Obesity Clinical Trial
Official title:
The Effect of a Low Glycemic Index Diet and Glycemic Load on Body Weight: A Systematic Review and Meta-analyses of Randomized Controlled Trials
A low glycemic index (GI) diet has been associated with improved glycemic control in type 2 diabetes patients and a reduced risk of cardiovascular disease (CVD). Low glycemic load (GL) diets have been associated in cohort studies with a reduction in both diabetes incidence and CVD events, especially in overweight individuals, and have been recommended by the Canadian, American and European diabetes associations. Life style modification trials have shown that reducing body weight in overweight or obese individuals improves obesity-related risk factors. The process of a systematic review combines the results from many studies in order to arrive at a pooled weighted average of the true effect. The investigators propose to conduct a systematic review and meta-analysis of the highest quality evidence from randomized controlled trials to assess the effect of low GI/GL diets on body weight change. The results of this synthesis will inform clinical practice guidelines and lead to better health outcomes through informing healthcare providers and patients, stimulating industry innovation, and guiding future research.
Background: Obesity and overweight have been identified as a risk factor for developing type
2 diabetes, dyslipidemia, hypertension, CVD and certain cancers. Nutrition therapy is the
first line treatment for prevention of these health conditions and as a key component during
their initial and long term management. Diets based on low GI foods and low GL have been
associated with risk reduction of type 2 diabetes and CVD. Current clinical practice
guidelines suggest the replacement of high GI foods for low GI foods based on the
improvement on glycemic control and CVD risk factors. However, there is no enough evidence
to support the association between low GI/GL diets on body weight reduction.
Need for a review: The lack of high quality data in this area to support clinical practice
guidelines represents an urgent call for stronger evidence. The use of meta-analyses of
controlled feeding trials remains the "Goal Standard" of evidence for nutrition guidelines
development.
Objectives: To provide evidence-based guidance for public health policy, health claims, and
nutrition guidelines relating to low GI/GL diets, we will conduct a systematic review and
meta-analysis of controlled feeding trials to assess the effect of a low GI/GL diet on body
weight.
Design: The planning and conduct of the proposed meta-analyses will follow the Cochrane
handbook for systematic reviews of interventions. The reporting will follow the Preferred
Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines.
Data sources: MEDLINE, EMBASE and The Cochrane Central Register of Controlled Trials will be
searched using appropriate search terms.
Study selection: Intervention trials that investigate the effect of a low GI/GL diet on body
weight in humans will be included. Studies that are less than 12 weeks diet duration, do not
have a control group, include pregnant women, breast feeding women or children, or do not
report body weight data will be excluded.
Data extraction: Two independent investigators will extract information about study design,
sample size, subject characteristics, GI/GL diet, follow-up, and the composition of the
background diets. Mean ± SEM values will be extracted for all outcomes. Standard
computations and imputations will be used to derive missing data. Risk of bias will be
assessed using the Cochrane Collaboration risk of bias tool.
Outcome: The primary outcome will be body weight.
Data synthesis: Pooled analysis will be conducted using the Generic Inverse Variance method
with random effects models stratified by metabolic phenotype (normal weight,
overweight/obese, metabolic syndrome criteria, diabetes, etc.). Random-effects models will
be used even in the absence of statistically significant between-study heterogeneity, as
they yield more conservative summary effect estimates in the presence of residual
heterogeneity. Paired analyses will be applied to all crossover trials. Heterogeneity will
be tested by the Cochran Q statistic and quantified by the I2 statistic. Sources of
heterogeneity will be explored by sensitivity and subgroup analyses. A priori subgroup
analyses will include GI/GL (absolute level, within-treatment change, and between-treatment
change), comparator (high GI diet, other), duration of follow-up, saturated fat intake
(absolute level, within-treatment change, and between-treatment change), dietary fibre
intake (absolute level, within-treatment change, and between-treatment change), total
carbohydrate intake (absolute level, within-treatment change, and between-treatment change),
design (crossover, parallel), risk of bias, and median baseline body weight. Meta-regression
analyses will assess the significance of categorical and continuous subgroups analyses.
Publication bias will be investigated by the inspection of funnel plots and application of
the Egger and Begg tests.
Evidence Assessment: The strength of the evidence for each outcome will be assessed using
the Grading of Recommendations Assessment, Development and Evaluation (GRADE).
Knowledge translation plan: Results will be disseminated through traditional means such as
interactive presentations at local, national, and international scientific meetings and
publication in high impact factor journals. Innovative means such as webcasts with e-mail
feedback mechanisms will also be used. Knowledge Users will act as knowledge brokers
networking among opinion leaders and different adopter groups to increase awareness at each
stage. Four Knowledge Users will also participate directly as members of nutrition
guidelines committees. Target adopters will include the clinical practice, public health,
industry, research communities, and patient groups. Feedback will be incorporated and used
to guide analyses and improve key messages at each stage.
Preliminary findings: We conducted a systematic review and meta-analysis of the effect of
dietary pulses on glycemic control in 41 controlled feeding trials. We found that pulses
alone or in low GI or high-fibre diets improved markers of glycemic control. Although the
improvement was clinically significant, it came at the expense of substantial inter-study
heterogeneity. We also conducted a systematic review and meta-analysis of prospective
cohorts on the associations of GI and GL with coronary heart disease events. We found that
high GI and GL diets were significantly associated with coronary heart disease events in
women but not in men. Knowledge translation from these projects has already begun. These
data have provided a rationale for a large trial of the effect of a low GI diet in type 2
diabetes to address some of the identified sources of heterogeneity, as well as the
rationale for a trial of the effect of lowering the GL with canola oil on glycemic control
and cardiovascular risk factors. These data were also used in the development of the 2013
CDA Clinical Practice Guidelines (CPG) for Nutrition Therapy.
Significance: The proposed project will aid in knowledge translation related to the effects
of a low GI/GL diet on body weight, strengthening the evidence-base for dietary
recommendations and health claims. It will improve health outcomes through informing
healthcare providers and patients, stimulating industry innovation, and guiding future
research.
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