Drinking whole milk appears to be associated with reduced likelihood of obesity in children, according to a new meta-analysis of observational studies published online December 18 in the American Journal of Clinical Nutrition.
Criticism of the findings, however, posits that inadequate attention to confounding factors — especially excluding total daily caloric intake as a possible confounder — mean reverse causality is just as plausible as any other explanation for the findings.
"Simply put, there is no way to remove residual confounding from discussions around nutrition research, and this study didn't try to quantify total daily energy intake," obesity specialist Yoni Freedhoff, MD, told Medscape Medical News. Another problem, he added, is that "there is no proven mechanism for why whole milk consumption would lead to lower weights." Freedhoff is an associate professor of family medicine at the University of Ottawa, Canada, and he was not involved in the study.
Among the hypothesized mechanisms proposed by the authors are caloric replacement of less healthy food, such as sugar-sweetened drinks, with milk fat or increased satiety from whole milk.
"Another possibility is that lower satiety from reduced-fat milk could result in increased milk consumption, causing higher weight gain relative to children who consume whole milk," write Shelley M. Vanderhout, RD, of the University of Toronto, Canada, and colleagues. But they acknowledge that confounding by indication and reverse causality are also possible alternative explanations for the findings.
"Parents of children who have lower adiposity might choose higher-fat milk to increase weight gain," the authors write. "Similarly, parents of children who have higher adiposity might choose lower-fat milk to reduce the risk of overweight or obesity."
Current nutritional recommendations advise that children under age 2 years consume reduced-fat milk, ranging from 0.1% to 2%, instead of whole milk (3.25% fat) to reduce obesity risk.
For the study, the authors combed through Embase, CINAHL, MEDLINE, Scopus and the Cochrane Library databases for all observational and interventional studies through August 2019 that assessed the relationship between the fat content of cow milk consumption and body mass index (BMI) z-score in healthy children aged 1 to 18 years.
Among the 28 studies that met the criteria, 20 were cross-sectional and eight were prospective cohort studies; none were clinical trials. Exposure measures used in the studies included food frequency questionnaire, multiway food record, and 24-hour food recall.
Outcome measures included "BMI z-score (zBMI), BMI, weight for age, body fat mass, lean body mass, waist circumference, waist-to-hip ratio, body fat percentage, skinfold thickness, and prevalence of overweight or obesity as defined by the the World Health Organization, Centers for Disease Control and Prevention, or International Obesity Task Force cutoffs," the authors report.
Ten of the studies found no link between cow-milk fat consumption and the child's likelihood of obesity. The other 18 studies found that drinking milk with a higher fat content was associated with lower adiposity.
The researchers included 14 of the studies, with 20,897 combined participants, in their meta-analysis. Analysis of those studies revealed the odds of being overweight or having obesity to be 39% lower in children who drank whole milk, compared with those who drank reduced fat milk (odds ratio [OR], 0.61; P < .0001). The authors also report that the heterogeneity of the studies was high.
"Observational evidence supports that children who consume whole milk compared with reduced-fat milk have lower odds of overweight or obesity," the authors conclude. However, they also write that "clinical trial data would have provided better evidence for the directionality of this relation," noting that only two prospective cohort studies attempted to control for confounding by indication by adjusting for baseline BMI.
A handful of the included studies controlled for energy intake, but the meta-analysis authors did not include this factor as an important confounder, noted Freedhoff.
"Total daily calorie intake is hugely important," he told Medscape Medical News, arguing that reverse causality is highly plausible "in that parents choosing low-fat milks for their children may be doing so in response to their children's weights or eating habits, and parental milk fat choice might also correspond with other specific dietary beliefs and patterns, which in turn might impact on weight."
The reliance on only observational research was the study's biggest limitation. Freedhoff expressed concern that the journal article "strongly suggests a causal relationship between the decline of whole milk consumption and the rise of childhood obesity" despite the authors' limitations discussion.
"I struggle greatly with studies like this and believe them to be contributors to the erosion of scientific literacy and to the rise of dietary fads and headlines," he said. "Plainly, these sorts of papers are harmful, not helpful, to discourse around diets unless there is an extremely clear discussion around their limitations," and even then he worries that the "very real and significant limitations will be lost to the press and the public."
The research was funded by the Canadian Institutes of Health Research. Study author Jonathon Maguire has previously received a research grant from Dairy Farmers of Canada and received donations of vitamin D supplements from Drops in another study. Vanderhout and the other authors have disclosed no relevant financial relationships.
Am J Clin Nutr. Published online December 18, 2019. Abstract
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Cite this: Can Whole Milk Reduce Risk for Obesity in Kids? - Medscape - Jan 06, 2020.