Your low-calorie diet might be eating away at your muscles (and your mood).

Karla Walsh; Nutrition review by Lisa Valente, M.S., R.D.
March 03, 2020

Remember the old mantra "calories in vs. calories out"?

"This theory has been totally disproven," says Rachel Fine, R.D., a registered dietitian and owner of the nutrition counseling firm To The Pointe Nutrition. "I often bring up the famous 2016 New York Times article about some of the contestants from The Biggest Loser, who after several years of follow up, had reported gaining back most—if not all—of the weight that had been lost on the show."

Getty / Supreeya Chantalao / EyeEm

Beyond weight gain, the study spelled out the negative impact of severe caloric restriction on metabolism, which can contribute to the common scale yo-yo.

"Creating an overall energy deficit is necessary for weight loss, but taking extreme measures to drastically reduce calorie intake is not advisable," says Michelle Hyman, M.S., R.D., C.D.N., a registered dietitian at Simple Solutions Weight Loss, nor is it sustainable.

Plus, if you're only considering how many calories you believe you're burning during spin class or your morning walk, you're ignoring all the hard work your body is doing behind the scenes. Experts estimate that your total daily energy use is broken down into three main parts:

  • 70 percent: Basal metabolic rate, or how much energy your body needs completely at rest
  • 10 percent: Thermic effect of food, AKA the number of calories your body burns simply digesting and absorbing what you eat
  • 20 percent: Physical activity

We know that millions of Americans consume too many calories, but how many of us are actually eating too few? And could this be one of the issues in our obesity epidemic?

"While I can't give an exact number, I would say it's a lot more people than you might initially think. Most don't even know they aren't eating enough calories, and many times it has nothing to with being under- or overweight," says Lisa Moskovitz, RD, CDN, CEO of the private nutrition practice NY Nutrition Group in the New York City metro area.

The amount of calories you need per day varies based on several factors, including height, age, activity level, gender, and medical conditions, but here's Moskovitz's general rule of thumb:

  • If you're sedentary, estimate 10 calories per pound (1,500 for a 150-pound person)
  • If you're mildly active, estimate 13 calories per pound (1,950 for a 150-pound person)
  • If you're moderately active, estimate 15 calories per pound (2,250 for a 150-pound person)
  • If you're highly active, estimate 18 calories per pound (2,700 for a 150-pound person)

For perspective, here's what 2,000 calories per day looks like on our heart-healthy Mediterranean meal plan.

Considering the average woman included in the latest National Health and Nutrition Evaluation Survey (NHANES) reported that she consumed 1,803 calories per day, there's a fair chance you might want to add more to your plate to see a lower number on the scale. (Or to gain the weight and energy you need if you're currently underweight!)

So how do you know if you're underdoing it? Here are seven signs you might not be eating enough calories, according to RDs.

1. You're Constantly Hungry

"Restrictive dieting can result in sharp decreases in the body's fat stores, which further results in hormonal deficiencies that control appetite, such as leptin. We often see dieters chronically hungry and generally out-of-tune with their intuitive feelings of hunger and fullness," Fine says.

Intuitive eating helps you manage portions naturally, no calorie-counting required, by tuning in to your body's innate feelings of hunger and fullness.

Related: I Tried Intuitive Eating for 2 Weeks—Here's What Happened

2. You Constantly Think About Food

Uber-restrictive diets impact the body and the brain. One landmark study from World War II found that those who are hungry do more than plan future meals. They anticipate them so much they can barely think of anything else: Men forced to lose 25 percent of their body weight by severely restricting calories would dream, fantasize, talk and read about food almost obsessively (and struggled to focus on anything else).

3. You're Cranky

Speaking of that brain, the neurotransmitters that send brain signals to maintain your mood need calories to run normally as well.

"Without enough nutrients that help regulate and promote feel-good neurotransmitters in our brain, you may start to feel more down and less happy overall. Irritability can also be a big issue in those who aren't fueling properly," Moskovitz says. (P.S.: These well-balanced recipes may help turn that frown upside down.)

4. Your Muscles Are Going MIA

Every body has several different metabolic zones in terms of calorie consumption, Moskovitz explains: A weight-gain zone, a maintenance zone, a weight-loss zone and a "starvation" zone. The latter puts you at risk for nutrient deficiencies, and if you're also not eating enough to cover your basal metabolic rate (that 70 percent level we mentioned above), your body can go into storage mode and instead of using the calories you eat for energy. That way, the fat is handy as a protective mechanism since the body thinks it needs it.

While you might expect the body might start eating muscle for energy on a body with few fat stores, it does the same on a body that has extra fat stores too—if it thinks it needs an alternate energy source.

"That means you'll lose lean muscle mass which can also impact your bone health. It could put you at risk for osteopenia, or even osteoporosis," Moskovitz says.

5. You Have No Energy

Your body will do everything it can to fight back against the self-imposed state of famine to defend a genetically determined set point weight, Fine says. Restricting calories will cause the body to conserve energy to vital processes, meaning that you'll feel fatigued sooner.

"Without enough calories, your body will not have the fuel it needs for daily activities, let alone exercising at the gym," Moskovitz says.

6. You're Not Sleeping Well

Even though your energy is dragging, you may find that you struggle to sleep if you're chowing down on too few calories.

"Your body can't rest when it's looking for more nutrients," Moskovitz says.

7. If You're a Female, Your Period Hits Pause

Over time, the body learns to adapt to this "new normal" of a calorie deficit and becomes remarkably efficient at prioritizing what's vital—and what's not, Fine says. That means breathing and blood circulation take priority, and any non-essential processes (such as reproduction) get demoted.

"Reproduction, while essential for the survival of a species, is not essential for the survival of an individual being. For females, a missing period can be a sign of not eating enough calories. The hormonal imbalances that ensue further result in a vast number of negative impacts including impaired bone health," Fine says.

So if you're trying to conceive in the near future and you believe you might be eating too few calories, work with a dietitian to design a meal plan that includes a sufficient number of calories.

Up Next: What to Eat—and Avoid—If You're Pregnant

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