In the current study, we found no association of adolescence total calcium, dairy and non-dairy calcium intake with early adulthood cIMT. However, adolescents with the intake of cream had increased cIMT in their early adulthood. Furthermore, no higher MetS in the early adulthood was observed in the adolescents with higher consumption of dairy and non-dairy calcium. Moreover, skim milk, non-dairy and total calcium intake, but not other dairy products, could increase TG. The intake of plain ice cream showed reverse association with WC after 10-year follow-up in young adults.
There are some studies examining the calcium and dairy products intake in childhood and its association with cardiovascular risk factors in adulthood (10, 12, 17, 32–35). Similar to our findings, the research conducted by Wu et al in 3-18 years old participants concluded that there were not any linear and nonlinear relationships between long term dietary calcium intake with cIMT, PWV, stiffness index, carotid artery compliance, and Young's elastic modulus (12). In terms of dairy products, although we could observe the relationship between long-term intake of cream and elevated cIMT, another research found no significant linkage between childhood dairy intake with adulthood coronary heart disease (CHD) and stroke mortality, while they observed that 400 mg/d calcium consumption had an association with decreased 40-60% stroke mortality (32). Interestingly, a U-shaped association in 20-year longitudinal study in China was seen between calcium intake of adolescents and hypertension in adults. Meaning that lower and higher intake of calcium in adolescence can cause high blood pressure in later life (33). We demonstrated that the consumption of plain yoghurt and butter can decrease SBP and DBP. Concerning another CVD risk factor, an evidence found no relationship between long-term calcium intake in youth with risk of type 2 diabetes in adulthood (34). Whereas, high school adolescents with dairy product intake demonstrated 38% lower risk of type 2 diabetes in middle-aged women (35). According to some other surveys claimed that there might be detrimental impact of calcium supplementation on CVD in adults (10, 17), all above mentioned studies did not indicate the harmful influence after long-term consumption of calcium and dairy products, whereas, long-term intake of cream probably could increase cIMT as well as the consumptions of dairy and non-dairy products might raise the components of MetS including TG and DBP as the current study found. Thus far, because there are conflicting conclusions between these evidence, and there has not been illustrated the effect of long-term intake of calcium supplements on cardiovascular causes yet (36), it could be due to the differences between the source of calcium intake.
On the other hand, there are some cross-sectional studies related to the linkage of calcium and dairy consumption with CVD risk factors needing to be mentioned. Although, we observed no long-term relationship of dairy and non-dairy calcium intake with MetS in our findings, we have previously observed that low-fat dairy products reduced risk of MetS in children and adolescents of the same population (TLGS) (37). Moreover, in another study, adults showed that higher calcium intake could diminish the risk of CVD only in women (38). With regard to food pattern changes, the Bogalusa Heart Study indicated that from childhood to adulthood, there was a decrease in consumption of milk, especially in males (39). It seems that adults are more likely to adhere to unhealthy dietary patterns than children as we also observed that adolescents had higher intake of dairy and total calcium in comparison with adults. Besides, in adults, it was observed that the intake of low-fat dairy had association with reduced PWV and cIMT (40). Although, cheese and milk consumption did not have the relationship with cIMT, 100g yoghurt/d could lower cIMT in elderly women (41). In contrast, it was seen that calcium consumption with supplements resulted in escalated cIMT in postmenopausal women with dyslipidemia (11). Overall, observing cross-sectional evidence seemed like that adulthood intake of calcium from dairy sources can reduce cIMT and other CVD risks.
The reason that there has not been the link between calcium intake and cardiovascular risks after a long time might be the independent role of circulating calcium levels from the sources of supplements, although it was claimed that circulating calcium can lead to artery calcification and clotting (10). In the current study, we evaluated this relationship in young adults which may be early to observe plaques and therefore increased cIMT, notwithstanding elevated cIMT and performing plaques can happen in early stage of life (42). Hence, calcium deposition has not been caused by the intake of dietary calcium and dairy products. It is worth mentioning that calcium begins its protective and lifelong role in cardiovascular risk factors from early in life so that it is necessary to consider diet from childhood to prevent them from chronic diseases in adulthood. Another hypothesis related to the influence of dairy products on artery structure is about lipid content of this product, in particular saturated fatty acid (SFA) having association with many negative health effects which may cause increased LDL levels, and thus an increased risk of CVD (43). Regarding this, we could observe the high consumption of cream consisting of SFA associated with elevated cIMT (44). However, findings have indicated that the link between SFA and CVD could not be considered evident than previously assumed due to the different combination of foods in saturated and unsaturated fatty acids, as well as contribution of significant quantities of other nutrients such as potassium and magnesium that may alter CVD risk (45).
The present study had some strength needing to be mentioned. First, it had a cohort design in which we observed the tracking intake of calcium and dairy products from adolescence to adulthood. Secondly, it was considered both cIMT and MetS which are strong risk factors for CVD since childhood and adolescence. Also, it is prominent to interpret the study in the light of limitations. We could only regard young adults as their cIMT were measured and this period of time is early to observe increased cIMT. Also, 10 years of follow-up was a short-term for observing the changes of artery structure. We did not consider other source of calcium as supplements that my change the results. Furthermore, changes the diet and food items during 10 years should have considered as it might affect the outcomes.