This follow-up study of a Chinese cohort of cognitively normal older adults aged 55 years and above in Singapore showed that those with daily consumption of milk or other dairy products were about half as likely to develop MCI or dementia compared to those with less dairy consumption. To the best of our knowledge, five prospective cohort studies published to date have reported the association between milk or dairy intake and risk of cognitive impairment with conflicting findings (20–24).
Our findings concur with two prospective cohort studies of Japanese men and women which showed a significant association between a higher level of milk and dairy intake with lower risk of Alzheimer’s disease but not vascular dementia in one study (20), and with vascular dementia in another study (21). On the other hand, one study of Caucasian and African Americans suggest that milk intake at midlife may be associated with greater rate of cognitive decline from midlife to late life (22). Additionally, two other cohort studies of Australian men and French women found that regular consumers of full-cream milk was associated with poorer cognitive function assessed by MMSE (23) and higher intakes of dairy desserts and ice-cream was associated with cognitive decline (24). A meta-analysis involving a total of 10,941 participants from four cohort studies and three cross-sectional studies concluded that there is an inverse relationship between milk consumption and cognitive disorders (Alzheimer’s disease, dementia, and cognitive decline/impairment) but this was limited to Asian participants who are noted to have relatively lower intake of overall milk and dairy products (14). Milk consumption is relatively low among Singaporeans (34), as it is among Japanese, which reinforces the evidence that among Asians, high dairy intake reduced the risk of MCI or dementia.
This may possibly be explained by genotypic differences among populations in lactase persistence (LP), a trait in which lactose can be digested throughout adulthood, and lactase non-persistence (LNP) which can cause lactose intolerance and influence dairy consumption. Asians are known to have higher prevalence of intolerance than other ethnicities. It has been reported that in all ethnic populations and especially among Caucasians, LP individuals tend to consume more cheese and recent total dairy intake (35). In LP, lactose is broken down by the enzyme lactase in the small intestine, resulting in the formation of D-galactose, which has been extensively shown to induce neurodegeneration through oxidative stress in animal models. Thus, Caucasians with a higher frequency of LP may be more susceptible to the neurotoxic effects of galactose-induced oxidative stress and cognitive decline. However, findings from a study fail to support the hypothesis that milk intake was associated with greater cognitive decline among Caucasians with the LP versus LNP genotype (22). Hence, more studies are needed.
There are many other proposed biological mechanisms for the protective effect of dairy intake against the risk of dementia. Prospective studies have reported that dairy intake was associated with a lowered risk of developing type 2 diabetes mellitus (36), hypertension (37) and obesity (38), and these same factors are also known risk factors for dementia (39). The reduced risk of cognitive impairment from milk intake could be attributed to modified neurovascular dysfunction, weight reduction and lowered metabolic risks (40, 41). Another possible mechanism could be the benefits from bioactive constituents present in milk and dairy products. It has been postulated that phospholipids in the milk fat globule membrane are active transporters of essential fatty acids that could improve brain health by reducing endoplasmic reticulum stress (42), which is known to increase the risk of cognitive disorders such as Alzheimer’s disease. Furthermore, milk and dairy products are important sources of vitamin B12 which is known to reduce plasma homocysteine levels (20). As low serum vitamin B12 and elevated plasma homocysteine levels are reportedly risk factors for the development of dementia (43, 44), milk and dairy consumption could lower the risk of dementia because of the presence of these nutrients. Numerous bioactive peptides are present in milk and dairy products that possess important biological activities and functionalities, including antihypertensive, antioxidative, immunomodulatory and other activities that influence physiological and metabolic functions with ultimate beneficial health effects (45). Recently, a novel oleamide identified from a fermented dairy product was demonstrated to be able to reduce amyloid beta accumulation via enhanced microglial phagocytosis, and to suppress microglial inflammation after amyloid beta deposition (46).
In this study, we found no statistically significant reduced risk of MCI or dementia in participants who reported high fish intake. Evidence from previous studies have suggested inconsistent results, as only four of eight observational studies that used incidence of Alzheimer’s disease or dementia outcomes reported positive findings (11). On its own, our data does not strongly support an association; however, our data taken together with existing data across studies does favour a role for fish consumption in delaying or preventing the onset of cognitive disorder in elderly individuals without dementia. A recent meta-analysis in 2015 of six prospective cohort studies reported the pooled relative risks associated with fish intake for dementia of 0.84 (95% CI = 0.71–1.01) and an associated 36% (95% CI = 8–56%) lower risk of Alzheimer’s disease (12). In our study population, we did not distinguish between fish types (high fat versus low fat, wild versus farmed) and preparation methods of fish (baked, fried, or steamed) which may influence the fatty acid content and obscure a true association. Inaccuracies in self-reporting frequencies of fish intake may also cause misclassification errors and bias risk estimation towards the null. Additionally, dairy and fish intakes were assessed only at baseline and possible changes after the baseline interview may attenuate estimates of risks.
In summary, our study provided strong support that among Asian older adults in this Singapore population, consumption of milk and dairy products but not fish consumption was significantly associated with lower risk of incident MCI or dementia. These findings have strong implications for future research and public health policy in terms of dietary interventions for the early prevention of dementia. Further research in populations with diverse genetic polymorphisms in LP and milk tolerance should be conducted to elucidate the ethnic population-specific effects of dairy consumption on cognitive decline and dementia risk.