Discussion
Using a unique investigation in tracking longitudinal dietary intake, this study revisited the importance of healthy dietary behaviors beginning in young adulthood to prevent or delay diabetes.8 15 27 While diet quality was improved with advancing age and diabetes awareness, especially when people learned their diabetes for the first time, the changes in food intake were not always concordant with evidence-based dietary recommendations. Therefore, individualized nutrition counseling and intervention need to be provided to people with diabetes at diagnosis and over time in order to promote longitudinal healthy dietary behaviors leading to diabetes remission and minimizing risks for developing diabetes complications.23 28
Forming well-balanced dietary habits should be emphasized beginning with young adulthood or even younger since it can continue throughout a lifetime and influence longitudinal health outcomes.8 29 In the current study, the diabetes groups compared with the control group consumed more adversely rated foods such as fried or salty plant foods, processed meat, or sweetened drinks in young adulthood or when they were free of diabetes. This dietary pattern may contribute to increasing their risk for diabetes along with other risk factors (ie, parental diabetes history, higher BMI, lower physical activity). Implementation of a realistic, practical and proactive nutritional intervention targeting at-risk young adults along with early detection using an appropriate nutritional screening tool is imperative.5 30
Over the years, the diagnostic criteria for diabetes and medical nutrition therapy (MNT) guidelines have been updated based on advances in research.5 31–34 Milestone research (ie, National Diabetes Prevention Program, Diabetes Control and Complication Trial) has reported the significance of diabetes prevention and management. However, the prevalence of T2D, especially early-onset T2D, is increasing, and the prevalence of optimal glycemic management continues to decline.35 36 Potential reasons may be related to translation time lags and food illiteracy.37 38 For instance, the diabetes group, especially the early-onset group, switched beverage choice from soft drinks to diet soda and fruit juices at Y7 when they first learned of their diabetes. This may be related to the people’s tendency to change self-care behaviors following disease diagnosis and symptom awareness without consideration of information source.17 39 40; many people at risk for or with diabetes tend to find healthy food substitutes when they learned of their health conditions. However, healthy eating does not mean to have ‘less tasteless’ food.17 40 Evidence-based individual counseling and education enable to provide support people to make better and enjoyable food selections without unpleasant feelings.5 Also, effective behavioral strategies with considerations of patient and disease characteristics are warranted to translate advances in nutritional sciences into MNT without time lags.6 38 41
This study raises questions about consuming dairy products. The APDQS rates whole-fat dairy products as adverse while low-fat dairy products are rated as beneficial. The case groups, however, consumed less whole-fat dairy products compared with the control group when they were unaware of their diabetes, while the difference disappeared after case groups learned of their diabetes. There are three possible explanations. First, dairy product consumption decreases with advancing age,17 42 43 and thus, the whole-fat dairy effect on developing diabetes may be diminished as participants’ age. In the current study, there was no significant net difference of differences on whole-fat dairy product consumption between case-control groups after participants learned of their diabetes and/or were getting older. Another possibility is a protective effect of black coffee, tea, and moderate alcohol consumption on diabetes development.7 44 Our data showed increases in coffee/tea in Y7 and Y20 across all groups. Also, the control group had modest alcohol consumption across all years while case groups decreased alcohol intake at Y7 and Y20.
Second, there is limited evidence about whether low-fat dairy products provide superior health benefits compared with whole-fat dairy products. A recent meta-analysis of randomized clinical trials evaluating the association between dairy product intake with the incidence of T2D concluded that intake of dairy products, especially low-fat dairy products, prevented diabetes via improving insulin resistance and reducing visceral fat and weight.45 However, the findings were not from experimental research that compared low-fat versus whole-fat dairy product consumption. The meta-analysis evaluated the dairy product effect on T2D with studies mainly focused on low-fat dairy products.
Lastly, the APDQS was created in 2007 and therefore reflects earlier advice when nutrition (eg, low-fat) and diet quantity (eg, energy intake) were emphasized.11 22 46 47 Over time, studies have shown the importance of healthy fat and high-density lipoprotein (HDL)-cholesterol and have initiated a re-evaluation of diets with full-fat dairy products.48 49 Although dairy fat may increase low-density lipoprotein-cholesterol, it also increases HDL-cholesterol and has uncertain protective effects on apolipoprotein C-III.48 Additionally, vitamin D (a fat-soluble vitamin) and calcium included in milk regardless of the level of fat content may play a protective role in diabetes development.49 That is, dairy fat may not be harmful to health as long as people are aware of the caloric intake and do not exceed their daily recommended total energy intake in order to prevent obesity.50 Further investigation is needed in this area.
This study extends prior CARDIA analyses that reported that changes in the APDQS were associated with subsequent diabetes risk as a binary outcome (event vs no event).2 8 The novel approach in the current study examined diabetes awareness with diet quality and food intake in a standardized fashion using serving size in a healthy cohort, which produced high-quality evidence supporting MNT.16 51 Using a nested case-control design with a relatively large sample size, the findings added high quality of evidence about how people make dietary change over time once they learned of their diabetes. Importantly, it shows a pattern of dietary changes in young adults with diabetes and provided a suggestion regarding the timing of nutrition education to be offered for young adults with early onset diabetes.52 APDQS temporarily increased and then decreased in the small early onset group, to a level that was even lower than the control group at Y20. Temporary improvement in diet quality may result from psychological distress (eg, being scared) due to diabetes diagnosis or transient efforts to make healthy lifestyles. Young adults, however, may be vulnerable of returning to long-term behavioral equilibrium (lower quality diet),40 53 and thus intensive and individualized intervention at appropriate timing is very important for this population. Additionally, the current study successfully identified certain food groups as a focus for future MNT to prevent or delay diabetes development and improve diabetes management while the difference in APDQS among groups seems very small. One possibility is that ‘slight fluctuations are sometimes not slight’. That is, younger individuals may provide compensatory physiological mechanisms to maintain euglycemia, but with small changes in dietary quality, along with BMI and age could affect diabetes onset, especially in those with high diabetes risk.54 Another possibility is that some specific advice, for example concerning whole-fat dairy, may be inaccurate. Further investigation using a rigorous research method is needed.
Our study has limitations. First, the CARDIA study was initiated by recruiting a biracial cohort in 1985–1986 to address racial disparities in CVDs,19 20 but greater diversity of people at risk for diabetes has occurred.4 55 Thus, research representing a current trend of population characteristics needs to be replicated. Second, few participants were classified as the early-onset group, thereby limiting the precision regarding estimates of subsequent changes made years later. To address this limitation, we calculated the net difference and the net difference of differences to provide an overall view of the differences between case and control groups over 20 years. Third, all changes in dietary quality and food intake were not observed due to unmeasured and unavailable variables. For instance, changes in dietary quality and food intake in the later-onset group were not included in the current study. Data from CARDIA Y35 including diet assessment will provide an answer soon. Fourth, we identified an unexpected result regarding alcohol intake. The case groups drank less alcohol than the control group and the difference was increased after diabetes awareness or with advancing age. Currently, there is a consensus about modest alcohol consumption (one or less drink per day) being beneficial for people with diabetes and prediabetes.7 This recommendation allows modest alcohol intake and avoids heavy alcohol consumption rather than encouraging the initiation of drinking.27 Message framing about alcohol consumption need to be carefully designed in order that the messages are delivered to the public as intended. Fifth, there is a concern about loss to follow-up, which is inevitable in longitudinal study design. However, in the sensitivity analysis of baseline characteristics comparing those with all follow-up diet data and those without follow-up diet data at either Y7 or Y20, there was no substantial difference with regard to the drop-out rate. Thus, the main estimate is unlikely to be considerably affected by selection bias due to loss to follow-up. Sixth, the CARDIA study did not collect autoimmune markers to determine type 1 diabetes vs T2D. Therefore, the type of diabetes could not be determined in this current investigation. Future research is needed to examine whether type of diabetes influences dietary behaviors.
Lastly, the present recommendation of macronutrient composition is a major change from previous dietary guidelines.22 23 33 Currently, the ADA recommends individualized meal planning based on current eating patterns, individual preferences, and metabolic goals.5 There is an increasing body of evidence suggesting that moderate consumption of carbohydrates (about 45% of total energy) and fat (36%~40% of total energy) may be beneficial to prevent and manage diabetes.5 7 While the reduction of energy intake was observed over time with no real difference across groups except Y0, CARDIA participants were not asked how they perceived their diet quality or whether they were aware of changing dietary guidelines. Further investigation about what motivates people to make dietary changes is warranted to identify a better way to deliver patient-centered MNT.