Obesity, pregnancy and ethnic differences
An increase in thoracic fat might lead to a stiffer thorax; the literature on this is not equivocal. Total lung capacity, residual volume, the VC and the FEV1 are not affected by obesity. In males fat deposition is predominantly central; the FRC decreases by increased abdominal fat both in the supine and sitting posture. There is therefore a greater chance that airway closure occurs in dependent lung regions during normal tidal breathing. Cotes et al. (ref. 1-3) demonstrated that obesity does have some negative effect on lung function indices.
Pregnancy similarly has minor effects on spirometric indices. The total lung capacity and the VC remain unchanged, but the FRC decreases by 10-25 %. This similarly may lead to airway closure in the normal tidal breathing range.
In children and adolescents lung function is positively correlated to the Quetelet index (body mass index, BMI: body mass/ height²); this might be because a higher index reflects more muscle mass. In boys there is no relationship between obesity and lung function, in girls an increased Quetelet index appears to be associated with somewhat ‘better’ lung function: the putative explanation is that fat deposition is central in boys, and peripheral in girls where it would not affect the lung (ref. 1). However, the body mass index is a biased index of obesity, as it correlates positively with standing height.
Differences between ethnic groups are dealt with elsewhere.
| Ref. 1 - Body weight and lung function | |
| 1 | Schwartz JD, Katz SA, Fegley RW, Tockman MS. Analysis of spirometric data from a national sample of healthy 6- to 24-year-olds (NHANES II). Am Rev Respir Dis 1988; 138: 1405-1414. |
| 2 | Fung KP, Lau SP, Chow OKW, Wong TW. Effects of overweight on lung function. Arch Dis Child 1990; 65: 512-515. |
| 3 | Cotes JE, Chinn DJ, Reed JW. Body mass, fat percentage, and fat free mass as reference variables for lung function: effects on terms for age and sex. Thorax 2001;56:839–844 |