A review of the relationship between dietary fat and overweight / obesity

s were assessed to determine whether the study was designed specifically to investigate the effect of diet composition on body weight. Animal studies were excluded. Studies were included if they contained a measure of body weight at baseline and post intervention (i.e. BMI, absolute weight loss or percentage of body weight loss). The effect of weight loss on CVD risk factors was not examined. Long-term follow up of a representative cohort is considered the ideal study design for answering questions related to aetiology and risk factors (National Health and Medical Research Council (NHMRC) 2000). To determine whether the association between dietary fat intake and the development of overweight and obesity is causal, it is recommended that issues such as biological plausibility and the existence of a dose-response relationship be considered. Randomised controlled trials are recommended for determining the effects of an intervention. Controlled trials (ad libitum and energy controlled), of at least 6 weeks in duration, were reviewed to assess the effectiveness of fat reduction compared to other dietary strategies on weight reduction or weight maintenance. National Heart Foundation of Australia February 2003 Relationship between dietary fat and overweight/obesity Page 6 The following criteria were considered in appraising the evidence (Sempos et al. 1999; Seidell 1998; NHMRC 2000): • Consistency in the evidence across a range of study designs. • The quality of the study, particularly relating to measurement bias. • The size of the effect. • Demonstration of a biologically plausible mechanism. • Relevance of the evidence to the Australian population. The following framework was used to assess the evidence: ‘Good Evidence’ ‘Moderate Evidence’ ‘Little Evidence’ Level of evidence Consistency across several study designs, including long term intervention studies Inconsistency across study designs; Use of surrogate measures; Limited number and type of studies Inconsistency across study designs; Limited number and type of studies Quality of the evidence Measurement bias adequately minimised Limited in quality Limited in quality Size of the effect Statistically significant. Effect possibly due to measurement bias Effect possibly due to measurement bias Mechanism Metabolic studies in humans Metabolic studies in humans Lack of metabolic studies in humans 3. Evidence from epidemiological studies 3.1 Ecological studies Several ecological or between-population studies have used aggregate level population data (usually national food balance data) to correlate dietary fat intakes with prevalence of overweight and obesity (Bray & Popkin 1998; Lissner & Heitmann 1995). A positive association between consumption of dietary fat (% E fat) and BMI have been reported in some studies, but not in others (Bray and Popkin, 1998; Lissner & Heitmann 1995). It is generally acknowledged that evidence from ecological studies is limited due to differences in physical activity levels, smoking, availability of food and affluence, as well as the variable quality of the dietary intake data (Willett 1999a; Bray & Popkin 1998; Lissner & Heitmann 1995). National Heart Foundation of Australia February 2003 Relationship between dietary fat and overweight/obesity Page 7 3.2 Cross-sectional studies Many cross-sectional studies have been conducted in individuals within the same population to examine the correlation between intake of dietary fat (mainly % E fat) and body fatness (mainly BMI). Most cross-sectional studies provide statistical evidence of an association (correlation coefficients ranging from 0.17 to 0.38) between % E fat and body fat in free-living populations (Lissner & Heitmann 1995). However, a number of studies have reported no association (Lissner & Heitmann 1995). Similarly, results from cross-sectional studies in children are inconsistent (Davies 1997; Gazzaniga & Burns 1993; Guillaume et al. 1998; Lissner & Heitmann 1995). Inconsistency in study findings may be explained by differences in diet methodology and in the measurement of confounders, such as energy intake, physical activity, age and cigarette smoking (Shah & Garg 1996b). Shah and Garg (1996b) identified four cross-sectional studies which controlled for all potential confounders. Two studies reported an inverse and no relationship between obesity and energy or fat intake, whereas the other two found obese individuals consumed more energy and more dietary fat (independent of energy intake) than lean individuals. However, % E fat explained only 1.6% of the total variation in body fat. Since indices of obesity and dietary fat intake are measured simultaneously, crosssectional studies are not considered sufficient to establish causation (Sempos et al. 1999).