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What is the relationship between body composition and caloric need?

Dear Alice,

In attempting to preserve lean body mass, what is the relationship of body composition to caloric need: Is it true that no matter how lean a person is, the same amount of calories is needed to preserve lean body mass or the leaner a person is, the more calories are needed to preserve lean body mass? Which one is true? And thank you.

Dear Reader,  

You’re right that body composition impacts the calories needed to maintain weight. Bodies are made up of water, muscle, fat, and bone, and the composition refers to the amount of lean body mass (muscle) and fat mass in the body. Keeping body composition within a certain range may be associated with lower risks for certain health conditions. There are several strategies for measuring body composition, but how accurate or descriptive the measurements may be is dependent a variety of factors. Whether you’re trying to maintain or reach a specific body composition, both the number and source of calories (protein, carbohydrates, fats) play a role. It might help to talk with a registered dietitian to get advice if you're considering making any changes to your diet. It can also be helpful to determine what may motivate these changes in the first place. In the meantime, keep reading to learn more about factors that impact body composition and how to measure it.   

Factors such as sex assigned at birth, age, diet, activity level, and genes all impact body composition. For instance, people assigned male at birth tend to have more muscle mass and less fat mass than people assigned female at birth. Also, as people age, lean muscle mass decreases, making it more difficult to maintain the same body composition. While eating a moderate amount of protein can help with developing and sustaining muscle mass, it’s also best paired with strength-training activity to increase lean body mass, build muscle, and burn fat. Adequate protein consumption through diet within two hours of physical activity may be most effective for building lean body mass. Other studies suggest that dairy may play a role in reducing fat while maintaining muscle. Ultimately, because muscle requires more calories to be sustained than fat, those who have more lean body mass tend to expend and need more calories to maintain muscle mass.  

For those who are looking to understand their own body composition, it can be measured in several ways, including:  

  • Underwater weighing (hydrodensitometry) involves submerging a person in a tank of water and having them expel the air out of their lungs. While this method is the most accurate for measuring body composition, it’s not easy to administer and can be expensive.  
  • Skinfold measurements measure the subcutaneous fat (fat under the skin) around specific body parts (triceps, waist, thigh, and back) using a device called a skin caliper. The quality of the skin caliper, the person performing it, and the formula used to calculate the percentage body fat impact the accuracy of the skinfold test.   
  • Bioelectrical impedance is a simple, non-invasive technique that uses electrical conductivity to estimate lean body mass. This test depends on upon hydration status because muscle holds most of the water in the body, meaning the more muscle a person has, the better the conduction.  
  • Infrared interactance uses a fiberoptic probe to measure subcutaneous fat and muscle at the biceps. Whether this method actually measures body composition is inconclusive as it’s relatively new.  
  • Magnetic resonance imagine (MRI), CT (computed tomography), and PET (positron emission tomography) scans create a visual display of specific body areas, showing deep fat compared to bone. This technique is expensive and hasn’t been proven to be more accurate than underwater weighing.  

These strategies vary greatly in cost and accuracy. Of the measurements listed, the skinfold measurement and bioelectrical impedance are the most affordable and accessible options for those interested in determining body composition. In fact, many gyms have staff that are trained to take skinfold measurements and have bioelectrical impedance devices to assess body composition.    

While guidelines about body mass can be a helpful starting point, everyone has unique needs when it comes to their health. Two people who are the same height and weight may need different amounts of energy or calories to maintain their weight, depending on their body composition. Also, sex assigned at birth, age, eating plan, activity level, and genes influence body composition, and therefore, caloric needs. If you’re thinking about making any changes to your life, it may be helpful to think about why you’re interested in making them. For example, in what way do you feel they may support or hinder any health goals, your lifestyle, or other values that you center in your life? The answers to these questions may help you determine how to make adjustments that best align with your goals. For more information, you may want to check out Go Ask Alice’s Nutrition & Physical Activity archive.  

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Last updated Jul 13, 2022
Originally published May 06, 2005

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