The Body Mass Index (BMI) and body weight do not give enough information about a patients body composition. Measuring the fat free mass gives us the extra information we need. The body composition can be measured in several ways. Unfortunately non of these techniques is a golden standard. That is why different techniques are combined in a full Nutritional Assessment.
How to measure?
- Protein status/ muscle mass: mid-arm muscle circumference and muscle area, grip strength, nitrogen balance, plasma protein and plasma urea;
- Fat reserves: triceps skinfold and BMI;
- Body water: bio-electrical impedance analysis, biochemistry, fluid balance, rapid weight change (due to ascites, oedema).
Read more about the composition of the human body in this article. Dieticians of the Maastricht UMC+, use different methods to assess a patient's body composition. For instance:
- Body weight
It is important to monitor the body weight on a regular basis, because it is an important indicator for the results of the nutritional therapy. A patient's body weight helps the dietician decide when to decrease the amount of treatment. The body weight is measured by using a digital scale. Preferably the patient wears no clothes or shoes and the weighing is done at a set time, after a toilet visit. If it is not possible to weigh the patient standing in an upright position, a weighing seat or weighing bed can be used.
The measured weight may vary by scale, therefore it is important to always use the same scale. In patients with oedema, ascites, dehydration or other disturbances in the fluid balance, the measurement is not very useful for determining the nutritional status. If it is not possible to measure the body weight at all, ask the patient or the patient's family for the regular body weight.
In the Maastricht UMC+, a wall mounted measuring tape is used for measuring height. The patient is measured without shoes, standing in upright position, looking straight forward and with their heels against the wall. If a patient cannot stand or sit, like in the ICU, the height can be estimated by measuring the arm span (or wingspan); the length from one end of an individual's arm (measured at the fingertips) to the other when raised parallel to the ground at shoulder height at a 90º angle. The average reach correlates to the person's height. Hereby help from a nurse is needed. Another way to estimate the height is by Lower leg measurement . According to Chumlea et al. (1985) knee height is a good alternative for measuring height of elderly and bedridden people. By using a nomogram knee height can be converted to height with a precision of ± 6 cm. You can find the calculations on this page.
Bioelectrical Impedance Analysis (BIA)
This indirect method for measuring body composition is based on the conduction of an alternating electrical current applied to the human body. BIA determines the electrical impedance, or the opposition to the flow of an electric current through body tissues. A flow of an electric current passes easily through tissues containing a lot of water and electrolytes, such as blood and muscles. Fat mass, air and bone are harder to pass. Therefore, the larger the fat free mass, bigger the capacity of the body to lead the electric current. This way, the water compartments of the body can be determined. The intracellular (ICW) and the extracellular water (ECW) together form the total body water (TBW). The TBW can be used to estimate fat-free body mass. and, by difference with body weight, body fat. On this page you can read more about this method.
If there is no bioelectrical impedance machine present, the other methods can also be used to estimate the amount of fat and fat free mass in the body. Together, they give us a clear idea of a person's nutritional status.
A skinfold calliper is used to determine the thickness of a skinfold. With the aid of skinfold measurement, a prediction of total body fat mass can be made. The measurements are based on the hypothesis that the body fat is equally distributed over the body and that the thickness of the skinfolds is a measure for subcutaneous fat. You can find more information on this page. On the page Anthropometry you van find more information about calculating the upper arm muscle circumference, the upper arm fat and muscle area, and the upper arm fat index.
Calculating ideal weight
A person's ideal weight can be calculated by measuring the wrist circumference or elbow breadth. The latter is a more valid method, because the wrist circumference is influenced by a person's body weight, and especially the fat percentage. On this page you can find more information on calculating ideal weight.
People who have undergone an amputation can have a different physique. It is often hard to assess how this affects a person's body weight. According to Brunnstom (1992) the contribution of the individual body parts on the total body weight is as follows:
- Head: 7%
- Trunk: 43%
- Whole arm: 6.5%
- Upperarm : 3,5%
- Forearm : 2,3%
- Hand: 0,8%
- Whole leg: 18,5%
- Femur: 11,6%
- Lower leg: 5,3%
- Foot: 1,8%
The Waist-Hip Ratio (WHR) and the waist circumference
Because not only malnutrition or a low body weight is a threat for your health, but also overweight. We also mention the Waist-Hip Ratio and the waist circumference. With this method patients with (an increased risk of) cardiovascular disease can be detected. You can read more about these methods on this page.
Fat Free Mass Index (FFMI)
Expressing Fat Free Mass Index (FFMI) and Body Fat Mass Index (BFMI) as percentages of body weight is unsatisfactory. It is more useful to express them as an index of height. Height indices of the BFM and the FFM are useful indicators of a person's nutritional status. You can read more about this method on this page.