How Low Is Too Low When It Comes to Calories?

A common question I get is: How low is too low when it comes to calories? 

After all, if the idea is that eating fewer calories than you burn = weight loss, then it makes sense that some may want to go as low as they can. 

Unfortunately, going too low can not only backfire on your weight goals, but it can also have potentially serious effects on your health. 

Let’s take a look at what calories actually are and what the side effects of going too low can look like. 

What Are Calories?

Scientifically speaking, a calorie is the amount of energy required to raise the temperature of 1 gram of water by 1°C (1.8°F). 

In food terms, calories (kcals) refer to the amount of energy that your body gets from consuming foods or beverages. 

While it can be easy to only think about calories in terms of whether we will gain or lose weight, calories are essential for keeping our bodies functioning properly. 

And when it comes to what our bodies use calories for, there are three main categories: 

• Basal metabolic rate: Your BMR is essentially the number of calories your body needs to perform its most basic, life-sustaining functions. Even if you slept all day, your body still needs energy in order to support vital functions of your heart, lungs, brain, liver, kidneys, and nervous system. 
• Digestion: When we eat, foods have to be broken down into smaller nutrients in order to be absorbed and used by the body. And in order to do this, our bodies require energy. The energy needed for digestion is also referred to as the Thermic Effect of Food. 
• Physical activity: whether it’s running a marathon or walking to the fridge, your body requires energy to fuel any movement that you do throughout the day. 

Now, weight loss is more complex than just calories-in, calories-out, but generally speaking, consuming excess calories can help us gain weight while eating fewer calories than we burn off can cause us to lose weight. 

Still, as you can see above, if weight loss is your goal, there’s still a certain number of calories that your body requires to perform its most basic and essential functions. 

And when we cut calories so drastically that we’re no longer meeting these needs, we can get into trouble. 

Potential Side Effects of Cutting Calories Too Drastically

So what are the potential side effects of consistently cutting your calories too drastically? 

Increased Risk of Nutritional Deficiencies 

Food is more than just a source of calories. 

What we eat also provides our bodies with essential nutrients like proteins, fats, carbs, vitamins, and minerals — all of which are needed in certain amounts to keep our bodies working properly. 

While even individuals eating an optimal number of calories for their lifestyle and goals can still fall short of meeting 100% of their nutrient needs, the risk for deficiency becomes an even bigger concern when calories are greatly reduced.

Some potential side effects of not getting the nutrients you need include:

• Brittle hair and nails, possibly even hair loss
• Low energy levels
• Difficulty concentrating
• Weakened immune system
• Weakened bones
• Irregular heart rhythms and palpitations

Working with a dietitian and logging your intake in Lose It! are both good ways to help make sure that you’re meeting your micro- and macronutrient needs for the day. 

It May Slow Down Your Metabolism

When you cut calories drastically, it’s a shock for your body. And one of the things it’ll do is go into a state called adaptive thermogenesis (sometimes referred to as “survival mode”). 

Basically what this means, is your body will try to slow down how many calories it’s burning to try to prevent weight loss.

While the state doesn’t last forever, studies have shown that even once calories are no longer restricted, your metabolism may still stay in a reduced state for a year or longer.

Another reason why your metabolism can slow down is that weight loss often involves loss of muscle. This can affect your metabolism as muscle tissue is “metabolically active” meaning that it burns calories.

To help out your metabolism, it’s important that you’re maintaining or even building muscle mass. And in order to build muscle, you need a combination of resistance exercise and enough calories to promote muscle growth. 

Curious about maintaining muscle while losing weight? Check out our article on protein pacing.

Can Affect Fertility

In women, drastic reductions in calories and overall inadequate calorie intake has been shown to negatively affect fertility. 

In particular, inadequate calories can lower levels of estrogen and luteinizing hormone (LH) levels, which are essential for ovulation. 

Signs to look out for include irregularities in your menstrual cycle, or a lack of menstruation. 

However, fertility disturbances may not always be symptomatic, which is why it’s important to be keeping your healthcare team in the loop on any changes to your weight or calorie intake.

Can Increase Risk for Gallstones 

Gallstones are hard, pebble-like pieces of material that can form in your gallbladder.

While some are asymptomatic, gallstones can block bile movement in your gallbladder. As a result, you could experience severe abdominal pain and may even need surgery. 

While the incidence of gallstones due to weight loss is overall low, research has shown that individuals following a very-low-calorie diet (800 calories/day or less) may be as much as 3 times more likely to have gallstones compared to individuals on a low-calorie diet (1,200-1,500 calories/day). 

In one study, out of the individuals who did develop gallstones, more than 60% of them had to have their gallbladders removed.

Now, we don’t quite understand why this happens, but it appears that quick weight loss from severe calorie restriction impacts the salt and cholesterol contents of bile, as well as the emptying of bile from the gallbladder — both of which can lead to gallstone formation. 

So How Do You Find That 'Sweet Spot' for Weight Loss?

This is where Lose It! can help as it takes your personal information (such as height, weight, and age), your physical activity, and your weight goals to calculate a good starting point. You can read more on how Lose It’s calculator works here. 

Essentially, to avoid going too low, you want to keep your calories between the amount needed for your BMR and the amount needed to maintain your current weight. 

Not ready to download the Lose It! app yet? You can also try the calculator below to get a basic idea of your ideal daily calorie intake. Note: This is intended to be a general guide only. For more individualized recommendations, please talk with your doctor or dietitian.

The Bottom Line 

While lowering your calorie intake is an essential piece of losing weight, it’s important not to cut so low that your body no longer has the fuel it needs to function properly. 

As food is so much more than just calories, one of the most important pieces of losing weight is making sure that you’re still getting the nutrition that you need. 

Using your Lose It! app and working with a registered dietitian are both great ways to help you find and maintain the right nutrient balance for your personal needs and goals.