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The Truth About Low-Carb Diets

The Truth About Low-Carb Diets

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Does a plate of pasta strike terror in your heart? A burger in a bun make you break out in a cold sweat? You're not alone.

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These days, we're skipping the bread basket in record numbers (the volume of bread, buns and rolls sold in U.S. stores fell by 9.1 percent between 2006 and 2011) and shunning starchy vegetables. It seems like a new low-carb eating plan bursts on the scene every few months. And nearly one in three adults report that they're cutting down on or completely avoiding gluten, according to market-research firm NPD Group. "Carb-phobia has really taken over people's minds," says Kim Larson, RDN, a spokesperson for the Academy of Nutrition and Dietetics.

But here's the thing: Eliminating carbs is risky business. "It's preposterous," says David Katz, MD, director of the Yale University Prevention Research Center and author of Disease-Proof. "All plants are carbohydrate sources, so eating no carbs means eating no plant foods, period." Carbs are important for your brain and body; the right ones even reduce your risk of disease.

Yet the misconception that the fewer carbs you eat, the better persists among health- and diet-minded Americans. "Everyone is jumping on this bandwagon," Larson says. "But the science just does not bear it out." In fact, studies now suggest that going low-carb for a long period of time may be harmful to our health.

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You can't live without carbs.

While many of us think of carbs as bread and pasta, they're in any food that comes from a plant, including fruits, vegetables, nuts, grains, seeds and legumes. "It's a huge, diverse class of foods," Dr. Katz explains.

To make carbohydrates, plants trap the sun's energy inside molecules of glucose—a simple sugar—then connect the glucose molecules together (sometimes along with the other two basic sugar building blocks, fructose and galactose) to create longer carb molecules such as sucrose and starch. When you eat that plant, your digestive system breaks the longer carbohydrate back down into glucose, which travels through your bloodstream into your cells. The cells process the glucose, releasing the captured energy and using it for fuel.

READ: The Truth About Seafood

Far from being poison, then, glucose sparks life. "If you're in a hospital and they need to get some energy into you, they'll use a glucose drip," says Joanne Slavin, PhD, professor of food science and nutrition at the University of Minnesota and chair of the 2010 Dietary Guidelines Advisory Committee on Carbohydrates. Dr. Katz adds, "If you don't have enough glucose in your blood, you're dead. It's that simple."

While you don't have to eat carbs to get glucose into your blood (your body can manufacture it if needed), "carbohydrates are the most efficient fuel source that we have," says Heidi Schauster, RD, a nutrition therapist in Boston. Low-carb advocates (most famously Robert Atkins, MD) point out that your body can also use other fuel sources, such as protein or fatty acids, to power itself. But many experts say that this method of converting fats into so-called ketone bodies, known as ketogenesis, is much less efficient than using glucose for energy. And when too many ketone bodies are produced, the body is in a state of ketosis. "Ketosis can be tolerated for a short time, but in severe cases it could have very serious effects in the long term for the brain, as well as the kidneys, liver and skeletal system," Dr. Katz says.

Besides providing much-needed glucose, plants are our only source of dietary fiber. Like other carbohydrates, fiber is composed of many bonded sugar units, but human enzymes can't chop it up, so it passes relatively unscathed through much of the digestive tract. And it benefits you in many ways. "People who eat more fiber have less cardiovascular disease," Slavin says. "They also weigh less and gain less weight over time."

READ: The Truth About Whole Grains

You probably know that fiber helps move food through your body, keeping you regular. In addition, some forms of fiber (including resistant starch, found in bananas, lentils and many other foods) are prebiotics, a type of carbohydrate that feeds the growth of "good" bacteria in your gut.

Not to mention, skipping carbs could mean falling short on essential nutrients. "Cutting out whole grains eliminates a good source of great nutrition, such as zinc, magnesium and B vitamins," Larson says. Steer clear of fruits and vegetables and you miss out on many vitamins and powerful antioxidants. These nutrients play a critical role in keeping us healthy long-term. "When we look at population-level studies of people who live well across the span of a lifetime," Dr. Katz notes, "they almost universally eat diets high in good carbohydrates."

The surprising danger of low-carb diets.

Going low-carb is far from a new concept. As far back as 1863, Englishman William Banting wrote a popular pamphlet describing how he lost 35 pounds on a meat-heavy, carb-light regimen. In the early 1970s, Dr. Atkins made a splash sharing a similar philosophy. But in the early 1980s, the U.S. Department of Agriculture told Americans that they should eat less fat for better heart health. Low-fat, high-carb products popped up everywhere, and we embraced our pasta bowls.

READ: Should You Go Gluten-Free?

Then came another about-face. In the last couple of decades, researchers have been telling us that fat isn't all bad—and that loading up on processed sugary sweets has been hurting our health and waistlines. Meanwhile, the Atkins diet made a comeback with Dr. Atkins' New Diet Revolution, and variations on protein-rich eating plans popped up (Zone! South Beach! Paleo!). Fast-forward to today, when books like Wheat Belly and last year's Grain Brain claim that carbs, especially wheat, can cause everything from dementia to belly fat.

No doubt, you can drop pounds on a low-carb diet in the short run. A 2012 review of studies (most of them lasting 12 months or less) found that obese people on lower-carb diets lost weight and improved their cardiovascular health (lower blood pressure, higher good cholesterol). But when low-carb diets are compared with other plans, the results are less impressive. A landmark Harvard study published in 2009 in The New England Journal of Medicine randomly assigned overweight people to one of four diets with similar calorie counts but different proportions of fats, carbs and proteins to see whether a low-carb diet (getting 35 percent of calories from carbs) had any benefits over diets with higher carb counts. Surprise: After two years, there was little difference in weight loss among the participants; all the diets led to improvement in cardiovascular risk factors as well. In other words, in the short term you'll likely lose weight on any diet that cuts calories.

But not all plans are equally good for you. New research suggests that following a carb-restricted diet for more than a few years could actually harm your health. A review published last year in Plos One looked at 17 prospective epidemiological studies (in which subjects fill out food diaries and are followed for years); these are less rigorous than clinical studies but are the only way to understand long-term effects of diet in the real world. When people eating the lowest-carb diets (typically 30 to 40 percent of calories) were compared with folks on the highest-carb eating plans (generally 60 to 70 percent of calories), it was found that the low-carb dieters had a 30 percent higher risk of death from all causes in the long run.

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"What we discovered was if you continue on the low-carb diet for many years, you could die younger," says lead author Hiroshi Noto, MD, of the National Center for Global Health and Medicine in Tokyo. "That may be because if someone eats a low-carb diet, most likely they resort to more fat or animal protein, which can be bad for your blood vessels." What's more, a study published earlier this year in Cell Metabolism suggests that having a diet high in animal protein in midlife (age 50 to 65) can significantly raise your risk of dying of cancer, compared with eating a low-protein diet.

The right way to eat.

"We know what a healthy diet is," Dr. Katz says. "All the studies that show better health outcomes over the long term are rich in plant foods—real plant foods, not highly processed, sugar-added, glow-in-the-dark crap." Think of Mediterranean, DASH and other ways of eating that have been linked to better health, adds Robert H. Eckel, MD, professor of medicine at the University of Colorado Anschutz Medical Campus and former president of the American Heart Association: "Those are not low-carb diets."

So what level should you shoot for? The Dietary Guidelines for Americans recommends that adults consume 45 to 65 percent of total calories from carbs. "Endurance athletes, who burn lots of energy, are at 65 percent, whereas diabetics who are limiting their carbs would be on the 45 percent end," explains Los Angeles dietitian Lori Zanini, RD, spokesperson for the Academy of Nutrition and Dietetics.

READ: 15 Ways To Boost Your Metabolism (Without Exercise)

To keep it simple, Larson recommends using food groups to guide your choices; aim for about four servings a day of whole grains (one serving equals 1/2 cup of cup cooked grains or whole-grain pasta, or one slice of whole-grain bread) and five to eight servings a day of fruits and vegetables (1/2 cup of berries, one piece of whole fruit and 1 cup of leafy greens all count as one serving). Limit foods and drinks with added sugars—these are the carbs linked to insulin resistance, heart disease and obesity.

If you've sworn off carbs for a while, it can be scary to put them back on your plate. "I tell my clients, 'Think about adding back in some grain foods for a couple of weeks to see how you feel,'" Schauster says. "Very often, after doing that, they realize, I have so much more energy, I get more out of my workouts, I don't feel cravings. A lot of things will shift."

Most of us relish the chance to enjoy juicy fruit and filling starchy veggies again, Larson adds: "When I show people what they can eat, there's such a sense of relief. They say, 'Really, I can do that?'"

READ: The Truth About Fruit

Should I be eating low-GI foods?

Glycemic index, or GI, refers to how carbohydrates affect blood glucose levels: Low-GI foods tend to raise blood sugar more slowly than high-GI foods. But the measurement doesn't take into account how much of the food is eaten in a typical serving, which can skew results, Slavin explains. Watermelon has a high glycemic index, for example, but since it's mostly water, you don't eat that many carbohydrates in one serving, so it doesn't actually raise blood sugar significantly.

For people who aren't diabetic, experts say that GI isn't all that helpful; instead, focus on adding carb foods that are also rich in fiber, which helps slow digestion and prevent blood sugar spikes.

Low-Carb Diet Smackdown.

South Beach, Dukan, Paleo—what the wheat?! We got the health scoop on some of the most popular protein-rich plans from Nanette Steinle, MD, assistant professor of medicine at the University of Maryland School of Medicine, and Walter Willett, MD, chair of the department of nutrition at the Harvard School of Public Health.

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Paleo

About 35 to 45 percent of calories from carbs.

The deal: The newest (and oldest!) carb-restricted plan, the Paleo diet claims to hark back to caveman days, encouraging dieters to eat like our ancient ancestors did.

The diet: Calling all carnivores—you can eat meat, and lots of it (be it beef or poultry). Fruits, nuts, eggs, seafood, seeds and veggies are also encouraged, but grains of any kind are a no-go, as are legumes and dairy.

The doctors say: While it's great to cut out processed carbs, emphasizing meat isn't optimal, says Dr. Willett; he suggests focusing on fish, nuts, seeds and some poultry.

Our rating: 3 stars

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Atkins

10 to 25 percent of calories from carbs in the initial phase.

The deal: Many credit Dr. Atkins' plan as the one that kicked off the low-carb diet craze.

The diet: During the first two weeks, you drop to an extremely low 20 grams of net carbs per day. (For perspective, a medium apple has about 21g.) After that, you can bump up your net carbs in 5g increments.

The doctors say: Severe carb restriction for long periods could result in dangerously high levels of ketone bodies in your blood. Plus, "many people replace carbs with lots of red meat and butter, which isn't good for overall health," Dr. Willett notes.

Our rating: 2 stars

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South Beach

About 30 percent of calories from "good" carbs in the initial phase.

The deal: Designed by cardiologist Arthur Agatston, MD, it's like Atkins' hipper, more modern sibling.

The diet: South Beach also has a carb-slashing first phase, but it's less restrictive than Atkins. It emphasizes choosing the "right" carbs (veggies, fruits, whole grains) and fats (like mono- and polyunsaturated).

The doctors say: Our experts like the focus on good carbs and fats, which means wholesome food is a priority. "Focusing on quality is the most important thing you can do to fuel your body right," Dr. Willett says.

Our rating: 4 stars

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Dukan

An estimated 27 percent of calories from carbs in the first phase.

The deal: Developed by French physician Pierre Dukan, this protein-heavy plan is known for its strict rules.

The diet: Vegetables are off-limits in the "Attack" phase, lasting two to seven days. In the next phase, you can add veggies. Then fruit, starchy foods, whole-grain bread and cheese get the go-ahead. In the final phase, all food groups are allowed in moderation.

The doctors say: "If you eat mostly meat on a regular basis for too long, you may raise trimethylamine-N-oxide (TMAO) levels in your body, which could up your risk of cardiovascular disease," Dr. Steinle says.

Our rating: 1 star

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