We conducted a cross-sectional study of a population-based sample of healthy, free-living Canadians to examine the relationship between dietary composition and BMI. Risk of overweight and obesity was decreased in all quartiles of carbohydrate intake compared to the lowest intake category. Spline analyses revealed that overweight and obesity risk was lowest among individuals who consumed 290 to 310 g/day carbohydrates (ranging from 47% to 64% of calories from carbohydrates), after accounting for age, sex, income, education, leisure-time physical activity, and total energy intake.
Overweight and obesity develops when there is a positive caloric imbalance, and is impacted by genes and the environment (17
). There is no consensus on the best dietary pattern to maintain optimal body weight, but multiple clinical trials have compared different dietary compositions in relation to weight loss (1
). Data from these clinical trials show that low-carbohydrate diets are more effective at inducing weight loss in the short-term (6 months) than high-carbohydrate diets, but there is no difference in their efficacy at the end of 1 year (1
). There is also evidence that the Mediterranean diet (approximately 40% energy from fat, high in whole grains, fruit, and vegetables) is as effective as low-carbohydrate and low-fat diets in weight reduction over 2 years (18
). These studies evaluated diets in relation to weight loss, and not the diets of free-living individuals. Thus, our study provides needed data on the association between dietary composition, in particular carbohydrate intake, and risk of overweight and obesity among a healthy, free-living population.
Overall diet quality, together with physical activity, is related to weight maintenance. Energy-dense foods are rich in refined carbohydrates and unhealthy fats. Snack-food items and soft drinks make up the majority of energy-dense foods (19
), while energy density is low in fruits and vegetables, poultry, fish, and whole grains (20
). In one study, participants who ate more fruit and vegetable were less obese (and had lower energy density diets) whether or not their diets were high-fat (>30% calories from fat) or low-fat (≤30% calories from fat) compared with people who ate fewer fruit and vegetables (21
). People consuming more energy-dense foods (20
) and less whole grains (22
) are more likely to have higher BMI than those doing otherwise. In our study, participants were least likely to be overweight or obese and had lowest BMI if they consumed between 47% and 64% of calories from carbohydrates. Participants consuming more carbohydrate ate more fruit and vegetables, fiber, and less saturated fat than those consuming less carbohydrate.
Overweight and obesity are risk factors for cardiovascular disease, and diet quality is also important in predicting cardiovascular disease (16
). In previous studies, the diet composition of groups with lower overweight and obesity was generally consistent with the American Heart Association dietary recommendations to minimize low-density lipoprotein cholesterol (23
), and Health Canada’s Food Guide (24
). Overall, participants in our study ate less fiber [13 to 22 g/day on average for the four levels of carbohydrate intake, vs the recommendation of 38 g/day for men and 25 g/day for women (24
)], and more saturated fat (7% to 14% vs the recommended <7% kcal energy from saturated fat). Thus, efforts to increase fiber and reduce saturated fat intakes, perhaps by avoiding energy-dense foods (26
), may provide important health benefits in this population.
A limitation of our data was that there was no information on carbohydrate type (simple or complex). In general diets composed mainly of simple carbohydrates, such as sugar, have deleterious health consequences, while diets made up mostly of complex carbohydrates are favorable for health (27
). Complex carbohydrates are a broad group of carbohydrates that are rich in soluble and insoluble fiber and include whole grain (27
). Whole grain contains a number of substances with potentially favorable health benefits, such as antioxidants, phytoestrogens, and fiber (28
), and is associated with lower energy intake (3
), improved metabolic profile, and lower risk of obesity (29
), central adiposity (3
), diabetes, and cardiovascular disease (30
). According to one estimate, Canadians consume less than one whole-grain serving per day on average (31
) and, as mentioned previously, participants in our study consumed less than the recommended levels of fiber, on average. We attempted to examine the effect of carbohydrates on overweight or obesity independent of fiber using multivariate modeling. As fiber intake is correlated with whole grain in the diet, this model indirectly assessed the association between refined grain and overweight and obesity. Carbohydrate intake remained inversely associated with risk of overweight or obesity with fiber in the model.
A second limitation of these data was that only leisure-time physical activity was assessed. Residual confounding because of unmeasured utilitarian and job-related activity was possible. Even so, participants in quartile 3 of carbohydrate intake (having lowest overweight and obesity risk) were more physically active than those in the lowest and highest carbohydrate intake categories, underscoring the central role of physical activity in relation to body weight. Third, as the study design was cross-sectional, diet, height, and weight were measured at the same time. It is possible that the participants had changed their diets as a result of being overweight or obese, and that few people consuming very low- and very high-carbohydrate diets were driving the associations. To minimize these biases, participants who reported chronic health conditions because of which they may have altered their diets, were excluded. In addition, the analyses were repeated excluding participants who reported high-protein and high-carbohydrate intakes because they may be trying to lose weight, or those who were obese. Results did not materially change in these analyses. Fourth, diet was assessed by 24-hour recall, which is a measure of recent food intake. Ten days or more of data are usually needed to get accurate estimates of usual intake from 24-hour dietary recalls (32
). To overcome this limitation, participants who reported that they ate more or less food than their usual intake on the day of the dietary assessment and those with implausible dietary intake were excluded; an average of two dietary recalls were used when these data were available (for about one third of participants). Moreover, recent (24-hour dietary data) and usual dietary intakes (from food frequency questionnaire) are correlated (33
). Because energy-adjusted nutrients were used, it was possible to assess diet composition, minimizing extraneous variation associated with age, sex, and measurement error (11
). Total energy intake was not associated with overweight and obesity risk probably because the error associated with measuring total energy intake in epidemiological studies is greater (34
) than the calories sufficient to cause overweight (17
). However, it is still possible to observe an association between overweight and obesity and diet composition because the latter is more accurately measured by dietary assessment methods used in epidemiological studies (35
It is challenging to find a statistical model that accurately captures the true association between carbohydrate intake and overweight and obesity. Traditional regression models impose the assumption that the relation between carbohydrate intake and overweight and obesity is linear, which may be incorrect. One option to overcome the strong linearity assumption is to categorize the exposure, as we did in these analyses. A drawback of this method is that because the data are grouped into categories, the relation at the extremes of the distribution may be misspecified. Yet another option is to use nonlinear regression methods, such as multivariate splines. The two main advantages of this technique are that it does not impose an assumption about the distribution of the association being evaluated (linear or otherwise) and it allows use of continuous (not grouped) data, overcoming the main limitations of more traditional multivariate methods. Results from the spline analyses suggest that the relation between carbohydrate intake and overweight and obesity prevalence is nonlinear. However, because of the potential for residual confounding by physical activity and the lack of more detailed information about carbohydrate type, our results need to be interpreted with caution and replicated in other data sets.
Carbohydrate intake was inversely associated with risk of overweight or obesity in this sample of free-living, healthy Canadians. Lowest risk of overweight or obesity was found among those consuming between 190 to 310 g/day carbohydrates (equal to between 47% and 64% calories from carbohydrate). Individuals with higher carbohydrate intake ate more fruits and vegetables and fiber, and less saturated fat, and a higher percentage reported being physically active, than those consuming less carbohydrate. Thus, this population may benefit by consuming whole grain instead of refined grain, more fiber, less saturated fat, fewer calories, and by remaining physically active.