DLM: BMI (2001)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

To better determine the influence of body weight on serum lipid and lipoprotein levels in a random sample of white American men.

Inclusion Criteria:
  • Participant in NHANES II conducted from 1976 through 1980, a representative sample of civilian, non-institutionalized residents of the US
  • White men.
Exclusion Criteria:
  • Women
  • Black men
  • White men with imputed data
  • Subjects taking meds to lower serum cholesterol
  • Subjects with questionable data.
Description of Study Protocol:

Recruitment

  • Participant in NHANES II conducted from 1976 through 1980
  • Stratified, multistage, probability cluster sample of households throughout the US.

Design

Cross-sectional.

Blinding

Not applicable.

Intervention

Examinations were conducted at 64 sites and included anthropometric measures, 24-hour recall and blood sampling.

Statistical Analysis

  • Mean serum lipid and lipoprotein cholesterol levels were calculated for men according to six BMI categories. Results were further divided into three age groups.
  • Regression formulated analysis of covariance model was the primary statistical analysis model. A separate analysis of covariance model was constructed for each group to avoid too much smoothing or linearization of the results.
    • Mean lipid and lipoprotein cholesterol levels for each BMI groups were adjusted for differences in cigarette smoking, dietary intake of saturated fatty acids and dietary cholesterol intake.
  • Sampling weights and design effects were incorporated into all statistical analyses
  • Adjusted data were also analyzed with simple linear regression to determine, using individual values, whether the correlation between the lipid or lipoprotein level and BMI was statistically significant
  • A second analysis for the significance of linear trends between categories of BMI was performed to determine whether a significant linear change in the mean of the lipid variable occurs for the successive BMI categories after changes attributable to standard deviation are taken into account.
  • All available data for each serum lipid variable were used in the statistical analysis
  • Significance was defined as P<0.01. 
Data Collection Summary:

Timing of Measurements

  • Weight
  • Height
  • Triceps and subscapular skinfolds
  • 24-hour recall
  • Blood samples for serum lipids.

Dependent Variables

  • Serum lipids: Analyzed using Lipid Research Clinics methodology
    • Total cholesterol
    • Triglyceride
    • HDL cholesterol.

Independent Variables

  • BMI categories
    • Less than or equal to 21kg per m2 
    • 21.1 to 23kg per m2 
    • 23.1 to 25kg per m2  
    • 25.1 to 27kg per m2  
    • 27.1 to 30kg per m2  
    • More than 30kg per m2
  • Age categories
    • 20 to 44 years
    • 45 to 59 years
    • 60 to 74 years. 

Control Variables

  • Cigarette smoking
  • Dietary saturated fatty acid intake (percent of energy)
  • Dietary cholesterol intake.
Description of Actual Data Sample:
  • Initial N:
    • 4834 had total cholesterol values.
    • 85% had HDL cholesterol values
    • 2,846 had fasting serum TG values; however 1,837 had data for TG
    • 83% had data for calculating LDL cholesterol
  • Age: See table
  • Ethnicity: White
  • Other relevant demographics: See table
  • Anthropometrics: See table 
Variable All Men 20-44 years 45-59  years 60-74 years
Age (years) 42 31 52 66
BMI (kg/m2) 25.5 25.1 26.3 25.7
Present smokers (%) 40.1 43.8 41.0 26.7
Energy intake (kcal)                                                                                                          2,487 2,727 2,328 1,937
Energy intake from saturated fatty acids (%) 13.3 13.2 13.5 13.1
Dietary cholesterol (mg per day) 438 447 442 403
  • Location: National survey, US.

 

Summary of Results:

Using linear trend analysis, changes in BMI (from 21.2 to 23kg per m2  to 27.1 to 30kg per m2 in men 20 to 44 years of age were associated with:

  • A total cholesterol level 0.59mmol per L (23mg per dL) higher (P<0.01)
  • A non-HDL cholesterol level 0.70mmol per L (27mg per dL) higher (P<0.01)
  • LDL cholesterol level 0.59mmol per L (23mg per dL) higher (P=0.03).

In middle-aged men (45 to 59 years) and older men (60 to 74 years) changes in BMI (from 21.2 to 23kg per m2 to 23 to 27.1kg per m2) were associated with smaller but still significant differences in:

  • Total cholesterol levels: Higher by 0.31mmol per L (12mg per dL) P<0.01 and 0.28mmol per L (11mg per dL), P<0.01, respectively
  • Non-HDL cholesterol levels: Higher by 0.37mmol per L (14mg per dL) P<0.01 and 0.25mmol per L (10mg per dL), P<0.01, respectively
  • LDL cholesterol unchanged.

The BMI associated differences in TG (higher by 0.70 to 1.33mmol per L; 62 to 118mg per dL, P<0.001) and HDL cholesterol levels (lower by 0.18 to 0.39mmol per L; 7 to 15mg per dL, P<0.001) were of similar magnitude in all age groups.

Author Conclusion:
  • Excess body weight is associated with deleterious changes in the lipoprotein profile. Higher BMI was associated at all ages with ­ higher TG,  lower HDL cholesterol and higher total and non-HDL cholesterol levels.
  • In young men, the higher total cholesterol was reflected mainly in the LDL cholesterol level; in middle-aged and older men, in the non-HDL fraction
  • Programs to reduce CHD by improving lipid levels should include more emphasis on achieving and maintaining ideal body weight
  • Since HDL cholesterol is inversely related to CHD, the fall in HDL cholesterol with BMI should impart additional risk for CHD. Thus, obesity may be a­ risk for CHD in two ways:
    • By the concentration of atherogenic, apolipoprotein B-containing lipoproteins
    • By decreasing HDL cholesterol.
Funding Source:
Government: NHLBI; National Center for Health Statistics, Centers for Disease Control and Prevention, US Department of Health and Human Services
University/Hospital: University of Texas Southwestern Medical Center, Dallas
Reviewer Comments:

Non-HDL cholesterol defined in this study as the difference between the total and HDL cholesterol levels, represents cholesterol in all lipoproteins that contain apolipoprotein B (VLDL cholesterol).

Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? Yes
3. Were study groups comparable? N/A
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) Yes
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? Yes
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? Yes
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? N/A
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? N/A
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? No
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes