The excellent review by Bailly et al. ^{(Reference Bailly, Germain and Galusca1)} highlights various abnormal clinical conditions important to help rule out constitutional thinness, including eating disorders, associated pathology and over-exercise, along with a history of weight, physiological menses and weight gain resistance. However, two additional factors that can significantly influence health should also be considered in constitutional thinness: body fat metabolism and skeletal muscle strength. Dysfunction in these tissues may be at least or more common than those noted by the authors (who note that constitutional thinness individuals do not seem to be characterised by a very low body fat percentage despite their low BMI).

Body fat is an important metabolic tissue. Abnormalities include underfat and overfat, which impair health^{(Reference Maffetone, Rivera-Dominguez and Laursen2)}. While overfat, whose global prevalence may exceed 80 %^{(Reference Maffetone and Laursen3)}, is a common cause of chronic disease, physical impairment and raises the risk of infectious disease, both overfat and underfat are associated with immune impairment, in particular reduced glutathione and increased oxidative stress levels^{(Reference Margaritelis, Theodorou and Kyparos4)}. Both conditions can also influence the appearance of leanness.

Reliance of body weight and BMI to assess leanness can be deceptive as these measures may not accurately reflect body fat content^{(Reference Maffetone, Rivera-Dominguez and Laursen2)}. Forty percent or more of normal-weight, non-obese individuals may be overfat^{(Reference Maffetone and Laursen5)}. The waist-to-height ratio is an effective clinical tool to rule out overfat, while dual-energy X-ray absorptiometry can accurately determine percent body fat.

Sarcopenia, whose prevalence in the elderly may be as high as 50 %, is associated with significant loss of muscle mass yet is often concurrent with excess body fat (called sarcopenic overfat) leading to both a reduction of weight and impaired adiposity^{(Reference Maffetone, Rivera-Dominguez and Laursen2)}. Cachexia is also associated with significantly reduced muscle mass and can occur due to unhealthy changes in fatty tissue. Both sarcopenia and cachexia can influence the appearance of thinness.

Muscle weakness is a common clinical condition that raises the risk of adverse health outcomes including physical impairment, morbidity and all-cause mortality^{(Reference Duchowny, Peterson and Clarke6)}, with hand-grip strength an indicator of overall body strength and a predictor of health outcomes^{(Reference Wang, Bohannon and Li7)}. While muscle mass contributes significantly to weight and BMI, it is not necessarily associated with muscle strength as thin individuals with lean muscles can be strong due to the increased muscle fibre contraction.

The assessment of body fat and muscular strength is an additional factor important in patients presenting with concerns about excess thinness.

I am the sole author and declare no conflicts of interest.