Learn about the how, why, and when our bodies may change shape and size throughout our life cycle.

In today’s health-conscious society, weight is a topic that often dominates conversations, media headlines, and even academic discussions, but what exactly does it mean to understand the science of weight? As a Registered Dietitian, I find that so many of my patients focus solely on the numbers on their scales, but there are far more intricate details of body composition and influencing factors that literally make up who we are as human beings.

While we’ll be examining this topic as respectfully as possible, this can be a sensitive subject to some, so please consider this a trigger/content warning if you prefer to skip what we’re sharing.

Body composition

Body composition refers to the proportions of various components that make up the human body, including fat, muscle, water, bone, and organs [1]. While total weight provides a snapshot of overall mass, understanding body composition offers deeper insights into health status and risk factors.

Research indicates that different body compositions can have varying implications for health outcomes. For instance, excess fat accumulation, particularly around visceral organs, is associated with an increased risk of chronic conditions such as cardiovascular disease, type 2 diabetes, and certain cancers. Conversely, adequate muscle mass plays a vital role in metabolic health, mobility, and functional independence, especially as individuals age [2].

One of the undergraduate classes I teach is a 300-level course called Nutrition Throughout the Life Cycle where we learn that peak bone density is typically reached by age 30 and peak muscle mass generally occurs between ages 20 and 30. This means that these are the ages our bodies are working automatically, leaving us then to actively manage our bones and muscles as we move into our later adult decades of life. The strength of our bones plays a critical role in preventing osteoporosis and fractures later in life and the mass of our muscles is essential for maintaining mobility, metabolic health, and overall strength as we age [3]. Both are influenced by a combination of factors including diet, physical activity, and lifestyle choices. By understanding and optimizing these factors, individuals can enhance their bone and muscle health, laying a strong foundation for a healthier future.

Understanding body composition can be useful in managing our health and fitness goals, but access and cost might prevent some from getting the true picture. For example, the “gold standard” and most accurate method for assessing body composition is the Dual-Energy X-ray Absorptiometry (DEXA) scan which gives a comprehensive view of body composition, making it a valuable tool for tracking changes in muscle and fat over time. Some medical facilities, including hospitals and specialized radiology centers, offer DEXA scans, though insurance may not cover the cost, so be sure to consult with your doctor to help you determine the suitability and frequency of these scans based on your personal health needs.

If you fall into one of the following categories, you might consider scheduling regular DEXA scans [4]:

• Women aged 65 and older: This group is at higher risk for osteoporosis and fractures, making regular bone density assessments crucial.
• Postmenopausal women under 65: Especially those with additional risk factors for osteoporosis, such as a family history of the disease, smoking, excessive alcohol use, or low body weight.
• Men aged 70 and older: Men in this age group are also at increased risk for bone density loss and should consider screening.
• Individuals with a history of fractures: Those who have experienced fractures from minimal trauma may have underlying bone density issues that need to be assessed.
• Patients with conditions or treatments affecting bone health: This includes those with rheumatoid arthritis, chronic kidney disease, or those taking medications like steroids that can impact bone density.

Daily calorie needs

In the United States, we tend to use a 2,000-calorie diet as a standard reference, but individual needs are so much more nuanced than just what is listed on the back of a nutrition label [5]. There are many ways and reasons that a Registered Dietitian may need to rely on body weight, especially those in clinical settings where keeping a person alive via tube feeding is a crucial part of understanding caloric needs. RDs may calculate and recommend certain energy (calorie) requirements based on formulas such as those listed below that take into account an individual’s life cycle and various factors such as age, sex, weight, height, and physical activity level:

• Basal Metabolic Rate (BMR): BMR represents the energy required to maintain basic physiological functions at rest, including respiration, circulation, and cellular metabolism. It can be estimated using equations like the Harris-Benedict or Mifflin-St Jeor equations, which factor in age, sex, weight, and height.
• Physical Activity Level (PAL): This method involves multiplying the BMR by an activity factor that corresponds to the individual’s level of physical activity. PAL values range from sedentary to very active, providing a more accurate estimate of daily energy expenditure.
• Estimated Energy Requirements (EER): The EER is calculated using standardized equations that incorporate age, sex, weight, height, and physical activity. These equations help determine the daily caloric intake needed to maintain energy balance.
• Total Daily Energy Expenditure (TDEE): TDEE includes BMR, physical activity, thermic effect of food (energy used for digestion), and adaptive thermogenesis (energy expenditure in response to changes in environment or diet). This comprehensive approach provides a detailed estimate of daily energy needs.
• Life Cycle Considerations: Energy requirements vary across different stages of life. For example, infants and children require energy for growth and development, while adults need energy primarily for maintenance. During pregnancy and lactation, women have increased energy needs to support fetal growth and milk production.

Metabolic health

In healthcare settings, accurately assessing body weight may be used for disease prevention, diagnosis, and treatment planning. While the body mass index (BMI) is a commonly used metric for estimating body fatness based on height and weight, it has limitations, particularly in distinguishing between fat mass and lean mass. Not to mention its history appears to be deeply rooted in eugenics [6]. Alternative methods such as waist circumference measurements, bioelectrical impedance analysis, and the DEXA scan mentioned above may provide more nuanced insights into body composition.

Weight is influenced by many factors, such as genetics, metabolism, environment, and psychology. While genetic predispositions may indicate that some individuals are more prone to certain body types or metabolic traits, lifestyle and environmental factors play a significant role in shaping weight outcomes. Socioeconomic status, access to nutritious foods, level of physical activity, stress levels, and cultural influences all contribute to variations in body weight across populations.

Psychological factors such as stress eating, emotional triggers, and disordered eating patterns can significantly impact weight management efforts. Understanding the complex interplay of these factors is essential for developing personalized interventions that address the root causes of weight-related issues.

Because of the above reasons, one of the ideas I continue to promote in my classes (and with my clients) is that not everyone in a smaller-sized (skinny) body is metabolically healthy, and not everyone in a larger-sized (fat) body is metabolically unhealthy. If you feel that a healthcare provider is biased against you because of your weight, or if you’re uncomfortable being weighed for any reason, you have the right to decline to get on the scale [7].

That said, there may be certain medical conditions where knowing the weight of a patient is a necessity. This may depend on if you have a heart- or kidney-related issue where you might be retaining fluids which can worsen your disease. You might also need to be weighed if you’re receiving anesthesia for surgery or if you’re undergoing treatment for an eating disorder. (You can still opt out of watching while you’re being weighed or knowing what your weight is in your chart.)

If you’re a clinician reading this, be sure to not only consider total weight but also body composition when evaluating health risks and formulating treatment plans, as well as honoring and being sensitive to your patient’s requests so you can tailor your interventions to address specific needs and optimize health outcomes in a mutually beneficial setting.

Weight bias

Weight bias can lead to weight discrimination which is the unfair treatment of individuals because of their weight. These unfair treatments can include health professionals spending less time, providing less emotional rapport with patients categorized as overweight or obese, or denying treatment altogether.

Weight cycling, commonly known as yo-yo dieting, involves repeated cycles of losing and regaining weight, which can have several negative health implications when people are constantly chasing—unsuccessfully, unsustainably, and unrealistically—an ideal weight, whether through their own pursuits or because of the sometimes medically unnecessary over prescribing of weight loss by many healthcare providers.

The science of weight is a multifaceted discipline that extends far beyond simplistic notions of dieting and calorie restriction. By understanding the complexities of body composition, recognizing the diverse factors that influence weight outcomes, and implementing evidence-based management strategies, individuals can achieve sustainable improvements in health and well-being. Embracing a holistic approach that addresses both physical and psychological dimensions is key to unlocking the full potential of any health goal.

References

1. The Utility of Body Composition Assessment in Nutrition and Clinical Practice: An Overview of Current Methodology – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8399582/
2. Puberty: Physical Activity and Growth – https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/body-composition#:~:text=In%20terms%20of%20tissue%20composition,short%20term%20within%20narrow%20limits.
3. Age at attainment of peak bone mineral density and its associated factors: The National Health and Nutrition Examination Survey 2005-2014 – https://pubmed.ncbi.nlm.nih.gov/31760214/
4. When Should I Get a Bone Density Test? – https://www.columbiadoctors.org/news/when-should-i-get-bone-density-test
5. Gordon, A. (2019, October 18). The Bizarre and Racist History of the BMI. Medium. https://elemental.medium.com/the-bizarre-and-racist-history-of-the-bmi-7d8dc2aa33bb
6. Development of Prediction Equations for Estimated Energy Requirements – https://www.ncbi.nlm.nih.gov/books/NBK591021/
7. Blind weigh-ins and how to say “no” to being weighed – https://withinhealth.com/learn/articles/blind-weigh-in