How Do Low-Carb Diets Work for Weight Loss?
Low-carb dieting seems to help sometimes with weight loss. But the science behind why is inconclusive.
Has the actor Rob Lowe recently let you in on his “little chocolate-peanut butter secret?” If so, you were likely watching an ad for the Atkins Diet, a meal plan company that promotes weight loss through a “low carb” diet.
Meal plans that cut back how many carbohydrates you eat have promised slimmer waists for decades. The science behind this claim has drawn on just as long and won’t be stopping soon. “The research will be ongoing forever,” says Jessica Alvarez, a registered dietician who studies metabolism and Type II diabetes at the Emory University School of Medicine. Though the studies are hard to do, it seems the dietary choice can lead to (at least temporary) weight loss. How exactly that happens, however, remains unclear.
The Fall Of One Theory, The Rise Of Another
The original biological premise behind a low-carb diet was fairly straightforward. Carbohydrates trigger the release of insulin, a hormone that prompts tissues to absorb sugars and produce fat. At the same time, too many of these nutrients will slow your metabolism while growing your appetite. In theory, if you eat fewer carbs, less insulin will pump through your body — triggering less fat production, a boosted metabolism, and reduced hunger. Unfortunately, those assumed sequences of events and their evidence (such as a changed appetite) haven’t borne out in research. “That hasn’t really held up in the literature at all,” says Heather Seid, a registered dietician who runs the Bionutrition Research Core at the Columbia University Irving Medical Center.
Instead, there’s more scientific support for another physiological mechanism that kicks in on the lowest-of-low-carb meal plans. Called the ketogenic diet, this program was developed by a Mayo Clinic physician for people with epilepsy. The program calls for participants to eat 90 percent of their calories from fat, with very little carbs or protein otherwise. Exactly how the pivot to low-carb, high-fat reduces seizures is not clear.
But the diet does cause weight loss, as it forces the body into ketosis, a metabolic state that digests fat rather than glucose for fuel. People on ketogenic diets lose weight for this reason but typically gain it back as soon as they stop the program. Additionally, “it’s not fun to eat,” says Seid. Even those with epilepsy might cycle on-and-off the program because it’s hard to stick to, she says. “It’s really demoralizing to follow a ketogenic diet for a long period of time.”
More Generic, More Uncertain
As for what other generic “low-carb” diets accomplish, research does indicate that these regimens can cause some degree of weight loss. For some people, shedding pounds can spark other desirable physical changes. For example, it can help people at risk of developing Type II diabetes — the illness that arises when your body stops producing or responding to insulin. Excess fat likely makes some tissues more resistant to insulin, and if someone can’t generate enough of the hormone to meet that demand, they might develop the disease. So, when it comes to returning the insulin supply-and-demand to a healthy balance, “weight loss is often that first step,” says Alvarez. For those looking to lose weight, for this reason, Alvarez says there’s been some research supporting low-carbohydrate diets as a possible tactic.
How weight loss happens in these less-stringent low-carb scenarios is still murky. Though often billed as diets that shift insulin production, these programs likely work by initiating a lesser degree of ketosis, Seid says. In fact, going a long time between meals is enough to trigger a little bit of fat-for-fuel burning. It’s something our bodies naturally do when running low on food. So if a low-carb diet just encourages participants to eat less overall, then maybe the weight loss comes from reduced-calorie ketosis — not reduced-carb ketosis. This issue surfaced in some of Seid’s research.
In their review of research into the efficacy of low-fat and low-carb diets, Seid and her team found that what mattered more than changing diet components was reducing total calories. A lot of research looks at food alterations (like low-carb) while also cutting calories, which makes it hard to pinpoint the cause of someone’s smaller pant size, Seid says.
Ideally, a study would only change what people ate, not how much they consumed. In one study, participants — who were consuming pre-set meals — lost more fat on a low-carb diet than they did on a traditional diet, even when both offered the same number of daily calories, Alvarez points out.
Cutting Carbs (In a Lab) Is Tough
The challenges of figuring out if (and how) these diets work are broader than just limiting what researchers assess. For one, “low carbohydrate” does not have a single definition. Sure, some more specific programs, like the keto diet, fall under this category, but the larger umbrella includes a range of interpretations. “That to me is one of the biggest issues or questions in our field,” says Alvarez. For example, the American Academy of Family Physicians labels a diet “low-carb” if less than 20 percent of the calories come from that nutrient. The Atkins Diet, on the other hand, bases its definition on weight: Those following the program might restrict themselves to 15 to 20 grams of carbohydrates a day for the first couple of weeks. Results across studies are hard to compare when participants adhere to different definitions of the diet.
At the same time, the various “low-carb” interpretations are helpful, Alvarez says. Not everyone can tolerate sticking to the same, most-restrictive versions. It’s worth knowing if mild carbohydrate reductions also cause weight loss.
The perfect study scenario is also challenging to sustain. To know exactly what participants eat, dietary research settings ideally supervise people 24/7. Otherwise, the temptations of day-to-day life are too powerful. For example, Seid and her team might try for precise reporting of what people eat during self-supervised meal plans by asking people to bring in their food containers.
“I can’t tell you how many times we receive back wrappers of things that weren’t in the diet,” she says. But complete surveillance studies are expensive, and funding for this kind of research has dried up significantly since the early 2000s. They’re also intimidating to join. Participants get sick of handing all dietary control over to researchers.
The day might come when dietitians and weight loss researchers agree about whether all those low-carb options out there do people any good. But in case it never does (or in the meantime), it’s important to remember that what you enjoy is likely to work best for you.
When low-carb diets (or really any diet scheme) doesn’t result in long-term weight loss, it might be because it’s hard to keep up. Other meal plans, like the Mediterranean Diet, might appeal to people more and bring their own health benefits, Alvarez says. What’s most important is finding what you can stick with.
“What I found in my clinical practice is that finding a diet that is sustainable for a long period of time, where they can have reduced calories and still feel fulfilled and happy, is the most successful diet,” Seid says. “And I use ‘diet’ with quotes around it.