Dietary Substitution of Whole Grains for Refined Grains Favorably Effects Fiber Intake and Energy Metabolism in Adults
Authors: Karl J. Philip , Meydani Mohsen , Barnett Junaidah B , Vanegas Sally M , Goldin Barry , Rasmussen Helen , Saltzman Edward , Jonnalagadda Satya S , Meydani Simin N , Roberts Susan B 
Published: November 2023

Whole grain‐rich diets are consistently associated with lower adiposity in observational studies. However, clinical trials have failed to substantiate this association or identify underlying mechanisms. The inconsistency has been suggested to be due to trial methodology including suboptimal dietary control and confounding due to body weight changes. This study was designed to determine the effects of substituting whole grains for refined grains in the diet on adiposity and energy metabolism parameters independent of body weight change. Following a 2‐wk provided‐food run‐in period (0 g whole grains/d, 21 ± 3 g fiber/d), 81 men and post‐menopausal women (40–65 yr, BMI < 35.0 kg/m2) were randomly assigned to consume one of two provided‐food weight‐maintenance diets for 6 wk. The diets differed in whole grain and fiber contents (WG [n = 41]: 207 ± 39 g whole grains/d, 40 ± 5 g fiber/d or RG [n = 40]: 0 g whole grains/d, 21 ± 3 g fiber/d), but were otherwise similar in energy and macronutrient composition, types of provided foods and meal structure. Plasma alkylresorcinols (AR), biomarkers of WG intake, were measured at the end of the run‐in (PRE) and intervention (POST) periods to assess diet adherence. Body composition was measured by air displacement plethysmography, resting metabolic rate (RMR) and respiratory quotient (RQ) by indirect calorimetry, and fecal energy content by bomb calorimetry PRE and POST. Hunger and satiety were assessed weekly using visual analog scales. PRE plasma AR did not differ between groups but were higher POST in WG versus RG (p < 0.001). Body weight was maintained throughout the intervention in both groups. Changes in fat mass, fat‐free mass, RQ, and fecal energy density did not differ significantly between groups. However, changes in RMR ([WG versus RG mean difference ± SE] 48 ± 23 kcal/d; p = 0.04), fecal weight (76 ± 12 g/d; p < 0.001), and fecal energy content (96 ± 18 kcal/d; p < 0.001) were greater in WG versus RG. Additionally, perceived hunger (p = 0.06) and prospective consumption (p = 0.07) trended towards being lower in WG compared to RG. Collectively, these findings indicated that dietary substitution of whole grains for refined grains, in amounts that exceeded national dietary recommendations while increasing dietary fiber intake to recommended levels, did not impact body composition over 6 wk during weight maintenance, but conferred favorable energetic effects that cumulatively resulted in a 135 kcal/d energy deficit and reduced hunger. These findings provide mechanistic evidence supporting the consistent inverse associations between whole grain intakes and adiposity documented in observational studies which have been unsubstantiated in previous clinical trials that lacked dietary control or were confounded by concomitant weight loss.

Support or Funding Information

Funded by General Mills and by the USDA/ARS under agreement #8050‐15000‐097‐01S.