glycemic diet in the Diogenes study?
A: After six months, people on the low-glycemic-index diet regained about two pounds less than those on the high-glycemic-index diet. But after a year, there was no difference. So even for regaining weight, glycemic index doesn’t matter much. Back to Top
Q: And in some studies, people might have eaten unhealthy high-glycemic foods like cookies, cakes, and candy.
A: Yes. That wasn’t a problem in our study, because we fed people all of their food, and we made sure that both the high- and low-glycemic foods were healthy. Back to Top
Q: Did you find any benefit for the lower-carb diets?
A: Yes. Triglycerides were about 25 percent lower on the lower-carb diets. That’s good because there is more and more evidence that high triglycerides raise the risk of heart disease.
What’s more, HDL cholesterol—good cholesterol—went up slightly, and diastolic blood pressure went down slightly. But there was no impact on insulin effectiveness. That was true whether or not the lower-carb diets had high- or low-glycemic carbs. Back to Top
Q: Could low-glycemic carbs help people with prediabetes or diabetes?
A: No one in OmniCarb had type 2 diabetes. If someone has a problem managing their blood sugar, then it’s more likely that foods with a lower glycemic index could help.
We did have some people with prediabetes in our study, and it didn’t matter to them if the glycemic index was high or low. But the study wasn’t designed to definitively answer that question.
Certainly, if you have prediabetes or type 2 diabetes, it’s good to reduce the amount of carbs. We’re just not sure about glycemic index. Back to Top
Other relevant links:
• A day’s worth of food on the OmniHeart diet. See: What to Eat According to the OmniHeart Study
• Triglycerides and plaque buildup. See: Why are Triglycerides Linked to Heart and Disease?
• What to do for optimal heart health. See: What Can People Do to Protect Their Heart?