We all know the most basic principle of weight loss: You need to burn more calories than you consume. And you have a long road of weight loss ahead of you, it can easily seem like the best way to fast-track this journey is to simply consume a whole lot less calories. That way, you’re not adding any new fat to your body, right?
Actually, it’s not that simple. What’s more, cutting too many calories can actually have the opposite effect, completely stalling your weight loss efforts.
“It’s a very common problem—that ‘starving’ yourself tends to not result in very effective weight loss,” says Shawn M. Talbott, Ph.D., L.D.N., nutritional biochemist and researcher in Salt Lake City, Utah, and past director of the University of Utah Nutrition Clinic.
RMR: Or, why you still need to fuel the fire
To understand why, it's important to understand resting metabolic rate (RMR), or the number of calories your body burns at rest. Let’s say you decide to cut your intake from 3,000 calories down to 1,000. The first thing that’ll happen if you drop your energy intake too low or too quickly is that your RMR will also drop.
“Your body basically says, ‘Okay, no calories coming in? I’m going into ‘sleep’ mode to save energy,’” Talbott explains. Evolutionarily, your body doesn't know how long that calorie deprivation is going to last, so it puts you in energy-saving mode to optimize survival.
This drop in RMR can be substantial—as much as 10%, Talbott says. In a larger guy, that’s about 250 calories less your body is burning naturally—which may not sound like a lot, but can make a ton of difference in weight loss over time, he points out.
It’s not just about the calories you need to move about in life, though. Your brain is fueled by glucose—the sugar molecule that is regulated by eating—and your body will do anything it can to keep your brain up and running. “If you continue at this too-low intake for more than a day, your body will start to break down muscle tissue to get at the amino acids stored here, which it can then convert to glucose,” Talbott explains. Break down enough muscle tissue—about a few pounds—and your RMR drops even further since, as we all know, muscle increases your metabolism.
There’s also an old saying Talbott points to: “Fat burns in the flame of carbohydrate.” Translation: Your body breaks down fat by oxidizing it in the mitochondria, the powerhouse of your cells. But in order to do this, you need energy from carbohydrates to shuttle the fatty acids into the mitochondria. No carbs, no fat burn.
What’s more: There are peripheral consequences of under-fueling yourself. “If your blood glucose drops too low, then you also face a cascade of problems like increased appetite and cravings; poor sleep, which will then elevate cortisol, leading to more cravings and more muscle loss; and fatigue, since food is energy, which prevents you from exercising intensely to burn fat,” Talbott adds.
What's the right way to cut calories?
“The trick with optimal weight loss is to eat just enough calories to keep your blood glucose levels within normal ranges, maximizing fat metabolism and RMR, and avoiding that cascade of concerns,” Talbott says.
The general rule of thumb is to cut about 25 percent of your baseline RMR—so 500 calories per day from a 2000-calorie diet. This is generally enough to get weight loss going, but not too much to set off the starvation alarm bells, he adds.
The exception to all of this? The strategy of intermittent fasting, where you temporarily cut all calories to “jumpstart your metabolic machinery,” as Talbott puts it. “I will often recommend intermittent fasting for people who seem like their metabolism is ‘stuck,’ so they don’t eat anything for a period of 18 to 24 hours, but then resume the ~25 percent RMR reduction as their regular weight loss target,” he adds.
Want to know more about weight loss? Check out our primer on everything you need to know about losing weight.