How to calculate the protein needs in under- and overweight persons?

In general, the underweight body (BMI<20) contains more protein per kg bodyweight and the overweight (BMI>25) / obese body (BMI>30) contains less protein per kg body weight. This results in an underestimation of protein needs in underweight persons and an overestimation of protein needs in overweight and obese persons. Therefore, protein needs can preferably be expressed as recommendation per kg lean mass. When information on lean mass is not available, protein needs can be calculated using actual body weight, adjusted weights or estimated fat free mass (FFM) with the equation of Gallagher. In this study, the research question was: ‘Which method of calculating protein needs equals the reference method (measured FFM x 1.5g protein/kg) best?’

Two populations were used to answer this question. The first was the ANAC population (Amsterdam Nutritional Assessment Center, University of Applied Sciences, Amsterdam): relatively healthy overweight and obese adults. The second was the VUmc population: an in- and outpatient population (all medical specialties) of the VU University Medical Center in Amsterdam. FFM was measured with air displacement plethysmography (ANAC) and bioelectrical impedance analysis (VUmc). Three methods to calculate protein needs were compared with the reference method: measured FFM x 1.5 g protein/kg:

A. Measured body weight x 1.2 g protein/kg
B. Adjusted weight of underweight and obese patients for the protein recommendation per BMI group:
– BMI<20 = protein recommendation × height (m)2 × 20
– BMI 20-30 = no adjustment of protein recommendation
– BMI>30 = protein recommendation × height (m)2 × 27.5
C. Estimated FFM (Gallagher equation) x 1.5 g protein/kg.
Mean, range and standard deviation were used to test the validity. An under- and overestimation of 5% was defined as a clinically relevant difference.

The deviation of method A was small in underweight and large in overweight and obese persons. In only 1% of the obese persons, the protein requirement was estimated correctly with method A. This improved to 15-33% with method B. Method C had the largest agreement with the reference method in the normal weight, overweight and obese group. Method A had the largest agreement in the underweight group.

There is a large variation in the results of these three methods to calculate protein needs, especially in overweight and obese persons. Protein needs can preferably be expressed as recommendation per kg lean mass. When information on lean mass is not available, protein needs can be calculated best using estimated FFM with the equation of Gallagher.