An increase in whole-grain consumption for a 16-week period did not significantly affect any of the biomarkers of cardiovascular health assessed here. These results are consistent with previous findings of Andersson et al
, who noted no benefit of consumption of a range of wholegrain foods (from mixed grain sources) on any marker of CVD risk(14)
. The 2007 study of Andersson et al
is also the only randomised, controlled whole-grain intervention other than the study reported here to have a robust power calculations to determine sample size. An earlier study of similar design showed a small but significant decrease in mean fasting insulin, but no other benefits to CVD risk were evident(13)
. Two previous studies have shown that inclusion of wholegrain foods alongside hypoenergetic dietary interventions reduced blood pressure(15)
, C-reactive protein and percentage abdominal fat(16)
significantly more than hypoenergetic interventions alone. Such studies support observational data suggesting whole-grain consumption is part of a healthy lifestyle, but do not demonstrate that increased wholegrain food consumption benefits CVD risk as a single, independent dietary factor.
The biomarkers of cardiovascular health used within the present study are all well-established biochemical and anthropometric markers of CVD risk, and embody the range of physiological processes thought to be involved in disease onset. We calculated a fasting marker of insulin sensitivity based on glucose, lipid and insulin concentrations (modified quantitative insulin sensitivity check index; QUICKI). Whilst this method has been shown to be significantly correlated with more robust measures of insulin sensitivity(23)
it may be less sensitive in detecting subtle diet-induced changes compared with direct measures such as the euglycaemic–hyperinsulinaemic clamp or intravenous glucose tolerance test. Likewise, a more detailed analysis of lipoprotein subclasses may be required to demonstrate changes in lipid metabolism.
We believe that the disparity in the findings from the observational data and the present intervention study warrants further consideration and may arise as a consequence of methodological and/or biological effects. We sought wherever possible to reduce the sources of methodological errors. Intervention studies may sometimes fail to show the expected effect because the subjects fail to comply with the dietary prescription or because of unexpected changes in the control group. FFQ are not as ideal as food diaries for assessing dietary or nutrient intake, but do provide a useful means of measuring changes in frequency of the consumption of specific food types over time, and also tend to be less time intensive for participants to fill out, thereby increasing compliance. The FFQ used in the present study was designed to quantify food consumption during the 7 d before measurement days. The shorter time frame for recollection may increase the precision of the measurement tool compared with FFQ which measure diet over preceding months or up to 1 year. Results from the dietary analysis suggested that the provision of specific foodstuffs and regular contact and motivation of participants by the research team resulted in good compliance to the prescribed intake levels of whole grain in each group. However, the participants appeared to include the wholegrain foods as a dietary addition as opposed to the dietary substitution that was explicitly requested in participant guidelines and investigator instruction. Therefore, the modality of whole-grain inclusion in the diet desired for the intervention may not have been achieved. Our dietary inclusion criteria for participants were based solely on low elective wholegrain food consumption. Future whole-grain-based dietary interventions in free-living individuals may benefit from more specific inclusion criteria (for example, choosing participants who regularly consume breakfast, or those who consume high amounts of (refined) grain products within their habitual diet). This dietary effect is also relevant to the development of dietary guidelines for whole-grain consumption in the general population. These guidelines must be designed to achieve replacement of refined-grain foods in the diet without increased overall food consumption.
Intervention studies may sometimes be underpowered to detect a significant effect, but this was a large study which estimated differences with high precision (i.e. narrow 95 % CI), where no clear trend was observed. The study may have been of insufficient duration to detect an effect, but comparison of the results at 8 and 16 weeks did not suggest a consistent trend which may have become significant with continued intervention. In other dietary interventions that ameliorate markers of CVD risk, lipid parameters are frequently modulated after only 2–4 weeks (for example, Cicero et al
, Madsen et al
and Jenkins et al
), suggesting that the time frame for the present study was more than adequate.
Accordingly, it may be more likely that the lack of effect of the wholegrain food intervention has a biological explanation. In this controlled intervention, we provided study foods in a very specific and structured manner, which may not reflect the consumption of wholegrain foods by habitual whole-grain consumers recorded in observational studies. The range of products was restricted and subjects had to make conscious changes in other parts of their diet in order to incorporate the study foods as prescribed. Breads and breakfast cereals were the most frequently consumed wholegrain foods and the concomitant increase observed in total dietary carbohydrate intake, plus the addition of food accompaniments such as spreads, milk, etc, may have contributed to the trend towards increased energy intake in the intervention participants. At the highest level of whole-grain intake there was a small but significant reduction (equating to about 0·5 per d reduction of consumption frequency) in fruit consumption, again implying that volunteers may have changed their dietary pattern in order to accommodate the high intake of whole grain required during the second stage of the intervention. These changes may have offset any health benefits of the wholegrain component. The net effect is that the diet of high whole-grain consumers in the present intervention study is different from that of high whole-grain consumers in observational studies, where ‘elective’ whole-grain intake is a marker of a broader diet and lifestyle that cannot be easily replicated in controlled, intervention studies. However, we believe this type of food-based dietary intervention in free-living, healthy individuals more appropriately models the impact that public health recommendations worded around increasing consumption to prescribed levels of generic wholegrain foods could have on the diet of the general population. Our findings that individuals within the present study tended to alter their diet will be useful in the design of future whole-grain-based dietary intervention studies.
Participants were given a range of wholegrain foods reflecting those generally available in the UK. The term ‘whole grain’ has been used to describe foods that contain more than 51 % whole grain in which the naturally occurring pro-portions of germ, bran and endosperm are retained(27)
. The majority of the foods provided to participants (see ), with the exception of bread, were not made from finely milled grain, thus it is unlikely that this aspect of processing would have influenced the results. While commercially available wholegrain loaves have similar glycaemic index values to refined loaves(28)
, previous work has shown that increasing the content of intact cereal grains in breads resulted in reduction in glycaemic index(29)
. Several of the other, less processed, foods used in the present study had glycaemic indices lower than that of wholemeal bread.
Finally, the lack of intervention effect may be a consequence of the study population. Pre-screening of participants for those with elevated fasting LDL-cholesterol would have better targeted a population at risk from CVD. However, we did not select a clinically high-risk population for the present study, since the outcomes of this research were based on benefitting dietary guidelines for the population as a whole. Instead, the present study focused specifically on a group of overweight individuals, representative of the population of the UK and other countries where overweight is now the norm, and likely to be at moderately increased risk of CVD (see ). However, the duration of this controlled intervention study represents a very short period of dietary change in the context of lifelong dietary exposures, and may be insufficient to change the lifetime disease trajectory for these individuals with a strong pre-existing risk factor.
Within the present study, we have tested whether infrequent whole-grain consumers respond with improved markers of cardiovascular health when substituting wholegrain food products into their diet, and have found no effect. The present study sounds a note of caution to the specific health claims for whole grain-rich foods and cardiovascular health. However, it does not undermine more general efforts to promote whole grains as part of a healthy diet for the general population across the life course, based on data from observational studies.