Should You Go On a Low-Calorie Diet?

dinner plate with sad face

If you’ve ever felt tempted to go on a low-calorie diet to lose or maintain your weight, you’re not alone. The internet is full of articles on cutting calories, calorie-controlled meal plans, and how to rev up your metabolism, but could there be a dark side to low-calorie diets? I’m breaking all of it down here to give you the lowdown on metabolism, figuring out the right calorie level for you, what happens when you go on a low-calorie diet, and (most importantly) what to do instead of starving yourself. 

If I eat less than my body needs, I should lose weight, right?

Theoretically, sure.

For many people, weight changes come down to calories in versus calories out. Eat less than your body needs – you’ll lose weight, eat about what your body needs – you’ll maintain your weight and eat more than what your body needs – you’ll gain weight.

BUT – it’s not always so simple.

What if you’re chronically eating a very low-calorie diet? What if you’ve been cycling through diets for years?

You might feel like you’ve reached a plateau. You’ve been eating the “right” way, but you can’t seem to make the scale budge.

Maybe you’ve even been tracking your calories in an app like MyFitnessPal and you’re eating less than 1200 calories per day. Isn’t that what health gurus online say is the magic number for weight loss?

What are you supposed to do? Go even lower in calories?


There are many things you can try, none of which include plunging yourself further into starvation by cutting calories, fasting for days on end, or doing a cleanse.

But first, here’s what you need to know about your metabolism

The word ‘metabolism’ is thrown around a lot — “I have such a fast metabolism,” or “eat 6 small meals per day to speed up your metabolism,” or “going through menopause wrecked my metabolism” — but what does that mystical word even mean? Is it like an engine that keeps your body moving? Is it a tiny creature running on a hamster wheel in your stomach? What do we feed it? Does it need to go for walks?

A few key terms:

  • metabolism – a general term to describe all of the life-sustaining chemical reactions in living things
  • basal metabolic rate or resting energy expenditure (BMR/REE) – the amount of energy your body needs to perform basic functions (like keeping your brain working, heart pumping, and lungs breathing) at rest
  • total energy expenditure or total daily energy expenditure (TEE or TDEE) – your BMR plus the energy you need for movement
  • activity factor (AF) – a number that is used to multiply or add to your BMR to get your TEE
  • low-calorie diet – a diet that contains fewer calories than a person needs to maintain their weight
  • very low-calorie diet (VLCD) – a diet with, at most, 800 calories per day, typically medically supervised and intended for extreme weight loss
  • calories in, calories out (CICO) – balancing the number of calories you take in with your TEE to maintain, gain, or lose weight (i.e. The concept that suggests if you burn more than you take in, you will lose weight, and if you burn less than what you take in, you’ll gain weight.)

Photo by Gesina Kunkel on Unsplash

How to figure out your energy needs:

Figuring out how many calories you need can be a fairly exact science, but it usually isn’t.

The two main ways to determine your calorie needs are:

  • indirect calorimetry – equipment that measures the oxygen and carbon dioxide a person breathes in and out and uses these numbers to determine how much energy is being used. This method is the most accurate way to determine BMR, but it is usually expensive and done in clinical settings.
  • predictive equations – estimate calorie needs using different factors such as sex, age, height, and weight, depending on the equation used. This is the most commonly used method for figuring out calorie needs, as it is widely accessible and does not require special equipment.
    • One of the most trusted REE equations for healthy adults is called the Mifflin-St Jeor equation (see below). Apps like MyFitnessPal often use this equation to recommend calorie needs to you.
      • for females: 10 x weight in kg + 6.25 x height in cm – 5 x age – 161 = REE
      • for males: 10 x weight in kg + 6.25 x height in cm – 5 x age + 5 = REE
    • Your REE is then multiplied by your AF, giving you the total number of calories needed to maintain your weight (TEE).
    • For weight loss, calories are often subtracted from your TEE to put you in a caloric deficit (i.e. taking in fewer calories than you need)

But – humans are complex creatures and we don’t live in vacuums or labs. Equations to estimate your calorie needs (like you’d see in apps like MyFitnessPal, in your dietitian’s office, or online) do exactly what they say they’ll do – they provide you with an estimate.

If you’re tracking your intake accurately and consistently eating fewer calories than your equation suggests, but your weight is still not budging, your BMR is probably much lower than you realize.

Reasons your BMR may be lower than you think it is:

  • Low activity. You may be fairly sedentary and/or overestimating your activity factor.
  • Chronic low-calorie dieting (more on that below)

Photo by Ursula Spaulding on Unsplash

What happens when I’m on a low-calorie diet or under-eating for a long time?

  • Decreased metabolism – In an effort to conserve energy, your body adjusts to low-calorie dieting by slowing down your BMR. Some research estimates that weight loss and/or low-calorie dieting can reduce energy expenditure by 20-25%.
  • Reproductive health issues – If you’re not getting enough calories and nutrients, your fertility and your sex drive can be negatively affected.
  • Gastrointestinal issues – Low-calorie dieting may leave you constipated, and very low-calorie dieting can lead to gallstones.
  • Poor mood/mental fog – Long-term and short-term low-calorie dieting can leave your brain with less of the fuel it needs to function optimally. Lack of food can also negatively affect the bacteria in your gut, which are responsible for making about 90% of your body’s feel-good chemical – serotonin.
  • Physical performance – Lower food intake can lead to lower energy levels, which can affect motivation to exercise or do other extra physical activity throughout the day. Not getting the proper fuel can also make it harder to recover from workouts and build healthy muscles and bones.
  • You’ll miss out on nutrients tooMost Americans eat plenty of calories, but most also get too little of the beneficial nutrients they need like calcium and vitamin D for healthy bones, potassium for vascular health, as well as fiber for healthy digestion and cholesterol. If you’re not eating enough calories overall, you most likely won’t be meeting your nutrient needs either.

What to do instead

1. See where you stand

Get your BMR tested or re-calculate your calorie needs. Track your foods for about 3 days to see how many calories you’re currently consuming.

2. If you’re under-eating, accept that you need to eat more to achieve optimal health

Yep. You read that correctly. Your body needs fuel, no matter what size it is. Eating what your body needs will put you in a position to:

  • boost your BMR
  • have more energy
  • workout harder and more often
  • establish a baseline calorie level that is sufficient if you’d like to reduce calories in the future

3. Make a plan

Add or subtract calories gradually instead of drastically changing them. This will allow your mind and body to adjust.

Adding calories slowly is also called reverse dieting and can be helpful if you’ve been low-calorie dieting for a long time and it’s affecting you negatively. (Either weight loss has stalled, you’re experiencing one of the side effects I mentioned earlier, or you’re just plain sick of it.)

If you’re looking to reduce calories for weight loss, for the love of all that is good, do it slowly. Don’t eat way under your body’s calorie needs and expect that approach to be sustainable. If you have a big weight loss goal, break it down into small pieces (especially if you’re plugging it into MyFitnessPal). This will most likely look like a weight loss rate of 1-2 lbs per week or a calorie deficit of 250-500 calories per day. (This seems to be the sweet spot for sustainability and enjoyment with my clients.)

No matter what your goal is, make sure you have support. An equation or app can’t get to know you. It can’t take your lifestyle, your preferences, and your history into account when assigning you a calorie level. A professional can. If you’re feeling lost, seek out a Registered Dietitian Nutritionist!

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