Although some combinations of macronutrients or types of foods may affect weight loss synergistically, when it comes to dropping pounds, there’s no denying that calories matter. Eat fewer of them and you’ll see a descending number on the scale.
The flip side of the coin, however, is that — as with everything in life — calorie cutting for weight loss requires balance. Our bodies need sufficient calories to function properly, and it’s definitely possible to eat too few of them — even when you’re trying to lose weight. Reducing calories too drastically for too long can actually derail your efforts to slim down, and even cause health problems.
Healthy calorie cutting for weight loss shouldn’t be a game of limbo (as in, testing how low you can go), but more like a puzzle. Assemble the right pieces in the right way, and you’ll lose weight and maintain your well-being.
Calories and Weight Loss: What’s the Relationship?
It’s common knowledge that eating fewer calories leads to fewer pounds (and eating more has the opposite effect) — but why exactly does this work? It helps to understand that calories are energy, and energy plays multiple roles in the body.
“When you take in calories as an energy source, your body will use most of those calories to sustain basal metabolic rate (BMR), or for biological processes like keeping your internal organs running,” explains Kalyn True, RDN, who practices in Houston. “Your body will also use some of that energy to digest the food that was just eaten (called the thermic effect of food or TEF). The remaining calorie intake should power your daily activities, or activity energy expenditure (AEE).”
The problem is, when we consume more calories than our bodies can use for these processes, there’s only so many places for them to go. “With additional energy left over, these leftovers are then stored for later usage,” True says. “The storage will either be glycogen or fat stores.”
Various factors affect the rate at which each person’s body metabolizes calories. Medical conditions, genetics, and age can all influence how quickly you reach a calorie deficit or surplus. Different types of foods may get processed differently, too. “Calories that come from protein and complex carbohydrates take much longer for your body to digest, so you have more time to use up those calories,” says the bariatric surgeon Felix Spiegel, MD, also based in Houston. “For foods high in sugar, those calories are absorbed quickly, which means you may store them as fat instead of using them for energy.”
What Counts as a Low-Calorie Diet?
A low-calorie diet doesn’t have to be complicated. “A low-calorie diet, in my opinion, is one that contains fewer calories than you generally consume,” says Bonnie Taub-Dix, RDN, of New York City, the creator of BetterThanDieting.com and the author of Read It Before You Eat It — Taking You From Label to Table.
For weight maintenance, the Dietary Guidelines for Americans 2020–2025 suggest a range of 1,600–2,400 calories for women and 2,200–3,000 for men — so you could consider anything below these numbers a low-calorie diet. But some popular diet plans take users to extremely low levels. (The HCG diet, for example, supplies as low as 500 calories per day, and the Master Cleanse offers a range of 600-1,200 calories.) Other low-cal eating plans, like the TLC diet diet hover higher, near the 1,200 to 1,600–calorie mark, or calculate a specific number based on your current weight and goal weight.
How a Low-Calorie Diet May Help or Harm Weight Loss
A low-calorie diet is a straightforward, research-confirmed path to weight loss, with plenty of journals, calculators, apps, and other resources available for calculating your progress. Plus, a standard lower-calorie eating plan doesn’t typically restrict any specific foods (or their timing), so it can be a flexible alternative to certain popular diets that tell you exactly what and when to eat.
That said, a slash-and-burn approach to calories may come with downsides. Meticulous tracking of numbers and portion sizes can be unhealthy territory for some people, ultimately creating disordered eating behavior. If you have a history of an eating disorder or a problematic relationship with food, it’s best to approach calorie counting with caution; enlist the help of a therapist or registered dietitian, if possible.
Regardless of your mental health history, a super-low-calorie diet may not be sustainable in the long term. “For most people, reducing calories alone is not going to be the answer to lifelong success at maintaining a desirable weight,” says Taub-Dix. “It also takes introspection, mindfulness, and some sort of physical activity to make this process successful.” Indeed, a study in the April 2018 Obesity Science and Practice found that improvements in mental health over 12 months were associated with greater weight loss success. And plenty of research supports the notion that physical activity enhances weight loss by burning more calories.
Why Cutting Calories Too Much Can Be Harmful
We all need calories for survival. If your body senses it’s not getting enough, it will fight the process with a mechanism known as “starvation mode” — opposing your sought-after weight loss outcome. “Starvation mode is a defense mechanism that the body uses to prevent fat loss and starvation. The idea is, your body wants to use your fat to keep you alive, so you don’t burn as many calories,” explains Spiegel. “This lowers your metabolic rate, which means you’re using less calories. If you cut back your calories too much, it hinders weight loss.”
To prevent weight loss plateaus, Spiegel recommends a goal of 1 to 2 pounds lost per week. “If you lose more than that, you’re losing body fluid and muscle mass.”
In other words, if you want the weight loss to be permanent, don’t rush the process.
Signs You’re Not Getting Enough Calories
Though a svelte body may be desirable, signs of malnutrition aren’t attractive. According to the Cleveland Clinic, the following are signs that you’re not getting enough calories:
- Fatigue or low energy
- Constant hunger
- Brain fog or inattention
- Hair loss or brittle hair
- Anxiety, depression, or mood swings
- Inability to get warm
How to Follow a Low-Calorie Diet Safely
If you’re looking to trim down by shaving calories, it’s critical to do so safely. Before you start a low-cal diet, consider discussing your plan with a registered dietitian or other healthcare professional who specializes in weight loss. (Bariatric doctors and certified bariatric nurses are other helpful providers.) And before and during your journey, True says to keep a food journal to help you evaluate and reflect on your diet choices. (It can also help you determine whether you’re eating enough.) Tracking apps like Lose It or LifeSum are other useful resources for tallying exact numbers of calories per day.
True recommends paying attention to the contents of your plate, for example by following the USDA’s MyPlate guidance for healthy eating. “A balanced plate would consist of half fruits or nonstarchy vegetables, one quarter of the plate being a plant or animal protein source, one quarter being a carbohydrate source, and one or two tablespoons of fat per mealtime.” This way, you can meet your nutrient needs without going overboard on calories.
Finally, remember that slow and steady wins the race. “The healthiest approach is a moderate decrease in daily calories and a moderate increase in daily exercise,” says Spiegel. “Don’t try to lose too much weight at once. Even decreasing your caloric intake by 200 to 300 calories a day and increasing your exercise by 20 minutes a day can go a long way for your long-term success.”
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