Meta-analysis or Systematic Review

M - Click here for explanation of classification scheme.

POSITIVE: See Quality Criteria Checklist below.

To evaluate the impact of various study design factors, including duration of the intervention, caloric deficit, exercise prescription and others as well as subject characteristics, including age, sex and initial body weight, on the rate of weight loss.

• Articles published between January 1995 and December 2009
• Articles reported weight loss results from studies with an intervention period of at least six weeks' duration in healthy adult subjects and used dietary counseling to induce a reduction in subject caloric intake by a fixed amount
• Placebo arms of weight loss drug studies were eligible for inclusion if results from all recruited subjects who completed the study were included
• Only data from treatment arms that provided calorie restriction were included in the analysis
• Informed consent was obtained.

• Treatment arms that involved the use of a weight loss drug
• Studies that uniquely reported results of an intent-to-treat analysis 
• Treatment arms involving diet restrictions but not caloric restriction.

Search Procedures
Literature search of published studies in English language conducted in PubMed using search terms weight loss and diet.

Study Quality Assessed
No.

Design

• A total of 35 weight loss studies published between 1995 and 2009 were identified that used dietary counseling to induce weight loss in healthy subjects
• Acceptable articles were published between January 1995 and December 2009, reported weight-loss results from studies with an intervention period of at least six weeks' duration in healthy adult subjects and used dietary counseling to induce a reduction in subjects' caloric intake by a fixed amount
• Variables that were associated with rate of weight loss among age, sex, initial body weight, frequency of dietary counseling, placebo use, exercise level, study length and prescribed energy deficit were examined using linear regression analysis.

Intervention Investigated
Dietary counseling to induce reduction in subjective caloric intake.

Statistical Analysis

• P<0.05 used to determine statistical significance 
• General linear model procedure was performed to determine which variables were associated with weekly change in weight.

Type of Information Abstracted from Articles

• Change in body weight in kg per week
• Study length
• Prescribed calorie deficit
• Frequency of dietary counseling
• Percentage of female subjects included in study
• Age
• Initial body weight of subjects
• Presence or absence of placebo
• Exercise
• Study length was the length of time between baseline and end-point body weight measurements.

Combination of Data

• Each individual variable abstracted from the articles was abstracted on a scale
• All study-related variables were converted to weekly values.

Analytical Methods

• General linear model procedure performed to determine which variables associated with weekly change in weight
• Interactions were examined between sex and exercise, energy deficit and exercise and frequency of counseling and energy deficit
• Quadratic effect of energy deficit and age were tested
• Terms not statistically significant were removed from the model by stepwise backward elimination
• Both linear and quadratic component of age in study model
• P<0.05 was considered statistically significant.

Articles
35 studies identified and included in present analysis, with a total of 64 data points.

Number and Type of Studies

• Majority of studies included dietary counseling without drug treatment
• Only seven data points provided placebo pills in addition to dietary counseling
• 25 data points included some recommendation to alter physical activity level or provided no information to participants on physical activity.

Study Characteristics

• Age ranged from 29 to 71 years and averaged 45.6 years
• Initial body weight ranged from 81.7kg to 129kg and averaged 95.7kg
• Prescribed energy deficits of 1,670 calories to 7,000 calories per week for eight to 52 weeks in duration
• The rate of weight loss observed in studies included ranged between 0.002kg and 1.13kg per week
• The average number of subjects represented by each data point was 19.9±3.1 (mode, 12; median, 13.5)
• Approximately 25% of data points represented study arms with up to10 subjects; 40.6% represented study arms with 11 to 15 subjects; 23.4% with 16 to 30 subjects; 6.3% with 31 to 50 subjects; 4.7% represented study arms with more than 50 subjects.

Key Findings

• The percentage of female subjects, the presence or absence of placebo and exercise were not major factors in the model to predict rate of weight loss and therefore were not included in the final analysis
• Interactions between sex and exercise, energy deficit and exercise and frequency of counseling and energy deficit did not reach statistical significance and therefore were removed from the model
• The final model included study length, age, initial body weight, prescribed energy deficit, frequency of dietary counseling and a quadratic effect of age
• Results of regression analysis shows:
  • As study length increases, the rate of weight loss decreases
  • Age, initial body weight, frequency of dietary counseling and prescribed energy deficit predict greater rate of weight loss.

Results of linear regression model of the predictive variables for rate of weight loss observed from weight loss studies published between 1995 and 2009

Parameter	Estimate	Standard Error	P-Value
Intercept	2.0738	0.4540	<0.0001
Length (week)	0.01775	0.002547	<0.0001
Counseling Sessions per Week	-0.08964	0.03735	0.0197
Deficit (calories per week)	-0.00005719	0.00001297	<0.0001
Age (years)	-0.06398	0.01979	0.002
Age x Age (years2)	0.0005568	0.0002002	0.0073
Initial Body Weight (kg)	-0.008814	0.02293	0.0003

 

• Findings show that predicted rate of body weight loss that can be expected increases with increasing initial body weight, subject age and prescribed energy deficit as well as frequency of dietary counseling
• Predicted rate of weight loss decreases with increasing study length (reduction in compliance with dietary restriction over time).

Government:

National Institutes of Health

Limitations

• A small number of men were included in studies
• It is unclear if dietary counseling was provided by an RD
• Factors on which studies reported
• Possible publication bias due to inclusion of only published studies.