Guide to the 4-Compartment Body Composition Model

Traditional "body fat scales" utilize a "2-Compartment Model", dividing body weight into "fat mass" (weight of body fat) and "fat-free mass" (weight of everything else). Body fat is of course important to monitor, but "everything else" can also reflect important signals regarding health, and that's the topic of this article!

 

So what does "everything else" include? Some professional Bioelectrical Impedance Analyzers use a "4-Compartment Model" that divides body weight into (1) Water, (2) Protein, (3) Minerals, and (4) Fat (i.e. separating "everything else" in the 2-C model into water, protein, and minerals). Each of these outputs is utilized differently!

 

Body water is crucial for storage and transportation of nutrients, closely related to metabolism. How can results be interpreted?

 

The most commonly accepted "normal" definition for body water is "Total Body Water÷Fat-Free Mass=73.3%".

 

What does it mean if Total Body Water exceeds this percentage? In short, the body is holding onto more water than "normal". In the case of relatively sedentary office workers, this water retention could be caused by insufficient physical activity and too much sodium intake.

 

However, for those with physically active lifestyles, the body may be holding onto water due to good adaptation! For example, medium-to-high intensity aerobic exercise utilizes higher plasma volume, and muscle activity promotes storage of muscle glycogen; storage of such energy would be generally seen as beneficial in this case.

 

What if Total Body Water is lower than "normal"? There are various possible reasons why the body is holding onto less water, such as lifestyle or dietary habits. It could be a "good" reason, such as good sodium excretion ability (metabolism), but it could also be a "bad" reason, such as insufficient hydration or low carbohydrate intake, resulting in lower muscle glycogen reserves.

 

The key point regarding Total Body Water is that interpretation isn't just about the number itself; the overall context may drastically change interpretation.

 

An important disclaimer is that the above descriptions apply onto healthy populations. Various health conditions such as those affecting blood pressure, kidney function, cardiac function, insulin resistance, metabolism, and the relationship between these factors all may require additional diagnosis and evaluation from qualified professionals.

 

Total Body Water (TBW) is comprised of a combination of Intracellular Water (ICW) and Extracellular Water (ECW) . "Edema" refers to an imbalance between ICW and ECW. As such, "high TBW" does not necessarily reflect "edema".

 

The body uses glycogen as a source of energy during activity. The body transforms carbohydrates into glycogen after digestion after the immediate energy needs are satisfied, and each gram of stored glycogen requires 2.2-2.6g of water. For those with high-intensity activity requirements, the body gradually learns to store glycogen with increased efficiency. As such, body water can be used as a way to observe stored water, and how well the body is adjusting to workout intensity, schedule, and diet.

 

 

Nitrogen balance is an important indicator of the body's metabolic protein levels. Protein within normal range generally indicates a metabolic level comparable to that of most healthy populations. In the long term, maintaining a healthy diet and lifestyle will keep this value relatively stable, achieving better metabolic levels.

 

If the protein result is lower than normal range, it may be a sign of negative nitrogen balance, which can lead to muscle tissue loss as well as poor immune system function. In this case, it would be recommended to supplement protein in daily diet, while continuing to measure and observe results to see if they can increase to normal range.

 

Patients with Chronic Kidney Disease (CKD) generally suffer from Protein Energy Wasting (PEW) syndrome, but this is a diagnosis that would require further evaluation by professionals. Even if protein is below normal range, it might not be directly caused by disease!

 

There is indeed positive correlation between protein and skeletal muscle mass! In other words, higher skeletal muscle mass generally means higher protein results as well.

 

A minor adjustment in dietary proportions may be necessary. A carb-heavy diet may increase body water ratio, if protein intake is relatively low. In the long-term, lower protein in the body will gradually reduce the activity of cellular enzymes in the body, and as slower metabolism generally has a variety of adverse effects, it's recommended to adjust diet to increase protein intake.

 

"Minerals" are comprised of both bone and non-bone material, affected by an individual's physical activity, lifestyle, and nutritional factors.

 

In addition to nutritional factors, physical activity also plays an important role affecting mineral content. Activity patterns common among athletes tend to promote bone synthesis, and mineral mass tends to be higher among such populations compared to the average person.

 

Looking at long-term trends, mineral proportion in healthy populations tends to be fairly stable, only decreasing after middle age or menopause.

 

Aging generally causes a gradual decrease in mineral content. In addition to age, insufficient physical activity and imbalanced nutrition can also lead to mineral loss. In addition to nutritional supplements and more frequent physical activity, sufficient sun exposure and vitamin supplements are important ways to stay healthy.

 

Note that mineral deficiency is not directly related to osteoporosis! Osteoporosis should instead be diagnosed through bone density testing such as DXA.

 

Osteoporosis can only be diagnosed by measuring bone density and related T-score. However, in terms of physiological information, mineral content below standard range could be caused by low physical activity, imbalanced nutrition, or micronutrient deficiencies. As such, using mineral mass and observing if it remains within normal range is important for reducing risk of related diseases, through adequate and balanced nutritional intake and regular physical activity.



In summary, professional body composition analyzers can provide much insight beyond fat level! But perhaps most importantly, observing long-term trends is more important than a one-time snapshot. Lack of radiation or other side-effects makes BIA an excellent measurement option in this regard, allowing for regular follow-up!