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How to improve and measure your body composition

You may have started a new diet and exercise program and feel you are making progress. But when you step on the scale, nothing has changed. Does that mean your efforts “aren’t working?”

Not necessarily.

Or, maybe your doctor tells you that your body mass index (BMI) is high. Do you know what that means, and is it necessarily bad news?

It depends.

The scale and BMI are rudimentary tools for gauging your overall body composition, meaning they don’t tell you enough to determine if you are at a healthy weight.

Here’s an example. A professional athlete may be six feet tall (183 cm), weigh 225 pounds (102 kg), and be in fantastic physical condition with a 32-inch waist (81 cm) and bulging muscles. A busy doctor may be six feet tall and weigh 225 pounds with a 39-inch waist (99 cm) and a bulging midsection.

They have the same weight and the same BMI, but very different health assessments. What’s different? The two men have different amounts of muscle tissue and fat mass. The athlete has a healthy body composition. The doctor perhaps does not.

Knowing your body composition gives you a better picture of your overall health than just knowing weight or BMI.

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Short-term intermittent fasting for fat loss

Time-restricted eating or short-term intermittent fasting, of less than 36 hours, can help with fat-mass loss with minimal to no loss of muscle mass. However, with longer fasting periods, there is a concern about losing muscle mass.

One study shows that six weeks of alternate-day fasting led to a 7% overall weight loss, most of it fat mass.25

Another randomized trial reports more significant fat-mass loss with intermittent calorie restriction than with traditional dieting.26

And an analysis of eight studies looking at time-restricted eating or alternate-day fasting plus resistance training demonstrates maintenance or even an increase in muscle mass.27

Although not all studies agree. For instance, one randomized controlled trial that did not dictate how much or what the subjects ate didn’t demonstrate a benefit for lean mass in those doing 16:8 time-restricted eating.28 One potential lesson is that your diet quality and total caloric intake are still important when engaging in time-restricted eating.

Older studies with longer fasting periods report a negative nitrogen balance — a marker of muscle loss.29 This suggests that the duration of fasting may play a critical role in your body-composition response.

What should we make of this evidence? Time-restricted eating and short-term intermittent fasting may help reduce calories without significant muscle-mass loss. But longer fasts may lead to muscle-mass loss, and what you eat during your “eating window” may impact your body-composition response. Given the limited amount of research in this field, we need studies that assess the long-term effects of intermittent fasting on body composition.


What to avoid to improve body composition

When looking to improve your body composition, what you don’t eat may be just as important as what you do eat. Here are three things to avoid to improve your body composition.

1. Alcohol
Alcohol provides calories without nutrition which can make it difficult to sustainably reduce caloric intake. In addition, alcohol can diminish inhibitions and self control, leading to excessive calorie consumption.30
2. Sugar
Sugar provides calories without nutrition and stimulates the brain’s reward center, encouraging you to eat more and stimulating your cravings.31
3. Ultra-processed foods
Engineered foods like chips, crackers, and treats are designed to make you want more. They tend to be high in fat and carbs, with little protein or nutrition. Their satiety index is very low, and thus you can easily overeat them.32

Summary

High protein diets are most beneficial for adding muscle mass. High protein, low carb diets are also beneficial for losing fat mass and improving satiety.

Other dietary approaches that reduce calories, including intermittent fasting, may also improve body composition, but too much calorie restriction for too long may negatively impact muscle mass.


Exercise

In addition to proper nutrition, exercise can be a powerful tool for improving body composition. Here’s what science has to say about the best exercises.

Time to pump that iron.

How to improve and measure your body composition - the evidence

This guide is written by Dr. Bret Scher, MD and was last updated on January 30, 2023. It was medically reviewed by Dr. Michael Tamber, MD on July 20, 2021.

The guide contains scientific references. You can find these in the notes throughout the text, and click the links to read the peer-reviewed scientific papers. When appropriate we include a grading of the strength of the evidence, with a link to our policy on this. Our evidence-based guides are updated at least once per year to reflect and reference the latest science on the topic.

All our evidence-based health guides are written or reviewed by medical doctors who are experts on the topic. To stay unbiased we show no ads, sell no physical products, and take no money from the industry. We're fully funded by the people, via an optional membership. Most information at Diet Doctor is free forever.

Read more about our policies and work with evidence-based guides, nutritional controversies, our editorial team, and our medical review board.

Should you find any inaccuracy in this guide, please email andreas@dietdoctor.com.

  1. Resting metabolic rate is essentially the amount of calories you burn at rest. You can learn more about it in our podcast with Kirsty Woods, PhD.

  2. Frontiers in Endocrinology 2019: Osteoporosis and sarcopenia increase frailty syndrome in the elderly[overview article; ungraded]

  3. Journal of Nutrition and Metabolism 2014: Measuring outcomes in adult weight loss studies that include diet and physical activity: a systematic review
    [review of non-randomized studies; weak evidence]

    American Journal of Medicine 2014: Muscle mass index as a predictor of longevity in older adults
    [non-controlled study; weak evidence]

  4. Indian Journal of endocrinology and metabolism 2018: Lean metabolic syndrome: An emerging concept
    [overview article; ungraded]

    Journal of Clinical Endocrinology and Metabolism 2014: Risk of developing diabetes and cardiovascular disease in metabolically unhealthy normal-weight and metabolically healthy obese individuals
    [observational study; weak evidence]

    Nutrition, Metabolism, and Cardiovascular Diseases 2006: Normal weight obese (NWO) women: an evaluation of a candidate new syndrome
    [observational study; weak evidence]

  5. Advances in Nutrition 2019: Recent advances in the characterization of skeletal muscle and whole-body protein responses to dietary protein and exercise during negative energy balance
    [overview article; ungraded]

    Medicine Science Sports and Exercise 2010: Increased protein intake reduces lean body mass loss during weight loss in athletes
    [randomized trial; moderate evidence]

  6. American Journal of Clinical Nutrition 2003: Essential amino acids are primarily responsible for the amino acid stimulation of muscle protein anabolism in healthy elderly adults
    [randomized trial; moderate evidence]

  7. A meta-analysis of RCTs demonstrates that eating more protein than average leads to better lean body mass.

    Journal of Nutrition 2020 The role of protein intake and its timing on body composition and muscle function in healthy adults: A systematic review and meta-analysis of randomized controlled trials
    [ strong evidence]

    Observational studies show those who have greater protein intake have greater muscle mass.

    American Journal of Clinical Nutrition 2016: Relation between mealtime distribution of protein intake and lean mass loss in free-living older adults of the NuAge study

    [nutritional epidemiology study with HR<2, very weak evidence]

    American Journal of Clinical Nutrition 2008: Dietary protein intake is associated with lean mass change in older, community-dwelling adults: the Health, Aging, and Body Composition (Health ABC) Study
    [nutritional epidemiology study with HR<2, very weak evidence]

    British Journal of NUtrition 2016: Dietary protein intake is associated with better physical function and muscle strength among elderly women
    [nutritional epidemiology study with HR<2, very weak evidence]

  8. British Journal of Sports Medicine 2018: A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults [systematic review of randomized trials; strong evidence]

    Nutrients 2019: Nutrition and supplement update for the endurance athlete: Review and recommendations. [overview article; ungraded]

    Journal of Sports Science 2011: Dietary protein for athletes: from requirements to optimum adaptation [overview article; ungraded]

  9. “Reference body weight” is a rough approximation of your lean body mass – the part that needs protein. You can look up your reference body weight here or use the simple chart below to estimate your protein needs.

    British Journal of Sports Medicine 2018: A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults [strong evidence]

  10. Some studies report added muscle-building benefit up to about 3.5 grams of daily protein per kilo of bodyweight, which is about 285 grams of protein for a 180 pound (81 kilo) individual.”

    Nutrition Reviews 2020: Dose-response relationship between protein intake and muscle mass increase: a systematic review and meta-analysis of randomized controlled trials
    [systematic review of randomized trials; strong evidence]

  11. In this case, height is a proxy for reference body weight. Reference body weight is a way of estimating how much lean body mass a person of a specific height would have and thus how much protein they need on a daily basis.

  12. Frontiers in Endocrinology 2019: Long-term effects of a novel continuous remote care intervention including nutritional ketosis for the management of type 2 diabetes: A 2-Year non-randomized clinical trial [nonrandomized study, weak evidence]

  13. Nutrition and Metabolism (London) 2004: Comparison of energy-restricted very low carbohydrate and low fat diets on weight loss and body composition in overweight men and women [randomized trial; moderate evidence]

  14. Nutrition and Metabolism (London) 2020: Effects of weight loss during a very low carbohydrate diet on specific adipose tissue depots and insulin sensitivity in older adults with obesity: a randomized clinical trial [randomized trial; moderate evidence]

  15. Journal of Nutrition 2015: A lower-carbohydrate, higher-fat diet reduces abdominal and intermuscular fat and increases insulin sensitivity in adults at risk of type 2 diabetes [randomized trial; moderate evidence]

  16. Journal of the International Societ of Sports Nutrition 2020: Effects of a ketogenic diet on body composition and strength in trained women[randomized trial; moderate evidence]

  17. American Journal of Physiology, Endocrinology, and Metabolism 2017 “Calories in, calories out” and macronutrient intake: the hope, hype, and science of calories
    [overview article; ungraded]

  18. Medicine, Science, Sports and Exercise 2017:
    Effects of weight loss on lean mass, strength, bone, and aerobic capacity
    [randomized trial; moderate evidence]

  19. 2014: Lifestyle intervention involving calorie restriction with or without aerobic exercise training improves liver fat in adults with visceral adiposity
    [randomized trial; moderate evidence]

  20. Obesity (Silver Spring) 2015 Changes in body composition over 8 years in a randomized trial of a lifestyle intervention: the look AHEAD study
    [randomized trial; moderate evidence]

  21. In a 12-day randomized crossover study, people were allowed to eat as much as they wanted on a high protein, normal protein, and a low protein diet. During the high protein portion of the trial, they consumed 500-550 calories less than they did during the normal protein and low protein portion of the trial:

    The American Journal of Clinical Nutrition 2013: Protein leverage affects energy intake of high protein diets in humans [randomized trial; moderate evidence]

    A systematic review of randomized controlled trials found that higher protein diets tend to promote weight loss, due in part to reducing hunger and increasing satiety:

    Journal of the American College of Nutrition 2004: The effects of high protein diets on thermogenesis, satiety and weight loss: a critical review [systematic review of randomized trials; strong evidence]

  22. The American Journal of Clinical Nutrition 2013: Contribution of gastroenteropancreatic appetite hormones to protein-induced satiety [randomized crossover trial; moderate evidence]

    Nutrition Journal 2014: Effects of high protein vs. high fat snacks on appetite control, satiety, and eating initiation in healthy women [randomized trial; moderate evidence]

    Advances in Nutrition 2015: Controversies surrounding high protein diet intake: satiating effect and kidney and bone health [review of observational studies; weak evidence]

  23. Journal of the American College of Nutrition 2004: The effects of high protein diets on thermogenesis, satiety and weight loss: a critical review [systematic review of randomized trials; strong evidence]

  24. Obesity Reviews 2014: Do ketogenic diets really suppress appetite? A systematic review and meta-analysis [strong evidence]

  25. Journal of Diabetes and Metabolic Disorders 2013: The effects of modified alternate-day fasting diet on weight loss and CAD risk factors in overweight and obese women [nonrandomized study, weak evidence]

  26. Practicing cyclical intermittent energy restriction every two weeks led to 26.4 pounds (12 kilos) of fat-mass loss compared to 17.6 pounds (8 kilos) for chronic calorie restriction. Although this protocol is different from many intermittent fasting studies, it demonstrates the success and feasibility of cycling energy restriction. While continuous caloric restriction is usually not sustainable long-term, two weeks of caloric restriction alternating with energy balance might be doable for a greater number of people.

    International Journal of Obesity (London) 2018: Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study [randomized trial; moderate evidence]

  27. Nutrients 2020: The effects of intermittent fasting combined with resistance training on lean body mass: A systematic review of human studies
    [review of randomized and non randomized studies study; weak evidence]

    As an example, the following randomized controlled trial demonstrated people following 16:8 time-restricted eating plus resistance training decreased fat mass and maintained muscle mass. The diet contained 22% of calories from protein.

    Journal of Translational Medicine 2016:
    Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males
    [randomized trial; moderate evidence]

  28. JAMA Internal Medicine 2020: Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial[randomized trial; moderate evidence]

  29. American Journal of Nutrition 1979: Loss of body nitrogen on fasting
    [non-controlled study; weak evidence]

    Klinische Wochenschrift 1975: Nitrogen loss in normal and obese subjects during total fast
    [non-controlled study; weak evidence]

  30. Current Obesity Reports 2015: Alcohol consumption and obesity: An update[overview article; ungraded]

  31. Frontiers in Psychiatry 2018: Sugar addiction: From evolution to revolution[overview article; ungraded]

  32. Public Health Nutrition 2019: Ultra-processed foods: what they are and how to identify them[overview article; ungraded]

  33. Journal of Applied Physiology 1985:
    Effects of aerobic and/or resistance training on body mass and fat mass in overweight or obese adults
    [randomized trial; moderate evidence]

    Combining resistance training and high-intensity cardio may help you gain muscle mass and lose fat mass. “Interventions that combine high-intensity aerobic and high-load resistance training exert beneficial effects that are superior to any other exercise modality at decreasing abdominal adiposity, improving lean body mass, and increasing cardiorespiratory fitness”.

    Obesity Reviews 2021: What exercise prescription is optimal to improve body composition and cardiorespiratory fitness in adults living with obesity? A network meta-analysis[review of observational and randomized studies; weak evidence]

  34. Sports Medicine 2021: The effect of resistance training in healthy adults on body fat percentage, fat mass and visceral fat: A systematic review and meta-analysis [systematic review of randomized trials; strong evidence]

  35. These recommendations are based upon consensus among fitness professionals and are not based on scientific studies.

  36. Active rest means staying active even if you’re not purposefully exercising. Go for a walk. Do some light stretching. Play a physical game for fun. Just don’t sit on the couch all day!

  37. Journal of Applied Physiology 1985: Effects of aerobic and/or resistance training on body mass and fat mass in overweight or obese adults[randomized trial; moderate evidence]

  38. Doing higher intensity exercise for shorter periods of time can be an effective alternative to longer duration, moderate cardio. See below for more information about high-intensity interval training (HIIT).

  39. Nutrition, Metabolism, and Cardiovascular Diseases 2013: Is high-intensity exercise better than moderate-intensity exercise for weight loss?[non-controlled study; weak evidence]

  40. BMJ Open 2016: Waist-to-height ratio as an indicator of ‘early health risk’: simpler and more predictive than using a ‘matrix’ based on BMI and waist circumference [retrospective observational study; very weak evidence]

  41. Obesity Reviews 2014: Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis [strong evidence]

  42. Journal Diabetes Science and Technology 2008: Body composition methods: comparisons and interpretation
    [overview article; ungraded]

    Journal of Investigative Medicine 2018: Advanced body composition assessment: from body mass index to body composition profiling
    [overview article; ungraded]

  43. Contrast Media and Molecular Imaging 2019: Assessment of body composition in health and disease using bioelectrical impedance analysis (BIA) and dual energy x-ray absorptiometry (DXA): A critical overview
    [overview article; ungraded]

  44. Medicine, Science, Sports and Exercise 1999:
    A new air displacement method for the determination of human body composition
    [overview article; ungraded]

  45. American Journal of HUman Biology 1992: Estimation of body fat from skinfold thicknesses in middle-aged and older men and women: A multiple component approach
    [overview article; ungraded]