Section
7
Measuring Body Composition in Population Health Research: Case Studies
Case Study 2 Assessing Adiposity in Infancy to Predict Risk of Developing Overweight and Obesity
Background
A longitudinal observational study in pediatrician offices and pediatric health clinics where infants receive routine clinical care is designed by a team of investigators to evaluate changes in adiposity in infants and toddlers aged 0–2 years. Interest in this topic arises from findings that rapid early life adiposity gain is associated with an increased likelihood of later health problems, including cardiovascular disease, insulin resistance, and overweight and obesity. The goal of this study is to describe patterns of growth (changes in adiposity) during the first 2 years of life and observe how these patterns relate to developing overweight and obesity at age 2 years.
Considerations
The study team understands that children grow rapidly during the first 2 years of life, and changes in length and weight will vary considerably across children. Because the body composition assessment methods would need to be conducted at multiple time points after baseline, the methods must be sensitive to detecting changes in body composition over time. The study team will also need to enroll and measure a large sample (n=2000) of children to model growth-related fat trajectories with sufficient sensitivity to inform on differences in the accumulation of fat.
The study team notes that methods would need to be performed at routine clinical appointments to avoid the need for extra visits, thereby minimizing burden on families. Pediatric clinics serve as an ideal location for this type of data collection because privacy is ensured, and length, weight, and head circumference measures are components of the routine well-child care (health supervision) visits which are scheduled as follows: the first visit is 3 to 5 days after birth, and subsequent visits are at 1, 2, 4, 6, 9, 12, 15, and 18 months and at 2 years. Study data collection would not need to occur at all these visits but at a minimum of six visits, including at an early and late (2-year) visit. Sufficient time during the clinic visit needs to be allowed for study team members to conduct the additional measurements. Acceptability of the measures to parents who will provide consent is another key consideration.
Method Selection
The study team recognizes that length and weight are the most common measurements taken, as they are the least intrusive, generally require little training and equipment, and can be conducted quickly and with minimal costs. However, length and weight and their relative indices such as weight-for-length and weight-for-age percentiles do not provide information on body composition, specifically FM or FFM. Such indices assume that a higher percentile reflects additional or excess FM and fail to consider the contribution of the FFM component to weight. The study team finds this to be potentially problematic because a higher index could reflect greater lean mass rather than FM.
The team considers other potential methods to more specifically assess total body fat. Skinfold thicknesses of the triceps, subscapular, iliac crest, and mid-thigh can be used to monitor changes in subcutaneous fat, which is a very good proxy for total body fat at these ages. These methods are noninvasive and often acceptable to parents. However, skinfolds would be difficult to acquire in the context of routine pediatric visits due to their complexity. These methods require data collector skill and training to conduct and additional time to acquire, especially in older infants and toddlers where compliance with measurement is more challenging. The study outcomes would be skinfold thicknesses of each of the four sites plus the sum of all four skinfold thicknesses, the sum of suprailiac and subscapular skinfolds as an index of central fat, and the sum of triceps and thigh as an index of peripheral fat. Methods that can estimate whole-body FM, such as DXA and ADP, would not be feasible in the pediatric clinic setting for many reasons including lack of accessibility, cost, the need for trained/technical staff, and time.
Ultimately, the study team finds skinfold assessment to be a strong option but decides against it for several reasons. Because the team will need to conduct measurements during multiple routine well-child care visits on a large sample of children, the additional staff time and resources needed to train the team as well as collect five separate measures makes the use of skinfold thicknesses less feasible. They ultimately select assessment of weight and length because it is the least intrusive of adiposity assessment methods. It can also be conducted quickly, making it much more feasible to conduct at multiple time points and with a large sample of children. The trajectories of change for weight-for-length and weight-for-age z-scores from first visit (early after birth) to 2 years would be modeled to identify patterns of increasing adiposity, including a pattern of rapid weight gain, which is a change in weight-for-age z-score >0.67 during the first 2 years. A score of 0.67 represents the difference between centile lines on standard growth charts, and an increase of 0.67 can be interpreted as an upward centile crossing through at least one centile line. The study can also assess differences in trajectory patterns and how they relate to developing overweight or obesity at age 2 years.