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analysis of randomised controlled trials and cohort studies
AbstractObjective To investigate the relation between total fat intake and body weight in adults and children.Inclusion criteria Randomised controlled trials and cohort studies of adults or children that compared lower versus usual total fat intake and assessed the effects on measures of body fatness (body weight, body mass index, or waist circumference) after at least six months (randomised controlled trials) or one year (in cohorts). Randomised controlled trials with any intention to reduce weight in participants or confounded by additional medical or lifestyle interventions were excluded.Data extraction Data were extracted and validity was assessed independently and in duplicate. Random effects meta analyses, subgroups, sensitivity analyses, and metaregression were done.Results 33 randomised controlled trials (73589 participants) and 10 cohort studies were included, all from developed countries. Meta analysis of data from the trials suggested that diets lower in total fat were associated with lower relative body weight (by 1.6 kg, 95% confidence interval 2.0 to 1.2 kg, I2=75%, 57735 participants). Lower weight gain in the low fat arm compared with the control arm was consistent across trials, but the size of the effect varied. Metaregression suggested that greater reduction in total fat intake and lower baseline fat intake were associated with greater relative weight loss, explaining most of the heterogeneity. The significant effect of a low fat diet on weight was not lost in sensitivity analyses (including removing trials that expended greater time and attention on low fat groups). Lower total fat intake also led to lower body mass index (0.51 kg/m2, 95% confidence interval 0.76 to 0.26, nine trials, I2=77%) and waist circumference (by 0.3 cm, 95% confidence interval 0.58 to 0.02, 15671 women, one trial). There was no suggestion of negative effects on other cardiovascular risk factors (lipid levels or blood pressure). GRADE assessment suggested high quality evidence for the relation between total fat intake and body weight in adults. Only one randomised controlled trial and three cohort studies were found in children and young people, but these confirmed a positive relation between total fat intake and weight gain.Conclusions There is high quality, consistent evidence that reduction of total fat intake has been achieved in large numbers of both healthy and at risk trial participants over many years. Lower total fat intake leads to small but statistically significant and clinically meaningful, sustained reductions in body weight in adults in studies with baseline fat intakes of 28 43% of energy intake and durations from six months to over eight years. Evidence supports a similar effect in children and young people.IntroductionThe optimal intake of total fat was debated at the Joint Food and Agriculture Organization of the United Nations/World Health Organization expert consultation on fats and fatty acids in human nutrition held in November 2008. It was agreed that any effect of total fat intake on body weight was crucial to making global recommendations (in the context of increasing overweight and obesity, in particular in low and middle income countries undergoing rapid transition in nutrition). Overweight and obesity increase the risk of many cancers, coronary heart disease, and stroke.1 2 3Although a previous systematic review found no randomised controlled trials of lower total fat intake that aimed to assess effects on body weight,4 metaregression within a systematic review assessing randomised controlled trials on the effects of step I and II diets (diets designed by the National Heart Lung and Blood Institute national cholesterol education programme to reduce the risk of cardiovascular disease in the general public and those at increased cardiovascular risk, respectively), found a strong relation between total fat intake and body weight.5 That review,fake rolex oyster perpetual lady datejust, however, included studies of as short as three weeks' duration and studies in which weight loss was a goal of the intervention, which may have overstated any relation because advice was to lower both fat and energy intake, and it excluded many trials of reduction in total fat intake that did not fit the step I or II criteria. To fulfill the requirements of the development process for the new guideline, a systematic review was needed of all available evidence of longer term effects of total fat intake on body fatness, in studies not intending that participants lose weight. WHO therefore commissioned a systematic review and meta analysis to assess the relation between total fat intake and indicators of body fatness (including obesity, waist circumference, and body mass index) using all appropriate randomised controlled trials and cohort studies in adults and children. The expert advisory group aimed to generate a recommendation on the population impact of total fat intake in the development of obesity. The group agreed that populations recruited specifically for weight loss studies and interventions intended to result in weight loss would be excluded. This was because they were potentially confounded by the implicit objective of reducing calorie intake to produce weight loss and would therefore lead to an overemphasis on studies carried out in highly selected obese populations in North America and Europe, which may have limited application to non obese populations or those in developing countries or in countries in transition.MethodsWe followed the methods of the Cochrane Collaboration, with a view to development of WHO guidance according to WHO's process for the development of guidelines.6 7We included randomised controlled trials and prospective cohort studies in apparently healthy children or adults from any country. Randomised controlled trials had to compare an intervention intended to reduce total fat intake by reducing percentage energy from fat or total fat in g/day with a usual fat intake arm and to continue the intervention for at least 26 weeks. The intervention could include dietary advice or provision of foods or whole diet but should not be confounded by effects of other lifestyle or medical interventions. We included studies where a low fat diet was compared with usual diet, or a low fat diet plus any non dietary intervention was compared with usual diet plus the same non dietary intervention, but we excluded studies where a low fat diet plus any non dietary intervention was compared with usual diet alone. Studies of interventions where weight loss was intended (in either or both arms or in any proportion of participants) were excluded. Some measure of body fatness (body weight, body mass index, or waist circumference) had to be assessed as change from baseline or at study end. Cohort studies had to assess the relation between total fat intake at baseline and change in a measure of body fatness from baseline over at least one year, or body fatness at least a year later.Study identificationWe ran two independent searches to June 2010. The first search through Medline, Embase (both on OvidSP), and the Cochrane Central Register of Controlled Trials focused on randomised controlled trials in adults (see supplementary table 1a8). The other search through Medline, Embase, and CINAHL focused on randomised controlled trials and cohort studies in adults and children (see supplementary table 1b9). Neither search was limited by language. For further trials we checked the bibliography of a related systematic review.10 We also asked members of the NUGAG subgroup on diet and health for relevant local studies, and six regional offices of WHO sent official letters to member states to solicit relevant studies carried out in their countries.Study assessmentFor each search we independently assessed the titles and abstracts and determined the eligibility of full text papers for inclusion (each independently and in duplicate). We also independently and in duplicate extracted data and assessed the validity of the studies. Differences between the reviewers were resolved by discussion and, if necessary, in consultation with a third reviewer.For randomised controlled trials we extracted data on participants, interventions, outcomes (body weight, body mass index, and waist circumference) and characteristics of trial quality. Where possible we also collected data on potential effect modifiers, including health status; cardiovascular risk; age; sex; country; baseline body mass index; mean years in trial; number of participants randomised and analysed; type of comparison; total and saturated fat intake during intervention period; energy, carbohydrate, sugar, protein, and alcohol intake; baseline total fat intake; information on intention to treat analyses; and difference in total fat intake (as a percentage of energy) between randomised arms at the latest time point. For dichotomous outcomes in each randomised arm we extracted the number of participants experiencing an outcome and total number of participants randomised. For continuous outcomes we extracted the number of participants assessed and the means and standard deviations of the change in, or final readings of, each treatment arm; where data were available on both change and final readings we used the data on change. Data were extracted at the latest time point within 6 11 months, 12 23 months, 24 59 months, and 60 or more months.For cohort studies we collected data on setting, design, measurement of the exposure to total fat, characteristics of the participants, similarity at baseline between high and low fat exposure groups, participant flow, and endpoint criteria. We also collected any assessment of the relation between outcome (body weight, body mass index, or waist circumference) and total fat intake, and the most adjusted and any non energy adjusted assessments.Assessment of validityFor the randomised controlled trials we used the Cochrane criteria6 to examine study validity, including sequence generation; allocation concealment; blinding of participants, staff, and outcome assessors; incomplete outcome data; and selective outcome reporting. Additional review specific criteria included presence or not of dietary interventions over and above alteration of total dietary fat,old rolex oyster perpetual datejust fake, and similarity or not of type and intensity of intervention in both arms.For cohort studies we used the Newcastle Ottawa criteria11 modified after consideration of the crucial factors for this review. These criteria included whether the design used a control group that was chosen from the collated cohort (internal) or chosen as a separate group (external); number lost to follow up; baseline similarity of the most and least exposed groups; factors adjusted for in analysis; and method of assessment of total fat intake. Moderate risk of bias was suggested where less than 20% were lost to follow up, up to two major factors were unadjusted for in the design or analysis, and diet was assessed using a 24 hour recall or diet diary: all other studies were at high risk of bias.Data analysisWhere P6 We excluded intervention arms not appropriate for this review or that were less appropriate than other arms. When two arms were appropriate for different subgroups then we used the control group once with each intervention arm, but we did not pool the subgroups overall. When outcomes were assessed at more than one time point, for general analyses we used data from the latest time point available, but all relevant time points were used in subgroups by study duration. We used the I2 test to examine heterogeneity, with more than 50% heterogeneity considered to be important.6 12 The possibility of publication bias was examined by funnel plots.13 14Prespecified subgroupings included mean follow up time, difference in fat intake between intervention and control groups (dose effect, difference of up to 5% of energy from fat, >5% to 10%, >10% to 15%, and >15%),rolex lady datejust fake, total fat intake of control group, year results were first published, and sex. After discussion of the preliminary results with the NUGAG subgroup on diet and health at its second meeting in March 2011, we added the following subgroupings for randomised controlled trials in adults: difference in level of attention given to participants in intervention and control arms, goal for percentage of energy from total fat in intervention arm, intervention resulted in more or less than 30% energy from fat, and baseline body mass index (mean 2)). A difference in the level of attention given to the intervention and control groups meant any variation, including any difference in the amount of health professional or training time or number or timing or duration of follow ups. Journal referees requested that subgroupings by health status (healthy people not recruited on the basis of health status, those with risk factors such as high lipid levels or breast cancer risk, and current illness such as diabetes, coronary heart disease, cancer, or intestinal polyps) and by degree of energy reduction in the low fat group compared with the control or usual fat group were added. The NUGAG subgroup on diet and health also requested metaregression, which was run in STATA/IC 11.2 to assess the effects of study duration, amount of reduction in total fat in the intervention arm compared with the control arm (as percentage of energy), and fat intake in the control group as subgrouping suggested that these factors may relate to degree of weight loss (multiple regression model, all three assessed together). Finally, the NUGAG subgroup on diet and health requested that data on serum lipid levels and blood pressure outcome be collected from the included randomised controlled trials on adults to ensure that the reduction in fat was not associated with harmful effects on other major cardiovascular risk factors, and that data on changes in other dietary components be collated to help with understanding any mechanisms of action.Sensitivity analyses assessed the effects of running fixed effects meta analyses and excluding the largest randomised controlled trial, trials not free of systematic differences in care (or that were unclear), trials not free of dietary differences other than fat (or that were unclear), trials without or with unclear allocation concealment (an important indicator of study validity), and trials using intention to treat analyses.Where feasible we intended to use meta analysis on the results of cohort studies, but given the small number of included cohorts and differences in the durations between studies, the measures of body fatness, and the assessment methods of correlation between total fat intake and outcomes, meta analysis was not appropriate. Instead, we used vote counting to describe the relation between total fat intake and body fatness in each study. We noted a positive or negative statistically significant relation between total fat intake at baseline and change in weight or body mass index over time.We rated the quality of evidence using GRADE (Grading of Recommendations Assessment, Development and Evaluation, which provides an explicit and comprehensive method to rate quality of evidence in health)15 in GRADEpro Version 3.2 for Windows, which was debated, agreed, and the wording refined by the NUGAG subgroup on diet and health.ResultsThe search for randomised controlled trials in adults identified 22012 titles and abstracts and the search for randomised controlled trials in adults and children and cohort studies identified 10208 (fig 1). Supplementary tables 1a and b give the search strategies in full. Of 465 full text papers assessed, 34 randomised controlled trials (33 in adults,fake rolex datejust oyster perpetual, one in children) and 13 prospective cohort studies (10 in adults and three in children) were included.Fig 1 Flow of papers through studyRandomised controlled trials in adultsOf the 33 randomised controlled trials in adults (73589 participants), 20 were in North America, 12 in Europe, and one in New Zealand, with none from developing or transitional countries. The duration of the trials varied from six months to more than eight years. In four trials the participants were all men, in 15 all women, and in 14 both sexes (one of which reported outcomes by sex), with varying mean ages and states of health (low, moderate, or high risk of cardiovascular disease or breast cancer). See table 1 and supplementary table 2 for the characteristics of the trials in adults.
AbstractObjective To investigate the relation between total fat intake and body weight in adults and children.Inclusion criteria Randomised controlled trials and cohort studies of adults or children that compared lower versus usual total fat intake and assessed the effects on measures of body fatness (body weight, body mass index, or waist circumference) after at least six months (randomised controlled trials) or one year (in cohorts). Randomised controlled trials with any intention to reduce weight in participants or confounded by additional medical or lifestyle interventions were excluded.Data extraction Data were extracted and validity was assessed independently and in duplicate. Random effects meta analyses, subgroups, sensitivity analyses, and metaregression were done.Results 33 randomised controlled trials (73589 participants) and 10 cohort studies were included, all from developed countries. Meta analysis of data from the trials suggested that diets lower in total fat were associated with lower relative body weight (by 1.6 kg, 95% confidence interval 2.0 to 1.2 kg, I2=75%, 57735 participants). Lower weight gain in the low fat arm compared with the control arm was consistent across trials, but the size of the effect varied. Metaregression suggested that greater reduction in total fat intake and lower baseline fat intake were associated with greater relative weight loss, explaining most of the heterogeneity. The significant effect of a low fat diet on weight was not lost in sensitivity analyses (including removing trials that expended greater time and attention on low fat groups). Lower total fat intake also led to lower body mass index (0.51 kg/m2, 95% confidence interval 0.76 to 0.26, nine trials, I2=77%) and waist circumference (by 0.3 cm, 95% confidence interval 0.58 to 0.02, 15671 women, one trial). There was no suggestion of negative effects on other cardiovascular risk factors (lipid levels or blood pressure). GRADE assessment suggested high quality evidence for the relation between total fat intake and body weight in adults. Only one randomised controlled trial and three cohort studies were found in children and young people, but these confirmed a positive relation between total fat intake and weight gain.Conclusions There is high quality, consistent evidence that reduction of total fat intake has been achieved in large numbers of both healthy and at risk trial participants over many years. Lower total fat intake leads to small but statistically significant and clinically meaningful, sustained reductions in body weight in adults in studies with baseline fat intakes of 28 43% of energy intake and durations from six months to over eight years. Evidence supports a similar effect in children and young people.IntroductionThe optimal intake of total fat was debated at the Joint Food and Agriculture Organization of the United Nations/World Health Organization expert consultation on fats and fatty acids in human nutrition held in November 2008. It was agreed that any effect of total fat intake on body weight was crucial to making global recommendations (in the context of increasing overweight and obesity, in particular in low and middle income countries undergoing rapid transition in nutrition). Overweight and obesity increase the risk of many cancers, coronary heart disease, and stroke.1 2 3Although a previous systematic review found no randomised controlled trials of lower total fat intake that aimed to assess effects on body weight,4 metaregression within a systematic review assessing randomised controlled trials on the effects of step I and II diets (diets designed by the National Heart Lung and Blood Institute national cholesterol education programme to reduce the risk of cardiovascular disease in the general public and those at increased cardiovascular risk, respectively), found a strong relation between total fat intake and body weight.5 That review,fake rolex oyster perpetual lady datejust, however, included studies of as short as three weeks' duration and studies in which weight loss was a goal of the intervention, which may have overstated any relation because advice was to lower both fat and energy intake, and it excluded many trials of reduction in total fat intake that did not fit the step I or II criteria. To fulfill the requirements of the development process for the new guideline, a systematic review was needed of all available evidence of longer term effects of total fat intake on body fatness, in studies not intending that participants lose weight. WHO therefore commissioned a systematic review and meta analysis to assess the relation between total fat intake and indicators of body fatness (including obesity, waist circumference, and body mass index) using all appropriate randomised controlled trials and cohort studies in adults and children. The expert advisory group aimed to generate a recommendation on the population impact of total fat intake in the development of obesity. The group agreed that populations recruited specifically for weight loss studies and interventions intended to result in weight loss would be excluded. This was because they were potentially confounded by the implicit objective of reducing calorie intake to produce weight loss and would therefore lead to an overemphasis on studies carried out in highly selected obese populations in North America and Europe, which may have limited application to non obese populations or those in developing countries or in countries in transition.MethodsWe followed the methods of the Cochrane Collaboration, with a view to development of WHO guidance according to WHO's process for the development of guidelines.6 7We included randomised controlled trials and prospective cohort studies in apparently healthy children or adults from any country. Randomised controlled trials had to compare an intervention intended to reduce total fat intake by reducing percentage energy from fat or total fat in g/day with a usual fat intake arm and to continue the intervention for at least 26 weeks. The intervention could include dietary advice or provision of foods or whole diet but should not be confounded by effects of other lifestyle or medical interventions. We included studies where a low fat diet was compared with usual diet, or a low fat diet plus any non dietary intervention was compared with usual diet plus the same non dietary intervention, but we excluded studies where a low fat diet plus any non dietary intervention was compared with usual diet alone. Studies of interventions where weight loss was intended (in either or both arms or in any proportion of participants) were excluded. Some measure of body fatness (body weight, body mass index, or waist circumference) had to be assessed as change from baseline or at study end. Cohort studies had to assess the relation between total fat intake at baseline and change in a measure of body fatness from baseline over at least one year, or body fatness at least a year later.Study identificationWe ran two independent searches to June 2010. The first search through Medline, Embase (both on OvidSP), and the Cochrane Central Register of Controlled Trials focused on randomised controlled trials in adults (see supplementary table 1a8). The other search through Medline, Embase, and CINAHL focused on randomised controlled trials and cohort studies in adults and children (see supplementary table 1b9). Neither search was limited by language. For further trials we checked the bibliography of a related systematic review.10 We also asked members of the NUGAG subgroup on diet and health for relevant local studies, and six regional offices of WHO sent official letters to member states to solicit relevant studies carried out in their countries.Study assessmentFor each search we independently assessed the titles and abstracts and determined the eligibility of full text papers for inclusion (each independently and in duplicate). We also independently and in duplicate extracted data and assessed the validity of the studies. Differences between the reviewers were resolved by discussion and, if necessary, in consultation with a third reviewer.For randomised controlled trials we extracted data on participants, interventions, outcomes (body weight, body mass index, and waist circumference) and characteristics of trial quality. Where possible we also collected data on potential effect modifiers, including health status; cardiovascular risk; age; sex; country; baseline body mass index; mean years in trial; number of participants randomised and analysed; type of comparison; total and saturated fat intake during intervention period; energy, carbohydrate, sugar, protein, and alcohol intake; baseline total fat intake; information on intention to treat analyses; and difference in total fat intake (as a percentage of energy) between randomised arms at the latest time point. For dichotomous outcomes in each randomised arm we extracted the number of participants experiencing an outcome and total number of participants randomised. For continuous outcomes we extracted the number of participants assessed and the means and standard deviations of the change in, or final readings of, each treatment arm; where data were available on both change and final readings we used the data on change. Data were extracted at the latest time point within 6 11 months, 12 23 months, 24 59 months, and 60 or more months.For cohort studies we collected data on setting, design, measurement of the exposure to total fat, characteristics of the participants, similarity at baseline between high and low fat exposure groups, participant flow, and endpoint criteria. We also collected any assessment of the relation between outcome (body weight, body mass index, or waist circumference) and total fat intake, and the most adjusted and any non energy adjusted assessments.Assessment of validityFor the randomised controlled trials we used the Cochrane criteria6 to examine study validity, including sequence generation; allocation concealment; blinding of participants, staff, and outcome assessors; incomplete outcome data; and selective outcome reporting. Additional review specific criteria included presence or not of dietary interventions over and above alteration of total dietary fat,old rolex oyster perpetual datejust fake, and similarity or not of type and intensity of intervention in both arms.For cohort studies we used the Newcastle Ottawa criteria11 modified after consideration of the crucial factors for this review. These criteria included whether the design used a control group that was chosen from the collated cohort (internal) or chosen as a separate group (external); number lost to follow up; baseline similarity of the most and least exposed groups; factors adjusted for in analysis; and method of assessment of total fat intake. Moderate risk of bias was suggested where less than 20% were lost to follow up, up to two major factors were unadjusted for in the design or analysis, and diet was assessed using a 24 hour recall or diet diary: all other studies were at high risk of bias.Data analysisWhere P6 We excluded intervention arms not appropriate for this review or that were less appropriate than other arms. When two arms were appropriate for different subgroups then we used the control group once with each intervention arm, but we did not pool the subgroups overall. When outcomes were assessed at more than one time point, for general analyses we used data from the latest time point available, but all relevant time points were used in subgroups by study duration. We used the I2 test to examine heterogeneity, with more than 50% heterogeneity considered to be important.6 12 The possibility of publication bias was examined by funnel plots.13 14Prespecified subgroupings included mean follow up time, difference in fat intake between intervention and control groups (dose effect, difference of up to 5% of energy from fat, >5% to 10%, >10% to 15%, and >15%),rolex lady datejust fake, total fat intake of control group, year results were first published, and sex. After discussion of the preliminary results with the NUGAG subgroup on diet and health at its second meeting in March 2011, we added the following subgroupings for randomised controlled trials in adults: difference in level of attention given to participants in intervention and control arms, goal for percentage of energy from total fat in intervention arm, intervention resulted in more or less than 30% energy from fat, and baseline body mass index (mean 2)). A difference in the level of attention given to the intervention and control groups meant any variation, including any difference in the amount of health professional or training time or number or timing or duration of follow ups. Journal referees requested that subgroupings by health status (healthy people not recruited on the basis of health status, those with risk factors such as high lipid levels or breast cancer risk, and current illness such as diabetes, coronary heart disease, cancer, or intestinal polyps) and by degree of energy reduction in the low fat group compared with the control or usual fat group were added. The NUGAG subgroup on diet and health also requested metaregression, which was run in STATA/IC 11.2 to assess the effects of study duration, amount of reduction in total fat in the intervention arm compared with the control arm (as percentage of energy), and fat intake in the control group as subgrouping suggested that these factors may relate to degree of weight loss (multiple regression model, all three assessed together). Finally, the NUGAG subgroup on diet and health requested that data on serum lipid levels and blood pressure outcome be collected from the included randomised controlled trials on adults to ensure that the reduction in fat was not associated with harmful effects on other major cardiovascular risk factors, and that data on changes in other dietary components be collated to help with understanding any mechanisms of action.Sensitivity analyses assessed the effects of running fixed effects meta analyses and excluding the largest randomised controlled trial, trials not free of systematic differences in care (or that were unclear), trials not free of dietary differences other than fat (or that were unclear), trials without or with unclear allocation concealment (an important indicator of study validity), and trials using intention to treat analyses.Where feasible we intended to use meta analysis on the results of cohort studies, but given the small number of included cohorts and differences in the durations between studies, the measures of body fatness, and the assessment methods of correlation between total fat intake and outcomes, meta analysis was not appropriate. Instead, we used vote counting to describe the relation between total fat intake and body fatness in each study. We noted a positive or negative statistically significant relation between total fat intake at baseline and change in weight or body mass index over time.We rated the quality of evidence using GRADE (Grading of Recommendations Assessment, Development and Evaluation, which provides an explicit and comprehensive method to rate quality of evidence in health)15 in GRADEpro Version 3.2 for Windows, which was debated, agreed, and the wording refined by the NUGAG subgroup on diet and health.ResultsThe search for randomised controlled trials in adults identified 22012 titles and abstracts and the search for randomised controlled trials in adults and children and cohort studies identified 10208 (fig 1). Supplementary tables 1a and b give the search strategies in full. Of 465 full text papers assessed, 34 randomised controlled trials (33 in adults,fake rolex datejust oyster perpetual, one in children) and 13 prospective cohort studies (10 in adults and three in children) were included.Fig 1 Flow of papers through studyRandomised controlled trials in adultsOf the 33 randomised controlled trials in adults (73589 participants), 20 were in North America, 12 in Europe, and one in New Zealand, with none from developing or transitional countries. The duration of the trials varied from six months to more than eight years. In four trials the participants were all men, in 15 all women, and in 14 both sexes (one of which reported outcomes by sex), with varying mean ages and states of health (low, moderate, or high risk of cardiovascular disease or breast cancer). See table 1 and supplementary table 2 for the characteristics of the trials in adults.
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