Low-Carb Diets: What’s Really Happening to Your Body?

Anyone who’s ever watched their weight (i.e., all people except the metabolically gifted) knows that carbohydrates have a bad rap. Almost all weight loss advice stems from avoiding bread and pasta like the plague—but why? We hear so much about how we should avoid an excess consumption of carbohydrates, but many of us don’t have a clue as to what a carb actually is, what it does, or how it affects us.

Here’s my experience with carbs: After studying in Italy for six weeks last summer and feasting on prosciutto, spaghetti carbonara, and gelato, I returned home feeling equal parts cultured and, well, disgusting. Wanting to reverse the damage, I embarked on a high-protein/low-carb diet, eating only vegetables and lean sources of protein. Desserts, breads, and even fruit were banned on this regime.

At first, I felt like a superstar. I was so sick of eating junk food in Italy, that I actually craved things like chicken and string beans. The protein kept me feeling full and I wasn’t even tempted to cheat. Within a week, I was fitting back into my clothes like I used to and felt fabulous. I was amazed at the fast results and owed it all to my food choices.

On about the tenth day, that all changed. I hadn’t been working out much because I had been busy getting settled back at home, so I decided to go for a run on one of my favorite trails. Because I’m an athletic person, the hilly five miles are usually a great workout but by no means super strenuous.

About halfway through, I experienced a feeling I had never encountered in all my years of exercise. It wasn’t the usual pain during a workout—you know the kind: aching legs, pounding heart rate—it was something else. I literally felt like the life had been sucked out of me, and I had absolutely nothing to give. I had zero energy to put one foot in front of the other. I felt scared, weak and fragile.

For the first time, I saw how severely a change in carbohydrate consumption could impact the human body. This got me to wondering: where should carbs come into play in weight loss and working out? Who should attempt a low- or high-carbohydrate diet? And for what reasons?

How Carbs and Your Body Work Together

“When you want to lose weight, there is every argument that reducing consumption of carbs will help, basically because reducing carbs is correlated with a net caloric deficiency, ” says Phil Clark, a pro runner and owner the NoLibs gym the Training Station.

But there’s more to it than that. You could reduce calories by cutting down on protein and fat, too, so what is it about carbohydrates, specifically, that impacts weight loss? Clark explains that it has to do with the way the body stores them. When carbohydrates are digested, they undergo a process that eventually results in glycogen, a molecule that serves as secondary long-term energy storage.

Here’s the catch: glycogen contains a lot of water, and water is heavy. So if you eat fewer carbs, you deplete your body’s supply of glycogen, and you’ll lose weight. “If your interest is just the number on the scale, it’s difficult to argue that reducing carbohydrates won’t make it go down,” says Clark.

But—of course there’s a but—we all know weight loss isn’t always correlated with fat loss, and the science here points to the fact that you’re mostly losing water weight. So if your aim is to become leaner in the long-term, eliminating carbohydrates is not the route to go. And, as Clark explains, since decreasing body fat requires a hefty amount of exercise, you can’t really exercise without carbs. See where we’re going with this?

Why Your Body Needs Carbohydrates

Your body relies on glycogen as an important energy source. It’s stored primarily in the muscles and liver, and is broken down into glucose during physical activity. The glycogen stored in the liver is released into the bloodstream so that it can be delivered to muscles as your body demands more fuel for energy.

“It is well established that active people and athletes need a high-carbohydrate diet. It’s not something we’re waiting for the science to prove,” Clark says. “You need glycogen stuffed in your muscles, and you need it constantly.”

So falling for the latest weight-loss fads (read: low carb diets) causes your exercise performance to suffer. If your glucose and glycogen stores are completely depleted, your body has no choice but to cannibalize itself. It converts skeletal protein to glucose. This explains why I felt like an absolute zombie when I attempted exercise with no carbs for fuel.

Who Should Eat Carbs—and Who Should Lay Off Them?

Bottom line: Active people should steer clear of low-carbohydrate diets. Active people aiming to lose weight should reduce calorie intake overall, rather than alter the sources of those calories. A standard recommendation is a diet consisting of 50 to 60 percent carbohydrates, 20 to 30 percent protein, and 10 to 15 percent fat.

What about sedentary people? If you sit behind a desk all day and don’t work out much, Clark says you won’t feel the negative repercussions of a low-carb diet as much as an active person would, and it isn’t nearly as dangerous. However, Clark stands by his recommendation to lower overall calories consumed instead of going to extremes and eliminating food groups.

“If you need to lose weight in a short amount of time, cutting out carbs is a nice trick,” he says. “But if long-term weight loss is your goal, it’s not the best solution.”