Substituting whole grain for refined grain: what is needed to strengthen the scientific evidence for health outcomes?

See corresponding articles on pages 589 and 635.

Worldwide, dietary guidelines have adopted the recommendation of consuming whole grain as part of a healthy diet. Ideally, dietary guidelines are based on convincing evidence, established by multiple randomized controlled intervention trials and observational studies in the target group of the recommendations. However, the scientific substantiation for the whole-grain recommendations has a slightly different history: it was initially justified by the relatively high contribution of cereal fiber and accompanying phytochemicals and then gradually supported by an increasing number of observational studies that linked whole-grain consumption to an array of beneficial health outcomes (1, 2). The rather consistent evidence obtained from observational studies has, to some extent, been supported by mechanistic studies that identified biologically plausible relations between whole-grain consumption and certain health outcomes in humans. However, results from randomized controlled trials (RCTs), which are usually considered to provide evidence on a cause-effect relation, on whole-grain intake and risk markers of diseases have provided less-consistent results.

In this issue of the Journal, the results of an extensive RCT on the substitution of whole grain for refined grain are reported in 2 separate articles (3, 4). The study was a 6-wk RCT in 81 healthy middle-aged men and postmenopausal women with a BMI (in kg/m²) <35 who were randomly assigned to a “whole-grain” or a “refined-grain” group that was provided a full weight-maintenance diet during the entire study period. An impressive amount of health outcomes were measured, including immune and inflammatory markers (primary outcomes), energy balance including resting metabolic rate, body composition, appetite, markers of glycemic control, stool data (weight, frequency, and energy content), gut microbiota, microbial products, and others. The authors claim that the study was “the first WG [whole-grain] intervention report that completely controlled the diet, maintained weight, and kept other dietary components except fiber constant” (3). The study showed increases in stool weight and frequency in the group supplied with the whole-grain compared with the group consuming the refined-grain diet, modest positive effects on gut microbiota and related factors, and no effects on markers of cell-mediated immunity or systemic and stool inflammation. The findings related to the energy balance variables showed positive effects on resting metabolic rate and stool energy excretion when refined grain was substituted with whole grain. Thus, the results overall add to the number of RCTs that show modest health effects on health outcomes when refined grain is substituted with whole grain.

The above-mentioned study is a good example of an RCT that very likely will be used to inform future updates of dietary guidelines in relation to whole grain—and at the same time, a good illustration of how the scientific evidence, especially from RCTs, could be strengthened further. First, any study on whole grain should be interpreted with caution. Studies on whole grain compared with refined grain have an inherent difference in the amount of cereal fiber, which is known to be closely connected to certain micronutrients and phytochemicals that may have potential health effects per se. This implies that when interpreting the results in relation to the potential health effects of whole grain compared with refined grain, due consideration should be given to differences in the intakes of dietary fiber and adhering substances.

Second, more attention should be given to the reporting of the procedure used in the estimation of whole-grain intake. Whole grain is a food ingredient and, as such, differs from nutrients that can be determined quantitatively by chemical analyses. The whole-grain content of food products has to be estimated by other means (e.g., from an ingredient list of the whole-grain products in the study) (5). The procedure for estimating the whole-grain content in various food items is seldom described in whole-grain studies, or in observational or intervention studies, which may leave uncertainties about the quality of the whole grain and considerable difficulties in comparisons between studies.

A main challenge in the reporting of whole-grain intake is the lack of a standard definition of whole grain. Various definitions for whole grain have been published by regulatory bodies, grain organizations, and others; and although there is general agreement that whole grain should include the bran, germ, and starchy endosperm in the same proportion as in the whole kernel, some differences exist (6). The need for a standard definition of whole grain and cereals considered to be part of the whole-grain family has been addressed more recently by several expert groups (1, 6, 7). In addition, suggestions of including specifications of the specific whole-grain cereal and the processing methods in future studies have been made because these factors are also known to be important for the physiologic and potential health effects of whole-grain products (8, 9). Together with the need for standard definitions of whole grain as such, there is also a need for uniformly adopted definitions of whole-grain foods (i.e., the amount of whole grain the food has to contain to be defined as a whole-grain food) (1, 7).

In the 2 articles reported in this issue on the effects of substituting refined grain with whole grain (3, 4), the estimate of dietary intake was calculated by subtracting the amounts of foods not consumed, as indicated on a food checklist, from the amounts of foods provided to participants with the addition of other foods consumed. The intake of nutrients and whole grain was calculated by using Nutrition Data System for Research software, and dietary adherence was assessed by several means, including a reliable objective biomarker for whole-grain intake. Considering the still-unresolved question about the role of whole grain in body weight and body fatness and the discrepancies between studies (10), it may seem surprising that studies that attempt to measure energy balance pay more attention to the measurements of energy metabolism and energy fecal losses than to the measurements of energy intake. Measuring energy intake directly by calorimetric methods to get a “true” energy balance would seem a feasible way in future studies. The body of scientific evidence needed for future recommendations for dietary guidelines in relation to whole grain could be strengthened considerably by more high-quality RCTs, with strict clarification on definitions and assessment of actual dietary intakes.