Cystic fibrosis (CF) is one of the most common life-threatening inherited disorders affecting 1/35000 individuals in the US. In the past decades, the median predicted survival of CF patients has increased significantly from 10-12 to over 37 years (1). One of the major factors contributing to increased survival has been the understanding of the role of optimal nutrition on overall health status of individuals with CF.

Forced expiratory volume in one second (FEV1) is currently the best predictor of pulmonary health in CF, and malnutrition in a person of any age with CF has a negative impact on lung function with a more rapid yearly decline in FEV1. Recent data from the Cystic Fibrosis Foundation (CFF) patient registries in the United States and Canada were analyzed to determine whether any association existed between pulmonary function as measured by FEV1 and Body mass index (BMI) for age. BMI-for-age percentile values \>50th percentile were associated with well-preserved lung function and lower values of BMI were associated with incrementally lower values of FEV1 (2). Thus, the CFF recommended that children with CF between the ages of 2 and 20 years maintain a BMI a≥50th percentile for age and sex. However, several publications have demonstrated that fat free muscle (FFM) wasting is evident even in individuals with a BMI \> 50th percentile, and it is unknown whether BMI percentile adequately reflects changes in body composition in this group of patients; therefore, BMI might not be the best marker indicating an optimal nutritional status.

Limitations of BMI as a marker of optimal Nutrition: fat mass versus muscle mass In individuals with CF, BMI is used to assess appropriateness of weight to height for age and sex. Since there is a strong association between BMI and overall health metrics, especially lung health, the CFF recommends that children with CF maintain a BMI ≥50th percentile for age and sex. However, an apparent loss of skeletal muscle may occur in CF patients who retain their normal body weight, indicating that BMI does not provide any information about body compartments and may fail to detect subtle alterations in body composition. Although several epidemiologic studies showed that CF patients who maintain BMI\> 50th percentile will have improved lung functions and survival, recent data indicate that the preserved FFM instead of BMI may have an impact on overall survival in CF (7). Because of the variability in the levels of both fat mass and fat-free mass, and the many combinations of fat mass index and FFM index associated with the same BMI, this weight-height index needs to be carefully interpreted. Because high levels of BMI-for-age are associated with substantial increases in fat mass, BMI is most useful as a measure of obesity. In chronic inflammatory diseases, a change in regional fat distribution is observed with a shift to a more central fat distribution. The latter is associated with an increase in low-grade chronic inflammation, insulin resistance, and cardiovascular risk (8). Therefore, BMI is a good index for assessment of obesity in otherwise normal children, but not adequate or sufficient in the CF population.

In summary, some pediatric patients with CF may have a decreased Fat-free Mass Index (FFMI) with a well preserved BMI of ≥50th percentile. Conversely, some patient might have a BMI\<50th percentile with normal FFMI associated with overall well-being. Current practice forces CF patients to consume high fat and caloric nutrients to maintain a BMI \>50th percentiles incurring the costs of causing anxiety, stress, daily struggle in the family with a potential increase in the pro-inflammatory state due to central fat accumulation. The outcome of the proposed study will contribute to the understanding of the relationships between BMI, muscle wasting and markers of health and wellbeing. If our study demonstrates that a well maintained FFMI rather than the BMI correlates better with good health status in CF, it may result in changes of current nutrition practice in CF at the national or even international level.