Report 1: Nutrition assessment: body composition & anthropometric measures
Purpose: When looking at the data collected during studies, nutrition scientists undertake preliminary analysis. This assignment is equivalent to the process during the drafting stages of the results and discussion sections in a research article. To assist you in understanding this data, you have been given some questions to explore the dataset and examine the nuances of using different body composition methods.
Background: Anthropometric and body composition methods are key components of assessing nutritional status. The methods used to assess body fatness vary from sophisticated reference techniques [e.g. total body water (TBW) by the doubly labelled water (DLW) and dual-energy X-ray absorptiometry (DXA)] to more simple techniques such as waist circumference (WC) and body mass index (BMI, kg/m2). There are numerous cut-offs and equations for interpreting the measures taken that are population specific. For nutrition scientists, it is important to understand these differences as it may lead to different conclusions i.e. the proportion of the population “at risk” (Mei et al. 2002, Wickramasinghe et al. 2005).
Task: You have been given a dataset of anthropometric and body composition measures from a group of children and adolescents in Northern Ireland (McGloin et al. 2002 and McCaffrey et al. 2008) and adults from Australia. You have been asked to undertake secondary analysis by answering the following questions: Questions or sections that you need to provide an answer for are in bold and yellow highlight to denote where you should change the information.
Q1: Conduct preliminary analysis and summarise the data in table 1, including the main differences in the results as a consequence of using different techniques to determine body fat.
The following steps summarise the process required to populate Table 1. During the weekly workshops we will further elaborate on the process to create a checklist.
- Use the equations in Appendix B and use SPSS to calculate:
- BMI cut-offs using IOTF; and CDC cut-offs; and WHO BMI cut-offs for adults
- Calculate body density using Durnin & Womersley equations
- body fat (% and kg) and Fat Mass Index using mean body density for the following equations: Siri / Weststrate & Deurenberg and Brozek (Appendix B)
- Check the data for normality (for the total group only) and complete appendix table A.
- Based on your findings from normality testing, complete Table 1 with mean ± SD or median (25th, 75th percentile) as appropriate, provide values for the total group and males and female.
- Provide a title and footnotes for the table where appropriate
- Provide a written summary to answer Q1
Q2: Briefly describe the statistical test you would use to test the differences between the results for the:
(a) differences in % body fat equations (Siri / Weststrate & Deurenberg Vs Brozek) (NB YOU DO NOT NEED TO DO THE TESTS)
(b) differences in % body fat equations (Siri / Weststrate & Deurenberg Vs Brozek) and gender differences (NB YOU DO NOT NEED TO DO THE TESTS)
(c) the two BMI cut-offs for <18yr olds (NB YOU DO NOT NEED TO DO THE TESTS)
Provide a written summary to answer Q2, your answer should indicate the independent variable and dependent variables required for these tests.
Q3: Using the Bland-Altman graph, what is the level of agreement between body composition methods for assessing body fat?
- Draw a Bland-Altman graph for body fat (Siri / Weststrate & Deurenberg as the reference method) and body fat from BIA (test method)
- Draw a Bland-Altman graph for body fat (Brozek as the reference method) and body body fat from BIA (test method)
- Provide legends and figure titles for the graphs
- Provide a written summary to answer Q3
Q4: Provide an overall conclusion that addresses the following:
a) Justify which method would you choose (bioelectrical impedance or skinfolds) to measure body fatness in apparently healthy children, adolescents and adults?
b) In terms of sensitivity and specificity, which of the BMI cut-offs for <18 yrs (IOTF or CDC/NCHS) is most appropriate to determine the proportion of the population classified as obese?
| Total (n 105) | Males (n 31) | Females (n 74) | |
| Age (years) | 20.0 (15.0, 20.5) | 15 (14, 17) | 20 (16, 21) |
| Weight (kg) | 59.6 (53.05, 66.15) | 65.6 (57.9, 76.65) | 58.45 (52.4, 63.23) |
| Height (m) | 1.67 (1.61, 1.73) | 1.76 (1.68, 1.8) | 1.63 (1.61, 1.68) |
| BMI (kg/m2) | 21.38 (19.66, 23.1) | 21.05 (19.57, 23.52) | 21.41 (19.84, 22.85) |
| Waist : Hip ratio | 0.77 (0.74, 0.81) | 0.8 (0.78, 0.82) | 0.76 (0.73, 0.79) |
| WHO BMI cut-offs (n 57) a | 0.0 (0.0, 0.0) | 0.0 (0.0, 1.0) | 0.0 (0.0, 0.0) |
| Overweight % (n 7) | 12.3% | 42.9% | 8.0% |
| Obese % (n 0) | 0.0% | 0.0% | 0.0% |
| IOFT BMI cut-offs (n 45) b | 0.0 (0.0, 0.0) | 0.0 (0.0 ,0.0) | 0.0 (0.0 ,1.0) |
| Overweight % (n 7) | 15.6% | 12.5% | 19.0% |
| Obese % (n 3) | 6.7% | 4.2% | 9.5% |
| CDC/NCHS BMI cut-offs (n 45) c | 0.0 (0.0, 0.0) | 0.0 (0.0, 0.0) | 0.0 (0.0 ,1.0) |
| Overweight % (n 7) | 15.6% | 12.5% | 19.0% |
| Obese % (n 3) | 6.7% | 4.2% | 9.5% |
| Bioelectrical Impedance measures d | |||
| % FM | 14.75 (9.8, 18.68) | 0.14 (0.11, 0.2) | 0.28 (0.23, 0.32) |
| FM (kg) | 0.26 (0.17, 0.31) | 9.4 (6.6, 14.4) | 16 (13.14, 19.99) |
| Fat Mass Index (kg/m2) | 0.09 (0.06, 0.11) | 0.05 (0.04, 0.06) | 0.11 (0.08, 0.12) |
| Skinfold measures e | |||
| Sum of skinfolds (mm) | 49 (32.1, 61.25) | 26.4 (23.9, 41.7) | 53.5 (41.0, 63.25) |
| Body density (g/ml) from Durnin & Womersley | 1.04 ± 0.03 | 1.06 ± 0.02 | 1.04 ± 0.02 |
| Siri / Weststrate & Deurenberg equation e | |||
| % FM | 24.2 (17.9, 28.97) | 13.89 (11.76, 19.58) | 27.4 (23.51, 30.0) |
| FM (kg) | 14.11 (10.39, 17.91) | 9.11 (6.9, 13.1) | 15.92 (12.79, 18.57) |
| Fat Mass Index (kg/m2) | 5.13 (3.85, 6.41) | 2.95 (2.2, 4.32) | 5.8 (4.75, 6.71) |
| Brozek equation e, f | |||
| % FM | 27.2 (22.64, 29.64) | 19.53 (15.61, 20.75) | 28.03 (24.95, 30.26) |
| FM (kg) | 15.71 (12.98, 18.08) | 12.99 (11.82, 14.97) | 15.98 (13.19, 18.2) |
| Fat Mass Index (kg/m2) | 5.61 (4.72, 6,65) | 4.48 (4.18, 5.06) | 5.91 (4.99, 6,7) |
a. WHO BMI cut-offs only for adults aged 20 years and older. 57 participants qualified out of 105; 7 of which categorised as overweight (3 males, 4 females).
b. IOFT BMI cut-offs for children <18 years old. 45 participants qualified out of 105; 7 of which were categorised as overweight (3 males, 4 females) and 3 as obese (1 male, 2 females).
c. CDC/NCHS BMI cut-offs for children <18 years old. 45 participants qualified out of 105; 7 of which were categorised as overweight (3 males, 4 females) and 3 as obese (1 male, 2 females).
d. Missing values for 3 participants (ID 66, 77 and 94).
e. Missing values for 1 participant (ID 98).
f. Brozek equation is used for adults aged over 18 years. 45 participants qualified out of 105. Display of Mean (±2SD) or Median (25th percentile, 75th percentile) was determined by
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