This study found that the overall quality of dietary intake consumed by a sample of individuals with T2DM was less than that of a healthy control population when measured using the Mediterranean Diet Score, the Alternate Mediterranean Diet Score and the Healthy Diet Indicator. Conversely, when individual macronutrient intakes were assessed, no differences were observed between those with and without T2DM. Given that the design of the current study is of a cross-sectional case–control nature, a cause and effect relationship between dietary quality and incidence of T2DM cannot be determined.
Lifestyle is considered a cornerstone both in the treatment and management of T2DM [30, 31]. The results of the current study show that individuals with T2DM consumed a diet significantly less like the Mediterranean dietary pattern compared to a non-T2DM population, as assessed by the Mediterranean Diet Score and Alternate Mediterranean Diet Score. In addition, there was a significant inverse association between the Mediterranean diet score and BMI as well as fasting plasma glucose and fasting plasma insulin, recognised as parameters of glycaemic control. Recent evidence from both observational analyses and intervention studies suggest that adherence to a specific dietary pattern that promotes metabolic health may be more beneficial than adherence to individual nutrient based recommendations . The Mediterranean dietary pattern is characterised by high intakes of fibre, lean meats and fruit and vegetables [9, 11]. Concern was initially raised as to the relatively high fat content of the Mediterranean diet, which can have up to 40% total energy derived from fat, and the effects that this could mediate on body mass and metabolic health. However, several epidemiological studies have shown that the Mediterranean dietary pattern is inversely associated with weight gain, BMI and T2DM risk [26, 33, 34]. Furthermore, evidence suggests that the Mediterranean dietary pattern has beneficial effects on lipid and glycaemic profiles [12, 35–37].
The current study showed that those individuals with T2DM had a lower score than the control group when dietary pattern was assessed using the Healthy Diet Indicator (HDI). Whereas several epidemiological studies and systematic reviews have examined the relationship between the Mediterranean dietary pattern and T2DM risk [7, 32, 33], to the best of our knowledge the current study is the first to examine dietary intake using the HDI in a sample of individuals with T2DM. The HDI is based on the WHO guidelines for diet and nutrition in the prevention of chronic disease and whilst the current study design prevents cause and effect inferences to be drawn, it is interesting that the T2DM cohort scored significantly lower than the control participant using this dietary pattern score. Conversely, no significant difference was found in dietary quality scores between the T2DM and control groups when measured using the Alternate Healthy Eating Index. While speculative, this result may be due to the fact that while the other DQIs use a predefined cut off point in the allocation of scores, the Alternate Healthy Eating Index allocates scores for a wider range of intakes.
Individuals with T2DM also scored significantly lower when micronutrient intake was assessed using the Micronutrient Adequacy Score, a result that highlights that although micronutrient adequacy was suboptimal in both groups, individuals with T2DM consumed a more nutrient dilute diet than their insulin sensitive counterparts. When the intake of nine individual micronutrients was assessed, individuals with T2DM consumed significantly less Vitamin D than the control group. Emerging evidence suggests that supplementing vitamin D in individuals with impaired glucose tolerance improves insulin sensitivity, however results from those with T2DM have been inconsistent [38, 39].
Much research has been carried out on the effects of different dietary patterns on body composition, metabolic health and disease risk. A Western dietary pattern with characteristic consumption of high levels of saturated fatty acids, red and processed meats, confectionery and refined grains has been associated with increased T2DM risk and deterioration of metabolic health, independent of weight status [2, 40, 41]. In the current study, food group analyses found that the T2DM group had a high consumption from the fruit and vegetable food groups yet failed to reach the recommended intake of 400 g of fruit and vegetables per day . The third highest food group consumption of the T2DM group came from meat and meat products. Total cereal grain consumption was notably higher than consumption of wholegrain cereals alone for the T2DM group. This suggests a greater consumption of refined cereal grains within this group. The T2DM group also consumed significantly more fats and oils, processed meats and bread than the control group. These findings represent features of a Western dietary pattern. Conversely, consumption of a prudent dietary pattern, which includes intake of fruit and vegetables, lean meats and wholegrains, characteristic components of a Mediterranean dietary pattern, has been found in other studies to be protective to metabolic health [2, 42].
Evidence suggests that even short term hyperglycaemia results in increased vascular intracellular adhesion molecules in individuals with T2DM, a risk factor for atherosclerosis [43, 44]. This supports a need for greater adherence to a dietary pattern that promotes increased amounts of fibre containing foods in order to regulate the hyperglycaemic response [45, 46]. Fibre independently affects metabolic health. The current analysis found that total fibre intake had a significant negative association with waist circumference. Similar results have also been found in previous analyses [47, 48]. Given the substantial evidence to support WC as an indicator of cardiometabolic disease risk , the current results suggest that fibre may play a role in the attenuation of obesity induced metabolic dysregulation.
Excess adiposity is associated with increased morbidity and mortality . Thirty-nine percent of the total population in this study were overweight. This is consistent with recent findings which concluded that 37% of a representative sample of the Irish population were overweight . At group level, the control group have a similar mean BMI, 27.6, when compared to a representative sample of the Irish population at 27.5. However, the T2DM group was found to have a BMI of 32.5 which is significantly greater than the control population. Given the known association between the Mediterranean Diet Score and BMI and weight status [26, 34], it must be noted that such differences in BMI between the two groups may have potentially contributed to the T2DM group having a significantly lower dietary quality score than the control group.
The strengths of the current study were that at group level there was no significant difference in age or activity level between the participants with T2DM and the control group, and that control participants were a representative sample of the national population in terms of BMI and habitual nutrient intake by comparison with the recently published national data . There are however, limitations associated with the current study. The first is the fact that validation of reported dietary energy intake data using the Goldberg method  revealed significant misreporting in this sample of Irish adults. Based on estimates of energy expenditure, seventy-four of the 111 subjects (66.6%) in this study were found to under-report their dietary intake, which is likely to explain the relatively low mean energy intake observed. However, the mean level of under-reporting was similar in both groups, suggesting that while mean nutrient intakes may be under-estimated in this cohort, any differences or lack thereof, detected between the two groups, remain. Studies suggest that the mis-reporting of dietary energy is a serious problem in any study of dietary intake . Moreover, obesity affects both the quantity and quality of reported dietary energy intake data [53, 54]. As 86% of the participants of this current study were overweight or obese (98% T2DM and 70% control) this will without doubt have contributed to the high level of energy under-reporting observed. Furthermore, a high level of under-reporting of energy intake has also been noted in individuals with T2DM [55, 56]. The second limitation is that although the two groups were similar in terms of age and activity level, a significant difference was found in BMI between the two groups. Thirdly, it must also be noted that some participants in the T2DM group may have previously received dietary advice from a healthcare professional which may have affected habitual dietary intake in this group. The fourth limitation is that there was no gender balance between the adults with T2DM and the controls. There were 39 male and 26 female participants with T2DM yet there were only 16 males and 30 females in the control group. Another limitation is that the cases and controls were not matched socio-economically. Whilst they were all recruited from the same catchment area in Dublin, Ireland, no objective measure of socio-economic status was used. A final limitation is that this study also has a cross-sectional design with a relatively small sample size allowing only observation of any associations between nutrient intake, dietary quality and metabolic health in a sample of individuals with T2DM. Further confirmation of these findings will require large scale randomized controlled trials.