The Truth About the Low-Carb High-Fat Diet

For years, we were told to fear fat. Filling your plate with the F word was seen as an express ticket to heart disease. The low-carb high-fat diet (or LCHF diet for short), which can also go by the Atkins diet brand name, is ridiculed for causing high cholesterol by giving people license to gorge on damaging red meats and full-fat cheeses. Meanwhile, carb-loading became a religion to endurance athletes hoping to avoid the feared hitting-of-the-wall.

Then, trends began to change. The common criticism of the Atkins diet was debunked: Popular science suggested that a low-carb diet that's high in fat diet actually improved HDL, or "good" cholesterol, and didn't worsen LDL, or "bad" cholesterol. And in the '80s, Stephen Phinney-a MIT medical researcher-noticed that the carb-loading math just didn't add up. Our bodies only have a limited store of glycogen, or the fuel in your muscles, about 2,500 calories of carbs in reserve at all times-and this can be quickly depleted on long runs. But our bodies have about 50,000 calories of fat stored-a much deeper pool to pull from. Phinney wondered if athletes could train their bodies to burn fat instead of carbs. Your body naturally burns carbs to keep your muscles moving-and carbs are the quickest form of fuel to convert into energy. But "think of glycogen as the gas in the tank of the car," says Pam Bede, R.D., sports dietitian for Abbott's EAS Sports Nutrition. When that gas is low, you need to refuel, which is where gels and GUs come in. If your body could burn fat, Phinney thought, you could go a heck of a lot longer before refueling.

So Phinney put a small group of elite male cyclists on a low-carb diet to test it out-forcing their bodies to tap into the fat stores. While plenty of studies show that a LCHF diet results in lower peak power and VO2 max-meaning it more or less makes you slower-he found that cyclists indeed performed just as well on a two and a half hour ride when they ate a diet low in carbs and high in fat as when they ate their traditional training diet.

Out of this, the low-carb high-fat diet was born. What is it? With an ideal meal plan, you're taking in roughly 50 percent of your calories from healthy fats, 25 from carbs, and 25 from protein, explains Bede. (The current government recommendation, for comparison, is 30 percent of calories from fat, 50 to 60 percent from carbs, and 10 to 20 from protein.)

The problem? Phinney's model was imperfect: When he tested cyclist's sprinting capabilities on the LCHF diet, he noticed fat-fueled athletes clocked in at a slower time than normal. Fast forward some 40 years, though, and medal-winning-triathletes like Simon Whitfield and Ben Greenfield have renounced the church of carbs in favor of a high-fat diet instead. Kim Kardashian famously followed the Atkins diet to shed her baby weight. Melissa McCarthy attributed her impressive 45-pound weight loss to a similar eating plan.

But with mixed research and confusing star-studded testimonials-does the diet work? And, furthermore, is it healthy?

Can It Improve Your Fitness?

The effect of a low-carb, high-fat diet on athletic performance has only been looked at in a handful of studies since Phinney's original experiment. And when it comes to high speeds, Bede says it makes sense why LCHF would slow you down: "Carbs are a fairly efficient way to burn fuel, so if you're running at high speeds and need that energy immediately, carbohydrates are going to be a better source of fuel," Bede explains. Because it takes longer for your body to access the energy in fat, you won't be able to perform as quickly.

If you're focused on distance and not speed, though, don't write off LCHF so soon. It actually helps with that moment every runner dreads: hitting the wall. "In endurance athletes, adapting as much as possible to use fat can help those who struggle with bonking. It can help delay that significant onset of fatigue, which is favorable because it enables an athlete to rely less on carbohydrate gels or fluid carbohydrates-and to go faster for longer," says Georgie Fear, R.D., author of Lean Habits For Lifelong Weight Loss. Another added bonus: You'll avoid the all-too-common side effect of gastric distress from race gels and GUs. (Gross! Avoid these 20 Foods that Can Ruin Your Workout too.)

But like much of the LCHF research, the scientific evidence is mixed-it's still a vastly under-researched area. The most promising study to date is expected to come out later this year from Jeff Volek, Ph.D., R.D., at the Ohio State University, the second most prolific researcher on the topic next to Phinney.

Beyond the research, there's also a growing wave of triathletes and ultra-runners who attribute their success to jumping on the fat-fueling bandwagon. Fitness coach Ben Greenfield finished the 2013 Ironman Canada in under 10 hours while consuming almost no carbs, while ultra-runner Timothy Olson set a record for fastest completion of the Western States 100-mile course on a LCHF diet. "Athletes I work with say that once they got used to the diet, they feel better than they ever have before, their performance is potentially better-but certainly no worse-and they don't have sugar cravings or mood swings like when they were trying to fuel with carbs," Bede says. (Sound familiar? Until you begin a low-carb high-fat diet plan, try these 6 Foods to Fix Your Mood.)

Whether or not it improves performance, teaching your body to pull from your fat reserves-which you can do by simply switching to the diet-does offer better blood sugar stability, Fear adds. This helps prevent hypoglycemia, or low blood sugar (which is the reason Hyvon Ngetich collapsed and had to-now famously-crawl across the finish at this year's Austin Marathon).

LCHF also helped strength athletes lose fat without compromising their strength or power, found a new study in Exercise and Sport Sciences Reviews. That means that while people may not have seen performance gains, performance didn't suffer-plus they lost weight, Bede explains.

But Can the Atkins Diet Really Help You Lose Weight?

While the now-popular weight loss angle has gotten slightly more scientific attention thanks to interested nutrition researchers, there is yet to be overwhelming evidence in either direction. But most of the limited research on weight loss and the low-carb high-fat diet has been in favor of it.

In theory, it makes sense that you'd lose weight: "Carbohydrates attract water, so part of the initial weight loss is shedding of water stores," says Bede. "More importantly, though, fat is very satiating. While it does have more calories per gram than a carbohydrate, you can only eat so much before you are full-similar to protein." With carbs, you can finish that whole bag of pretzels without meaning to. If you're avoiding refined carbs, you're also avoiding the cravings for more unhealthy foods that research has shown they cause.

A study last year in the Annals of Internal Medicine made one of the most convincing cases yet: Researchers found that men and women who switched to a low-carb diet lost 14 pounds after one year-eight pounds more than those who limited their fat intake instead. The high-fat group also maintained more muscle, trimmed more body fat, and increased their protein intake more than their carb-heavy counterparts. These results are promising not only because researchers looked at the diet long-term, but also because they didn't limit how many calories the participants could eat, debunking the idea that a LCHF diet only works as well as any other calorie-capped diet. (Find out more in When More Calories Is Better.)

Should You Try the Diet?

No one agrees that LCHF is perfect for everyone-or ideal for anyone for that matter. But whether you should even try it is up for debate among our experts. Fear, for example, isn't crazy about LCHF as a sustainable diet dogma. "I've just seen too many people end up sick, burned out, and feeling awful," she explains.

On the other hand, Bede has seen it work for many of her athlete clients. And the science agrees that there is little harm-other than to your speed-to trying it out. It probably will help you lose weight, and there is still a chance it'll help your distance or power performance.

And if your first instinct on hearing "restrict your carbs" is "yeah right," you don't actually have to be quite so rigid: The high-fat group in the Annals of Internal Medicine study made all of their weight-loss gains despite the fact that they never actually kept their carb goals as low as the study guidelines.

Plus, at its roots, the Atkins diet or any low-carb high-fat diet is all about healthy eating, which everyone can benefit from. "You're eating mostly fruits, vegetables, heart-healthy oils, with some full-fat dairy and a touch of whole grains-all of which are a recipe for optimal health," Bede says. And this brings up the point: "The benefit of the diet could potentially be in ditching the junk and loading up on the whole foods more than the actual fat itself." (See: Carbs Without Cause: 8 Foods Worse than White Bread.)

Just know that you have to give your body at least two weeks to learn how to use fat as fuel-a phase known as fat adaptation, Bede advises. "If you're continuously feeling fatigued during your run from a LCHF diet after that, you may not be responding well to it." Ideally, you'd try the diet before training starts so the adjustment period doesn't affect your mileage or time goals, she adds.

How to Achieve 50 Percent Fat, 25 Percent Carbs, 25 Percent Protein

Just like how you should skip refined carbs for whole grains in traditional diets, your fats on a LCHF diet should come from healthy sources as well: full-fat dairy, nuts, and oils. And while saturated fats, like those in cheese, have gotten the biggest reputation makeover, there is still a place for unsaturated fats in your diet as well. (Find out just how much in Ask the Diet Doctor: Importance of Polyunsaturated Fat.) The few carbs you do eat will ideally come from produce. And, most importantly, you need to still be eating enough protein.

And if the idea of ramping up your fat and lowering your carbohydrates sounds intense, know that Bede's ideal day doesn't veer that far off the typical healthy track. Check it out!

• Breakfast: 2 cups fresh spinach sautéed in 2 tbsp olive oil, served with one egg and 1/2 cup mixed berries
• Snack: 1/4 cup mixed, dry roasted nuts
• Lunch: 2 cups romaine lettuce with oil and vinegar dressing (2 tbsp each olive oil and balsamic) and 3 oz grilled chicken breast
• Post-Workout: A smoothie made with one scoop whey protein powder (Bede recommends EAS 100%), 1 cup water (to taste), 1/2 cup mixed berries, 1/2 cup chopped kale, and crushed ice.
• Dinner: 3 oz of a high-fat fish like salmon, brushed with 2 tbsp olive oil and grilled. Side of 1 cup steamed vegetables tossed with 1 tbsp butter.