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Why Eating Fewer Calories Won't Help You Lose Weight

US News & World Report - Health 12/8/2022 Elaine K. Howley
Woman cooking © (Getty Images) Woman cooking

Conventional nutritional wisdom has always recommended eating less and exercising more if you want to lose weight. In theory, this makes sense – burn more calories than you eat, and the pounds will come off.

But there’s often a significant drawback that develops when you go too far with an extreme diet, says Dr. Danny Shouhed, a board-certified surgeon and founding medical director of the Bariatric Surgery and Metabolic Weight Loss Center at Providence Saint John’s Health Center in Santa Monica, California.

“Patients go on these crash diets where they’re really depriving themselves of calories to the point at which your body goes into starvation mode, and that’s the part that becomes counterproductive,” he says.

By starvation mode, Shouhed is referring to an evolutionary mechanism that developed “thousands of years ago when we used to be hunters and gatherers and didn’t have refrigerators.” Back then, it wasn’t uncommon for our ancestors to feast one day but then not be able to find food the next or for several more days.

To cope with this uncertainty of food availability, the human body adapted to decrease the basal metabolic rate during periods of fasting. The basal metabolic rate, or BMR, is the amount of calories the body needs to keep life-sustaining functions such as breathing.

“It’s the same concept of how bears hibernate in winter or how a crocodile can go underwater for 60 minutes at a time without gills and requiring no oxygen,” Shouhed explains. “The crocodile has the ability to drop its heart rate from 60 to 5 beats per minute to conserve energy when underwater. When your body goes into starvation mode, it’s similarly trying to keep as much energy and calories as possible just to function daily.”

A Drive for Equilibrium

Everyone has a set point, or a weight range in which the brain wants to keep the body. This weight range varies from person to person and is determined by both genes and environment. The body is wired for survival, and as far as it’s concerned, dieting is a form of starvation.

But the body is malleable. Gaby Vaca-Flores, a registered dietitian and founder of Glow+Greens, a nutrition and skin care consultancy based in Santa Monica, California, says that when you shift into that starvation mode, it creates a “metabolic adaptation, which essentially means that your metabolism is slowing down as your body gets used to surviving off a smaller amount of calories.” This typically translates into a weight loss plateau.

The body’s relentless drive for equilibrium and the adjustable nature of the basal metabolic rate make it more difficult to shed weight, even if you’ve reduced your calorie intake. And naturally, when you inevitably stop following the restrictive diet, your body will stay in that starvation mode afterward (how long depends on the person and the circumstances).

“Unfortunately, once you hit that starvation mode for a certain period of time, that original set point for your basal metabolic rate that’s in your hypothalamus (a structure in the brain that controls metabolic functions) has changed indefinitely. So now your set point, or your BMR, actually finds a new equilibrium,” Shouhed explains.

Re-Gaining Weight After a Diet

If, for example, you once needed 1,200 calories to keep your body going each day, your body might be able to function on 800 calories after a period of calorie restriction. As soon as you begin consuming more than that amount, your body has to figure out what to do with the excess calories.

This is why so many people rebound after a weight loss and may actually end up weighing more than when they started the diet. “From a mathematical standpoint, now you’re actually going to end up even higher than where you first started,” Shouhed says.

This process is controlled by a variety of hormones, including ghrelin – the hunger hormone that your gut produces to signal your brain when it’s time to eat – and leptin – the satiety hormone that helps your brain understand when you’re full.

Under normal circumstances, these hormones are balanced in a way such that the body tries to maintain equilibrium as its natural set point. But when these hormones are disrupted by an extreme diet, that can throw the whole system out of whack and make it very difficult to keep the weight off after losing it.

Simply put, why many people “fail” at weight loss has nothing to do with willpower and everything to do with the way the body tries to keep itself alive.

This fact was demonstrated in dramatic fashion by a 2016 study conducted on contestants who had lost enormous amounts of weight very rapidly as part of the reality television show “The Biggest Loser.” The show, which ran on NBC from 2004 to 2014, put participants with obesity through vigorous workouts while on a low-calorie diet to see who could lose the highest percentage of body weight the fastest. The contestants all lost massive amounts of weight, but within a year of the end of the show, nearly all of them had gained back most, if not all, of the weight they lost. And some had even surpassed their starting weights with that rebound.

The study authors looked into why and discovered that the extreme diet and exercise regimen tipped these contestants into starvation mode and lowered their BMR. They also discovered that the metabolic rate did not return to previous levels even after they gained back all the weight they lost.

A subsequent study from the same team delved further into why this was happening. The researchers found that the body is constantly trying to maintain equilibrium.

The findings were disheartening for anyone who wants to lose weight, but they also validated what people who’ve tried every diet out there and regained the weight already knew: Unless you maintain those extreme calorie restrictions forever, diets don’t work over the long term, and there can be long-term consequences to not eating enough.

Other Effects of Extreme Calorie Restriction

In addition to altering how your metabolism runs, “severe calorie restriction often results in some degree of malnutrition, including micronutrient deficiency,” explains Candace Pumper, a staff dietitian with the Ohio State University Wexner Medical Center in Columbus. Micronutrient deficiencies can lead to a range of symptoms including:

Athletic and physical performance can also be negatively affected when you don’t eat enough calories. For some people, restricting calories too much can lead to hair loss. Women may experience irregularity of the menstrual cycle or a cessation of menstruation all together in severe cases.

Restricting Calories Can Affect Mental Health

Sleepiness, reduced vigor, mental exhaustion, mood swings, brain fog and other psychological symptoms can also crop up if you’re not eating. These can include diet backlash, which occurs when even the thought of a “forbidden” food is enough to trigger overeating, which can grow into a cycle of restriction and deprivation followed by overeating and guilt.

Dieting can also cause an erosion of confidence and self-trust. Certain foods as “good” or “bad” can make for difficult decision making at each meal time. By putting the focus on external factors, such as calorie counting or food rules, you lessen your ability to listen to your brain and body, and it becomes harder to hear signals like hunger, fullness and satisfaction.

Instead, you become more vulnerable to external cues telling you what or when to eat, such as advertising, available food and the time of day. You’re more likely to eat for emotional reasons or just because the food is there, even if you're not hungry.

While you certainly will lose weight by not eating, it’s not the best way to go about sustainable weight loss.

So, What Does Work?

Pumper says that “gradual versus rapid weight loss is associated with greater declines in fat mass and body fat percentage as well as significant preservation of RMR,” or resting metabolic rate, another term for BMR. Gradual weight loss usually means about 1 to 2 pounds per week for most people. This should be achieved through “moderate calorie restriction” rather than not eating or severely limiting your intake of calories, she says.

Weight loss takes lifestyle change, not just a quick-fix diet," adds Mia Syn, a registered dietitian based in Charleston, South Carolina, and author of “Mostly Plant-Based.” In order to stay healthy and look your best long term, you have to make many small changes.” These small changes should add up over time. Examples include watching portion sizes and making healthy swaps of higher calorie and less nutritious foods for fresh veggies and other nutrient-dense foods. There are lots of nutrient-dense foods you can eat, with fresh fruits and vegetables and whole grains being particularly good choices.

Making sure you’re getting adequate sleep and exercise is also important. Getting enough exercise is important for keeping your heart and lungs as well as your muscles strong. It can also help you keep your weight stable. You need sleep to repair your muscles after workouts and to support a healthy immune system, normal brain function and many other aspects of overall wellness.

How Many Calories Do I Need?

So how many calories should you eat each day? Well, that “depends on a few factors, including your basal metabolic rate and activity level,” says Syn. She recommends “seeing your doctor or a registered dietitian to help you calculate and set realistic goals for calorie consumption and physical activity to lose weight safely and effectively.”

That said, to lose 1 pound per week, you’ll need to create a calorie deficit of 500 calories per day, which totals 3,500 calories over the course of the week. One pound is equivalent to 3,500 calories. “However, calorie intake should not fall below 1,200 a day for women or 1,500 a day in men,” Syn says.

Vaca-Flores recommends keeping your calorie deficit to between 250 and 500 calories per day. “This should help you lose a steady 0.5 to 1 pound per week, which is sustainable for most people, and it lessens the risk of slowing down your metabolism.”

This guidance is in line with what the Academy of Nutrition and Dietetics recommends for adults looking to lose weight. They recommend that women should consume 1,200 to 1,500 calories per day and men should consume 1,500 to 1,800 calories per day, but there can be a lot of variation from person to person. “This information is for informational purposes solely,” Pumper says. “It’s not meant to substitute for professional nutritional advice or treatment,” so it’s always best to visit with a dietitian to get tailored advice for your specific situation.

How to Lose Weight While Restricting Calories

Finding the sweet spot between reducing caloric intake enough to trigger weight loss and dipping into that starvation mode where the body tries to preserve weight by slowing down its metabolic rate can be a complicated calculation. Eating less to lose weight is one strategy, but exercise should also be part of the equation.

Shouhed says that you shouldn’t rely on exercise to do the heavy lifting in losing weight, considering the biggest gains come from improving your diet. However, it’s true that exercise can help in several ways, not least of which is that it helps you retain muscle while you’re losing weight. In contrast, if you cut calories too drastically and don’t exercise, your body may begin burning muscle to fuel basic needs, which can further slow your metabolism because lean muscle burns more calories at rest than fat does.

To help retain muscle as you lose weight, Shouhed recommends ensuring that you’re eating enough protein. "You still need carbs and fat, but if you’re increasing the amount of protein that you’re having, you’ll feel fuller longer.” This can help you stick with a reduced-calorie approach while making sure you don’t feel excessively hungry or skimp on the macronutrients your body needs.

Lastly, Shouhed says any changes you make should be incremental. Start small and build up a piece at a time to create a healthier lifestyle. “Do it gradually. Don’t go to extremes.”

Copyright 2022 U.S. News & World Report

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