BODY COMPOSITION VS. BMI: WHAT THE SCALE DOESN’T TELL YOU ABOUT YOUR HEALTH!

Medically Reviewed by Dr. Rosmy Barrios - September 02, 2024

It’s no secret to most people that excessive weight serves as a risk factor for several chronic lifestyle conditions. As a result, weight loss is largely associated with health improvement in those that are overweight.  While technically true, this view is often overstated. Using weight as a tool for assessing one’s health is complex and the digits that appear on the scale do not give the full story. In actuality, maintaining an optimal weight does not always equate optimal health, as it depends on how the weight is distributed.

Weight distribution depends upon many of the complex cellular processes that ultimately dictate one’s metabolism and resultant body shape. These factors are a far more accurate reflection of one’s true state of health than total weight alone. Body composition gives a direct insight into how a person’s weight is distributed, which is why it is preferred by many healthcare professionals when assessing a person’s metabolism.

The following discussion takes a look at what body composition is, how it works, and why it is more accurate than a BMI in terms of assessing one’s overall health.

What is Body Composition?

Body composition refers to a measurement of the macro components of the body. This gives one an idea of how weight is distributed throughout different types of body tissues.

Body weight is composed of more than just fat. Most cells are essentially membranes made of fats and proteins, which house fluids, organelles (consisting of more proteins and fats) and many other chemical substances. The way that cells distribute fluids, fats, proteins and minerals is governed by cellular metabolism and is what makes up our body composition.

At a macro level, body composition is divided into water or fluids, bone tissue, muscle mass, and fat mass. Fat is further divided into brown and white fat. White fat accumulates just under the skin and is generally what people consider when thinking of their weight. Brown fat insulates our organs and is absolutely essential for protecting them.

Depending on the metabolic activity of these tissues; some areas of the body may be too thin and prone to wasting, while other areas may be too large and prone to generating excessive weight.

Metabolic variation across tissues of the body gives rise to different body shapes and profiles, some of which are associated with poorer states of health than others. For instance, someone may weigh a lot but end up having heavier bones or more muscle, which may be a sign of good health. Another person may have an ideal weight and still be carrying excess fat, having minimal muscle tissue and weak bones.

In this way, assessing body composition can help a skilled practitioner to understand more about the metabolism of the person and to assess if their weight is truly concerning or not.[1]

How is Body Composition Measured?

There are many methods for measuring body composition. Depending on the method, body composition can be measured in various degrees of complexity. The simplest measures divide the body into fat mass and fat-free mass. This is known as a two component model of body composition and is generally sufficient for practitioners to keep tabs on weight fluctuation.

More advanced methods include more components and can ascertain body water, bone mineral content, protein content and soft tissue mineral content. The most sophisticated techniques have managed to assess other factors relevant to health, such as muscle glycogen levels.[2]

Skinfold Testing

Most practitioners that work with body composition will use specialized skin calipers to take measurements of skinfolds at various body sites, recording variations over time.

This test is also used to assess the composition of the skin tested in terms of muscle, fat and fluids. The accuracy depends on the skill of the physician and is prone to an error margin of approximately 3% on either side[3]. Most agree that the error is minimal and that this is an accurate enough method for those that want an optimal body shape.

For more advanced analysis of body composition, more components need to be evaluated.

Advanced Body Composition Measurement Methods

Aside from the skin calipers test, other methods used to measure body composition require the use of sophisticated equipment and are most often employed in the context of sports medicine and nutrition[4]. Examples include:

• Dual-Energy X-Ray Absorptiometry (DEXA) Scan. This is one of the best technologies available for measuring body composition. It works by sending two low x-ray beams through the body, which interact with bone and soft tissue respectively and takes tissue-specific measurements[5]. Due to the physical properties of these tissues, a very accurate body composition can be calculated from the scan results[6]. This method may lose accuracy for assessing body composition in critically obese individuals (over 500lbs).[7]
• Hydrostatic Weighing. This method uses water displacement to take a reading of body volume. When matched against a person’s above-water weight, their under-water weight can be used to work out their body fat percentage[8]. This is because fat is less dense than bone and muscle relative to the water. Air mass is removed from the reading and is calculated from expiration testing and trapped air estimates[9]. Hydrostatic weighing is also sometimes referred to as underwater weighing and is very accurate for distinguishing fat mass from fat-free mass.
• Bioelectrical Impedance. This method can assess body composition through measuring the resistance of one or more bioelectrical currents as they pass through the body. Body fluids and tissues with more fluid in them such as muscle are more conductive and will show up as having less resistance. Tissues that have fewer fluids, such as fat, will have a higher degree of resistance. Using an estimate of total body water, body composition can be calculated from the results[10]. The accuracy of bioelectrical impedance has been disputed amongst experts[11]; particularly for use in clinically ill patients, epileptics, those with abnormal tissue hydration and those with electrical implants (e.g. pacemakers).
• Air Displacement Plethysmography (BOD POD). This method uses a BOD POD to measure body mass and volume to give an indication of body density. Body mass divided by volume can give a good estimate of both fat and fat-free mass. Inside the BOD POD, air is displaced when a person sits inside, which is used to generate their volume. A built-in scale weighs the person and various calculations pertaining to the pod’s internal environment help to assess thoracic air content alongside a few other parameters pertaining to body composition. [12]
• Total Body Counting of Potassium. Total body potassium can be measured with whole body counting. This method measures the whole body content of potassium 40 (K-40), a naturally occurring radioactive isotope of potassium. This form always constitutes exactly 0.0118% of total body potassium and therefore can be used to accurately calculate potassium levels. A highly specialized setup is required to take this reading. The patient lies between two sodium iodide sheets inside a radiation-shielded room. The gamma radiation being emitted from the K-40 is then recorded and the whole body potassium can then be calculated.[13]
• MRI Scan. MRI (Magnetic Resonance Imaging) uses a combination of magnetism and radio waves to get a very accurate picture of body tissue. A cross section of tissue can be analyzed from three different views (sagittal, vertical and horizontal) that can give in-depth information about fat and muscle distribution, as well as bone to a lesser extent[14]. Although MRI scans can be used to assess body composition with a high degree of precision[15], they are more often employed by researchers[16] and healthcare professionals to monitor health concerns such as tumors, heart abnormalities and brain lesions.
• CT Scan. Computerized Tomography is a very similar medical imaging technique to an MRI. Instead of magnetism and radio waves, it uses radiation to take measurements and produce an image. The images produced from a CT scan are typically of a lesser quality than those seen in an MRI; however, CT scans are more common in clinical practice as they are more cost-effective. In terms of assessing fat and muscle distribution, the difference in image quality is negligible[17]. Like an MRI, CT scans are usually used to monitor health concerns and the composition of specific problem areas, as opposed to assessing whole body composition. They are sometimes used to validate the results of other body composition tests.[18]
• Ultrasound. Another effective tool for assessing body composition includes ultrasonography. Ultrasound devices work by generating a sound wave that penetrates the body and reading the resultant “echo” as the sound reverberates off tissue.[19] The image produced by ultrasound devices depicts a precise greyscale rendition of all the tissues measured. In spite of this, the technique is not as accurate as an MRI or CT scan and is prone to misinterpretation. Nevertheless, ultrasound can be used to take quick measurements in a cost-effective manner[20]. Ultrasound tests are now available that are able to take accurate, cost-effective readings of skinfolds with minimal invasiveness and no radiation exposure.

Body Composition, Disease Risk and the “Obesity Paradox”

While excessive fat is associated with an increased risk for many chronic lifestyle diseases, it is not the only predictor of them. The complexity of body composition emphasizes this point.

Muscle wasting, incorrect fat distribution, fluid retention imbalances and weak bones are common to many individuals irrespective of their total weight. These factors are central to many diseases and may not be obvious at a glance. They also highlight how the metabolism of every person is unique, even between those with the same BMI.

The Obesity Paradox

Obesity is known to predispose people to metabolic disorders and cardiovascular diseases. This has led many to the conclusion that a lower BMI is healthier. When looking at BMI across the population however, the risk of disease and all-cause mortality is substantially higher in those with a very low BMI compared to those with a very high BMI; leading researchers to investigate what became known as the “obesity paradox.” [21] [22]

As it turns out, many individuals with a naturally high BMI are metabolically healthy[23]. Body composition analyses of these cases reveal that disease risk increases substantially in those with a lower fat-free mass. Excess fat does not serve to compensate for reduced muscle mass, placing many with little muscle mass at a higher risk of disease irrespective of their fat mass.[24] While muscle mass is crucial, both muscle and fat distribution play significant roles in health. Since muscle weighs more than fat, BMI is once again shown to have limited application.

Fat-Free Mass, Aging and Disease Risk

Sarcopenia (muscle wasting) appears to be the most common change in body composition seen in the aging process and the majority of chronic diseases, including obesity, cardiovascular disease and osteoporosis.[25] [26]

A very low fat-free mass and a very high fat mass each correlate with an increased mortality risk in both adults and children. Increased fat mass is however associated with a lower risk of acquiring demyelinating diseases and may help improve prognosis in genetically susceptible individuals[27].

Irrespective of fat mass, a moderately high fat-free mass and increased muscle strength have been associated with lower disease risk[28]. In those receiving chemotherapy, muscle loss is common and an increased fat-free mass is associated with better disease prognosis and treatment outcomes.

The aging process is also associated with decreased muscle mass and bone density as well as increased fat mass, around organ tissues and/or under the skin.[29]

In a very large Danish study, in-depth body composition correlations were drawn with regard to aging.[30] The study took the body compositions of just over 57 000 people, aged 50-64 years, and found the following:

• People with the low end of a high fat-free mass reduced the risk of mortality by 20-23%.
• Individuals with exceedingly high fat-free mass did not increase or decrease mortality risk.
• Mortality risk was reduced by 11-14% in those with a moderately high fat mass, but was increased by as much as 5-11% in those with an exceedingly high fat mass.

Body Types and Weight Distribution

Weight distribution also has a role to play in terms of body composition and disease risk. However, using body types as a means of diagnosing a health condition is fundamentally flawed. Research has revealed that those most at risk of cardio-metabolic diseases are those that have an irregular body type which does not fall into any known category. As a general rule of thumb, even weight distribution is a far better indicator of health than weight itself (whether excessive or deficient).

If weight accumulates unevenly, it is often a sign of poor health. A common sign of this would be the “apple shape” where weight tends to accumulate around the belly and sometimes in the thighs. Men are more prone to acquiring this shape during aging. Women are more prone to a healthier “pear shape” where weight accumulates on the hips and is more evenly distributed.[31] The hormonal differences between men and women facilitate this trend, suggesting that women are better equipped for handling a bit of extra fat while men are better equipped for building more muscle.

In metabolically healthy individuals with even weight distributions, body types (also known as somatotypes) have been identified. They include:

• Ectomorphs (skinny phenotype)
• Mesomorphs (curvaceous phenotype)
• Endomorphs (stocky phenotype)

Individuals with extreme characteristics from any of these groups were shown to bare an increased disease risk[32] [33]. Some advocate eating for one’s somatotype, however this view has been heavily critiqued. It is far more accurate to have one’s body composition assessed.

Bariatric Surgery, Severe Obesity & Body Composition Outcomes

Bariatric surgery is recommended for treating very severe cases of obesity for which all other options have failed. Body composition testing in patients that have undergone bariatric surgery shows that it is effective for normalizing excessive fat and water retention, but may promote muscle wasting.[34]

Factors Affecting Body Composition

Body composition is governed by our metabolism, which is largely affected by our environment, genes, diet and lifestyle.

The Effects of Diet on Body Composition

Adequate nutrition is a vital component of maintaining optimal health. A balanced diet offers the right proportions of all essential nutrients that can facilitate a healthy body composition.

A closer look at body composition across patients shows that excesses and deficits in tissues are likely to be linked to a form of malnourishment.[35] For example, deficits in the gut microbiome and the nutritional profile of obese individuals contribute to excessive fat mass[36]. These changes also correlate with age-related obesity and muscle wasting.

In those that are not malnourished, the below points summarize effects of dietary nutrients in relation to body composition:

• Carbohydrates

Carbohydrates confer mixed results on body composition, largely due to differences between types of carbs and their relation to other nutrients.

In general, a low carb diet, high in fat or protein, appears associated with lower fat mass[37]. When coupled with a near equal percentage of fat however, more carbs promoted less long-term weight gain in some individuals.[38]

Moreover, low glycemic load diets proved to reduce intra-abdominal fat (related to the “apple” body shape) by as much as 11% compared to high glycemic load diets.[39] This highlights the differences between starchy carbs and fibrous carbs.

• Fat

High fat diets do not necessarily cause increased fat mass in the body. Average fat intake in the US has been declining over the last few decades and has been clearly linked with a rise in obesity numbers.[40]

Very high fat (ketogenic) diets promote rapid weight loss, and diets that stick to a range of between 18 and 40% revealed no difference on fat mass in studies.

• Protein

Protein is typically associated with increases in fat-free mass, particularly muscle. In a study on 605 healthy Spanish participants, higher BMI’s and fat-free mass were associated with increased protein intake as well as higher levels of physical activity[41].

Many aged individuals battle to digest proteins and deficiency in relation to muscle wastage are commonplace in this population.

• Micronutrients

Micronutrient deficits, such as minerals and vitamins, are also associated with sarcopenia and reduced fat-free mass, especially bone mass. In children and adults, muscle fitness and consuming a micronutrient-rich diet, high in fresh fruits and vegetables, is associated with vastly improved bone mineral density.[42]

Exercise and Body Composition

Decreased physical activity has not only contributed to the steady rise in obesity over the years. Many people with reduced physical activity levels are prone to muscle wasting, irrespective of weight or diet.

In order to properly make use of the nutrients in our diets, our cells need to have an optimal metabolism and energy balance. Exercise is an often under-estimated part of maintaining health in this regard.

Consistent physical activity builds bone and muscle, reduces excessive fat or fluid stores, enhances cellular respiration, and promotes optimal cellular turnover and regeneration. All healthy cells theoretically benefit from the metabolic effects of exercise, which serves to normalize body composition.

All forms of moderate exercise tend to reduce stored fat, build muscle, promote oxygenation, and reduce the risk of chronic disease in healthy members of the population.[43]

Different types of exercise affect body composition and health uniquely:

• Aerobic exercise is excellent for improving cardiovascular health and is required to keeping the heart strong.
• Weight-bearing exercise is absolutely essential for building strong bones.
• Stretching and exercises that improve limb coordination are good for enhancing balance and aspects of cognition.

Exercise may have a negative impact on body composition in those that are severely malnourished, energy deficient or suffering from a severe infection. Constant intensive exercise is associated with hormonal disruptions that are likely to facilitate an unhealthy body composition.[44]

Other Metabolic Factors that May Affect Body Composition

Aside from diet and exercise, the following may also affect cellular metabolism and alter body composition:

• Hypothalamic-Pituitary-Adrenal Axis and Hormones. Hormonal disruption tended to result in excessive fat mass and unbalanced hip-to-waist ratios in both men and women.[45] Low energy intake has been shown to cause disturbances in the HPA axis that may promote excessive weight gain. Hence while severe restriction diets may induce rapid weight loss in obese people, but are not conducive to optimizing body composition.[46]
• Liver and Digestive function. Impaired digestion reduces nutrient absorption and increases the risk of undernutrition; which in turn further impairs digestion[47]. This tends to lead to reduced overall mass, particularly in severe cases of starvation. The composition of the liver also heavily affects body composition, as evidenced in studies looking at mild to severe liver cirrhosis. Liver damage is linked with reduced overall tissue mass and an increase in extracellular water.[48] Increased liver fat serves as a risk for insulin resistance independent of overall weight[49].  
• Mental-emotional well-being. Mood and self-esteem tend to affect physical activity levels and dietary habits. In conjunction with body composition disruptions, states of disease tend to lower the quality of life and greatly increase the risk of depression.[50] Some studies reveal that depressed individuals are inclined to have an increased fat mass[51]. Reduced self-esteem and a distorted body image in those with anorexia clearly contribute to reduced overall body mass.
• Genetics. While relatively little is known about this topic, genetics are known to shape metabolism, weight distribution and body composition[52]. Hereditary genes set the bar for one’s baseline metabolism and are often similar between relatives. Epigenetics refers to the impact of the environment on one’s genes and is also noted to alter metabolism and phenotype. Those exposed to similar environmental factors may show some similarities in terms of body composition, however these will be moderated by their hereditary traits, diet and physical activity levels.

Conclusion

Having a healthy weight is not necessarily the same as having an ideal BMI, which is why the scale may not tell the full story about your health. A healthy weight means that the weight is distributed evenly, and that there are no deficits or excesses in any type of tissue mass.

Body composition measurements of bone, fat, muscle and body fluids can give a far more accurate insight into weight distribution and are preferred over BMI’s by top healthcare specialists. Genetics, nutritional status and physical activity levels largely shape one’s metabolism and overall body composition. Environmental and lifestyle factors are also known to affect weight distribution.

Maintaining muscle appears to be the most important factor to focus in terms of managing weight for health purposes. Increased muscle mass is associated with more graceful aging and a reduced risk of both disease and mortality, irrespective of fat mass.

To learn more about the immune system and how it functions, talk to a doctor. To search for the best doctors and healthcare providers worldwide, please use the Mya Care search engine.

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