What's a calorie deficit?

Dietitian Jessica Tilton describes calories as the energy in food, but she also understands that for many of us, they represent so much more.

“What we eat is very important; it's personal,” she says. “We associate so many things with comfort, with love, with family members.  Long-term diet change presents challenges because of these associations.”

Perhaps that’s why the idea of a calorie deficit can feel so much easier said than done. 

Ahead, Tilton shares more about calorie deficits and how balancing your diet and physical activity can help you reach – and maintain – a healthy body weight. 

What is a calorie deficit? 

A calorie deficit occurs when you burn more calories than you consume, Tilton says. 

Your diet and physical activity levels can contribute to a calorie deficit. 

“A caloric deficit can be reached by consuming fewer calories, increasing physical activity levels or both,” Tilton says. “Most weight loss programs recommend a reduction in calories and an increase in physical activity as a sensible and, more importantly, sustainable way to promote weight loss.”  

Diet: Your body gets calories through the foods and drinks you consume. MD Anderson dietitians recommend eating meals rich in whole grains, vegetables, fruits, beans, nuts and seeds.

Physical activity: Your body uses calories to fuel everything from functions like breathing and digestion all the way to physical activities like exercising, Tilton says. The American Institute for Cancer Research suggests a minimum of 150 minutes of moderate exercise or 75 minutes of high intensity exercise each week.

How much of a calorie deficit is needed to lose weight? 

Each person has unique calorie needs. These needs are determined based on factors including age, sex, height, weight and physical activity levels. 

Not everyone needs to be in a calorie deficit. To find out if a calorie deficit is right for you, first determine if your weight falls into a healthy range. This can be done by speaking with your doctor or using at-home tools like a Body Mass Index (BMI) calculator or by measuring your waist circumference. 

If your results fall outside of the healthy range and you'd like to lose weight, Tilton says general calorie deficit recommendations suggest that eating 500 fewer calories per day may help with weight loss. For a more personalized recommendation, she recommends the National Institutes of Health (NIH) Body Weight Planner. This free online tool can tell you how many daily calories are needed to maintain your current weight, as well as how many are needed to reach your goal weight.

Calorie recommendations change based on weight. This means that the calorie deficit you aim for when you start losing weight will change as you make progress. 

This is why Tilton says the general guidance that says eating 3,500 fewer calories per week leads to a pound of weight loss isn’t necessarily true – at least long-term! 

“Weight reduction leads to a change in energy needs. Energy balance is dynamic and leads to change with weight loss,” Tilton says.

The size of a calorie deficit can also affect how quickly you will lose weight, Tilton notes. For example, a 100-calorie deficit will result in a slower weight-loss process than, say, a 500-calorie deficit. But together with healthy changes like increased physical activity levels, even a small calorie deficit can make an impact over time.

How long can I be in a calorie deficit? 

The amount of time you will need to be in a calorie deficit depends on your weight loss goals. Generally, Tilton says it is OK to stay in a calorie deficit until you’ve reached your goal weight as long as you feel comfortable.  

However, she notes that a long-term calorie deficit may suppress metabolism and slow down how quickly you lose weight.

Even in a calorie deficit, choose healthy options to ensure your body gets the nutrients it needs. You shouldn’t feel intense hunger or notice any signs of malnutrition, Tilton says. 

“With a well-balanced diet of whole foods and 150 to 300 minutes of moderate physical activity weekly, many people can remain in caloric deficit until they meet their weight loss goals,” Tilton says.

Is it safe for cancer patients to be in a calorie deficit? 

Cancer patients should speak to their care team about their weight loss goals and if they are safe to pursue while undergoing treatment.

Senior clinical dietitian Timothy Coleman and Tilton note that weight loss is usually not recommended for cancer patients in the following situations:

• While healing from surgery
• While receiving radiation therapy
• While receiving chemotherapy
• If they are at risk for malnutrition

It is important that cancer patients consume enough calories to prevent malnutrition, Tilton says. She notes that cancer itself can affect how many calories the body uses. Additionally, treatment-related side effects such as mouth and throat sores, appetite loss, diarrhea, nausea and vomiting can make it challenging for patients to meet their nutritional needs. 

MD Anderson patients can request to meet with a dietitian who can help them determine calorie needs and provide personalized nutritional support. 

Why am I not losing weight in a calorie deficit?

There are several reasons why you might not lose weight even though you are in a calorie deficit.

First, Tilton notes it can be challenging to accurately assess your own eating habits. In this case, it may be helpful to work with a dietitian who can help you take an honest look at your diet.

She also notes some health conditions and lifestyle factors can affect weight loss. These include: 

• Genetics
• Hormonal imbalances
• Conditions such as insulin resistance  and hyperthyroidism
• Taking certain medications 
• Stress
• Sleep quality

If you believe you’re struggling to lose weight because of a medical issue, Tilton says to make an appointment with your doctor. 

Weight loss can be tricky to navigate alone. If you have any questions or concerns, reach out to health professionals like your doctor or a registered dietitian for support. 

Request an appointment at MD Anderson online or call 1-877-632-6789.