Ethnic Differences in Early Onset Alzheimer's Disease

Neurodegenerative Disorders Research Pvt. Ltd., West Perth, Australia

Corresponding Author:

Panegyres PK
Neurodegenerative Disorders Research Pty Ltd
4 Lawrence Avenue, West Perth, Western Australia 6005
Tel: +61 8 9481 6293
E-mail: research@ndr.org.au

Received date: June 29, 2017; Accepted date: June 30, 2017; Published date: July 07, 2017

Citation: Chen HY, Panegyres PK (2017) Ethnic Differences in Early Onset Alzheimer’s Disease. J Alzheimers Dis Parkinsonism 7:346. doi:10.4172/2161-0460.1000346

Copyright: © 2017 Chen HY, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Visit for more related articles at Journal of Alzheimers Disease & Parkinsonism

Keywords

Dementia; Ethnicity; Risk

Introduction

In a recent report published by World Health Organisation, it was estimated that there were 46 million people living with dementia around the world and this number is projected to be tripled by 2050 [1]. With the increasing proportion of minorities among elderly populations [2] and a higher prevalence of dementia observed among Hispanics [3-5], Latinos [6] and African Americans [3-6], the burden of dementia in minorities is a challenge for many health systems. Being the most prevalent form of dementia and similar ethnic disparities were reported in Alzheimer’s disease (AD) [7], AD should be a priority for countries where populations are composed of a significant proportion of Hispanics and African Americans, like the United States.

Dementia and AD are mostly found in people aged 65 years and over. It can occur in people younger than 65 years and the numbers are alarming [8-10]. This may be due to a growing awareness of early onset dementia in the community as well as an improvement in the diagnostic tools. An accurate estimation of prevalence of early onset dementia/early onset AD, however, is not easy as different diagnostic methods and study designs were used [11]. Nevertheless, it still poses a threat to the health system and society that people with early onset dementia/AD have to face such a challenge at an early age which has a major impact on individuals and families, as well as society compared to those diagnosed at later stage of life [12,13]). The aetiology of younger onset dementia/AD is not fully understood; however, some similarities to the late onset are identified, including ethnic disparities between whites and other minorities [3]. Our studies suggested that native Americans, Alaskans and African Americans were at greater risk of developing early onset AD than other ethnicities [14].

Explanations for such ethnic disparities may be grouped into genetic factors, family history, socio-economic status and comorbidities associated with cerebrovascular dysfunction. It should be noted that most results are potentially biased due to small sample size, poor study design or possible misclassification of ethnicity. Studies illustrate that among younger onset dementia patients such assignment of ethnicity may be incorrect or uncertain in 30-50% [15]. The cultural and linguistic backgrounds of minorities may further reduce the accuracy of cognitive and behavioural assessments. These limitations should be borne in mind when interpreting findings.

We summarise the findings that explore the pathways between ethnicity and AD. Some studies suggest differences could be culturalthat is, some cultures perceive dementia-related symptoms as part of the aging process [16-20], which is beyond the scope of this review.

Factors Relating Ethnicity to Alzheimer’s Disease

Genetic factors

Research exploring genetic effects in AD has increased our understanding of patho-mechanisms. Genetic background contributes to the higher incidence of dementia in certain minority groups [5,21- 23]. One of the well-known genes is Apolipoprotein E (APOE). It is a major cholesterol carrier that supports lipid transport and has a role in injury and repair in the brain. There are three major forms of APOE-ε2, ε3 and ε4. APOE ε4 alleles increase risk for AD and cerebral amyloid angiopathy [24].

The APOE ε4 allele effect is evident at all ages between 40 and 90, but less marked after age 70; the presence of APOE ε2/ε3 alleles appear to have a protective effect and similar across different ethnicities [22]. Nevertheless, there are some ethnic differences identified. Farrer et al. found that, compared with those who did not have APOE ε4, white and Hispanic people who inherited APOE ε4 from one of the parents were at a greater risk of developing AD (in comparison to African Americans) [22]. For people who inherited an APOE ε4 allele from parents, whites and African Americans had higher risks of developing AD. Notably, the association between APOE ε4 and AD that is found among minorities is weaker than the association with whites. Nevertheless, this weaker association explains, in part, the greater prevalence of AD and other dementias in African Americans and Hispanics. Another study found that with one or more APOE ε4 allele, African American and Hispanics were more likely to develop AD, controlling for years of education [25].

Most studies suggest that African Americans and Hispanics have a greater risk of developing AD when an APOE ε4 allele is present. This increased risk cannot be explained by an effect of the APOE ε4 allele alone, proposing that some other factors are operational. Some studies indicate that the APOE genotype is related to ancestry; the high-risk allele among African Americans could be gradually attenuated as a result of mixture with other ethnicities or modified by environmental factors [26,27].

Family history

The association between family history of dementia and the risk of developing AD has been established [28-30], and authors have suggested that the effect is only present among people who carry APOE ε4 [31]. Studies examining the impact of family history on dementia are often biased, as the effect could be easily masked by genetic factors or other risk factors [29]. In the study by Green et al., there is evidence to suggest that, after stratifying for APOE genotypes, females with a family history were at a greater risk of developing dementia than males among whites [29]. A similar effect was found in African Americans; increased risks were also found in spouses as well as first degree relatives. These authors concluded that family history has a similar effect among whites and African Americans and ethnicity has a stronger effect than family history. However, self-reported ethnicity and lack of control for educational level may have contributed to the observed risks in these studies. A recent study found a family history of dementia increased the risk of AD and the risk is independent of the APOE ε4 allele [32]. Devi et al. observed a similar pattern among Hispanics; having relatives with AD provided an increased risk of AD compared to controls [33]. This increased risk was also found among whites but not among Africa Americans. Two studies suggest that this association is more obvious with female relatives [29,33].

Familial AD can be inherited through gene mutations; and these forms are more common in younger onset dementia [34]. Early-onset familial AD, which typically develops before the age of 65 years and accounts for only a small portion (<1%) of AD cases, is primarily caused by overproduction of Aβ owing to mutations in either the APP gene or genes encoding presenilin 1 (PSEN1) or presenilin 2 (PSEN2), essential components of the γ-secretase complexes responsible for cleavage and release of Aβ [23,35,36]. No relationship between ethnicities and these gene mutations has been observed.

Socioeconomic status (SES)

Many studies use ethnicity as a surrogate of SES-there is a strong relationship between low SES in African Americans and Latinos/Hispanics with the development of dementia [37-41]. A similar association is observed in measures of social integration, such as parental or early life socioeconomic position, childhood IQ, educational attainment, occupational characteristics, cognitive function and neurocognitive disorders [42,43]. Ethnic disparities and the risk of developing AD must consider the effect of SES. However, an examination of multiple prospective cohorts which investigated the role of cognitive performance, memory and executive function at baseline and over time found there were differences by race/ethnicity at baseline (whites higher than minorities); this pattern did not hold for cognitive decline after adjusting for years of education [44]. The effects of education and ethnicity were not consistent between studies.

The possible explanations for lower cognitive test scores for Blacks and Hispanics include the bias of the tests or the different types of tests [45-48] and lower levels of SES [49-51]. This is particularly so with lower levels of educational attainment and the quality of education, which might explain the substantial variation in cognitive test scores among ethnicities [52-54]. It is clear that early life experiences, particularly education, have a great impact on cognitive test scores in later life. Therefore, it is important to consider education and SES when assessing ethnic disparities in AD.

Comorbidities

Conditions like stroke, diabetes, hypertension, congestive heart failure, high fat intake, high cholesterol, smoking, alcohol misuse, atrial fibrillation, low folate and obesity were found to be associated with cognitive decline and dementia [42]. Other studies have confirmed the relationship between hypertension, heart disease, diabetes, stroke and AD [55-62]. Our study confirmed that hypertension is a risk factor for early onset AD [63]. Similar to SES, people with multiple conditions/ diseases, the risk of developing AD and other dementias is increased [64]. The Health and Retirement Study [39] found that in those people with cognitive impairment, the proportion of individuals with high blood pressure was higher than those without cognitive impairment for African Americans, Hispanics and non-Hispanic whites. The same relationship was observed for diabetes, stroke and heart disease. Nevertheless, high blood pressure and stroke were more common among African Americans, but diabetes was more common in both African Americans and Hispanics compared with whites. Hispanics seem to have less heart disease compared with the other two ethnicities.

Purnell et al. reviewed published works that examined the effect of cardiovascular risk factors on the risk of AD [65]. Although most studies reported a significant association between hypertension and AD, the definition of hypertension is not consistent nor follows generally accepted clinical criteria. If strict criteria were adopted, then future studies might shed light on this association.

The relationship between diabetes and AD risk was similar to hypertension; most studies did not observe a significant relationship and for those that did, diabetes often interacted with other factors, particularly hypertension [65]. A recent study reviewed cohort studies and pooled results to examine the relationship between diabetes and AD. They found that diabetes was associated with a higher risk of AD; however, this association was observed in both Western and Eastern populations-Eastern populations having higher risk [66]. This study did not examine other ethnic differences.

Cholesterol levels have also been investigated; however, the findings are less conclusive most studies have shown that high cholesterol levels increase the risk of AD; some did not. One study found the risk was reduced for people with a high level of cholesterol [67]. There is some evidence to suggest that high cholesterol levels may have variable impacts in different ethnic groups [68].

Again, most studies did not reveal a significant association between stroke and AD. A significant association was only observed when stroke interacted with other diseases variables [58].

Based on these findings, it seems that the relationship between the above comorbidities and the risk of AD are not conclusive. Nevertheless, it is believed that these conditions account for some of the ethnic differences in the prevalence of AD and dementia. For example, one study found that reducing ethnic and racial disparities in the incidence of Type 2 diabetes could reduce the incidence of cognitive impairment and dementia by 17% [69]. Studies in the 1990s found that the high prevalence of cerebrovascular diseases amongst African Americans and Hispanics probably explains the higher incidence of vascular cognitive impairment [70-72].

Conclusion

It is clear that most observational studies identified a greater risk of AD in African Americans and Hispanics compared with the non- Hispanic white population. The possible explanations are that these minorities have a high prevalence of at-risk genes that lead to dementia, low socioeconomic status associated with low or poor-quality education, and high prevalence of comorbidities-hypertension, diabetes, stroke and heart disease. Studies generally support one of these factors as operational for ethnic differences in dementia and most studies agree on the different prevalence of APOE ε4 among ethnic groups. Nevertheless, we posit that none of these factors alone can explain the excess risk of AD among African Americans and Hispanics compared to whites. To further elucidate these ethnic disparities requires prospective analyses of large cohorts of African Americans and Hispanics to tease out the effects of each risk factor and its mechanism of action in each ethnic group. Such a study will increase our understanding of the pathomechanisms of AD in general.

References

1. World Health Organisation (2015) World Alzheimer Report 2015: The global impact of dementia an analysis of prevalence incidence cost and trends. WHO, London.
2. Vincent GK, Velkoff VA (2010) The next four decades: The older population in the United States: 2010 to 2050. US Department of Commerce Economics and Statistics Administration, US Census Bureau.
3. Alzheimer's Association (2010) 2010 Alzheimer's disease facts and figures. Alzheimers Dement 6: 158-194.
4. Gurland BJ, Wilder DE, Lantigua R, Stern Y, Chen J, et al. (1999) Rates of dementia in three ethnoracial groups. Int J Geriatr Psychiatry 14: 481-493.
5. Tang MX, Cross P, Andrews H, Jacobs DM, Small S, et al. (2001) Incidence of AD in African-Americans, Caribbean Hispanics and Caucasians in northern Manhattan. Neurology 56: 49-56.
6. Mayeda ER, Glymour MM, Quesenberry CP, Whitmer RA (2016) Inequalities in dementia incidence between six racial and ethnic groups over 14 years. Alzheimers Dement 12: 216-224.
7. Manly JJ, Mayeux R (2004) Ethnic differences in dementia and Alzheimer’s disease: Critical perspectives on racial and ethnic differentials in health in late life. National Academies Press, Washington DC.
8. Harvey R, Skelton-Robinson M, Rossor M (2003) The prevalence and causes of dementia in people under the age of 65 years. J Neurol Neurosurg Psychiatry 74: 1206-1209.
9. Australian Institute of Health & Welfare (2012) Dementia in Australia. AIHW, Canberra.
10. Withall A, Draper B (2010) Alcohol-related dementia: A common diagnosis in younger persons. Alzheimers Dement 6: S188-S189.
11. Lambert M, Bickel H, Prince M, Fratiglioni L, Von Strauss E, et al. (2014) Estimating the burden of early onset dementia: Systematic review of disease prevalence. Eur J Neurol 21: 563-569.
12. Bakker C, de Vugt ME, Vliet D, Verhey F, Pijnenburg YA, et al. (2014) Unmet needs and health-related quality of life in young-onset dementia. Am J Geriatr Psychiatry 22: 1121-1130.
13. Deloitte Access Economics (2011) Dementia across Australia: 2011-2050. Deloitte Access Economics, Canberra.
14. Chen HY, Panegyres PK (2016) The role of ethnicity in Alzheimer's disease: Findings from the C-PATH online data repository. J Alzheimers Dis 51: 515-523.
15. Bentham P, La Fontaine J (2008) Services for younger people with dementia. Psychiatry 7: 84-87.
16. Ayalon L, Areán PA (2004) Knowledge of Alzheimer's disease in four ethnic groups of older adults. Int J Geriatr Psychiatry 19: 51-57.
17. Eiser AR, Ellis G (2007) View-point: Cultural competence and the African American experience with health care: The case for specific content in cross-cultural education. Acad Med 82: 176-183.
18. Gelman CR (2010) Learning from recruitment challenges: Barriers to diagnosis, treatment and research participation for Latinos with symptoms of Alzheimer's disease. J Gerontol Soc Work 53: 94-113.
19. Gray HL, Jimenez DE, Cucciare MA, Tong HQ, Gallagher-Thompson D (2009) Ethnic differences in beliefs regarding Alzheimer disease among dementia family caregivers. Am J Geriatr Psychiatry 17: 925-933.
20. Jett KF (2006) Mind-loss in the African American community: Dementia as a normal part of aging. J Aging Stud 20: 1-10.
21. Crean S, Ward A, Mercaldi CJ, Collins J, MCook, et al. (2011) Apolipoprotein E ε4 prevalence in Alzheimer’s disease patients varies across global populations: A systematic literature review and meta-analysis. Dement Geriatr Cogn Disord 31: 20-30.
22. Farrer LA, Cupples LA, Haines JL, Hyman B, Kukull WA, et al. (1997) Effects of age sex and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease: A meta-analysis. JAMA 278: 1349-1356.
23. Liu CC, Kanekiyo T, Xu H, Bu G (2013) Apolipoprotein E and Alzheimer disease: Risk mechanisms and therapy. Nat Rev Neurol 9: 106-118.
24. Saunders AM, Strittmatter WJ, Schmechel D, George-Hyslop PH, Pericak-Vance MA, et al. (1993) Association of apolipoprotein E allele epsilon 4 with late-onset familial and sporadic Alzheimer's disease. Neurology 43: 1467-1472.
25. Tang MX, Stern Y, Marder K, Bell K, Gurland B, et al. (1998) The APOE-epsilon 4 allele and the risk of Alzheimer disease among African Americans whites and Hispanics. JAMA 279: 751-755.
26. Sahota A, Yang M, Gao S, Hui SL, Baiyewu O, et al. (1997) Apolipoprotein E-associated risk for Alzheimer's disease in the African-American population is genotype dependent. Ann Neurol 42: 659-661.
27. Teruel BM, Rodríguez JJL, McKeigue P, Fuentes E, Hernandez MA, et al. (2011) Interactions between genetic admixture ethnic identity APOE genotype and dementia prevalence in an admixed Cuban sample: A cross-sectional population survey and nested case-control study. BMC Med Genet 12: 43.
28. Duara R, Barker W, Lopez-Alberola R, Loewenstein D, Grau L, et al. (1996) Alzheimer's disease: Interaction of apolipoprotein E genotype family history of dementia gender education ethnicity and age of onset. Neurology 46: 1575-1579.
29. Green RC, Cupples LA, Go R, Benke KS, Edeki T, et al. (2002) Risk of dementia among white and African American relatives of patients with Alzheimer disease. JAMA 287: 329-336.
30. Prince M, Cullen M, Mann A (1994) Risk factors for Alzheimer's disease and dementia: A case-control study based on the MRC elderly hypertension trial. Neurology 44: 97-104.
31. Huang W, Qiu C, von Strauss E, Winblad B, Fratiglioni L (2004) APOE genotype, family history of dementia, and Alzheimer disease risk: A 6 year follow-up study. Arch Neurol 61: 1930-1934.
32. Scarabino D, Gambina G, Broggio E, Pelliccia F, Corbo RM (2016) Influence of family history of dementia in the development and progression of late-onset Alzheimer's disease. Am J Med Genet B Neuropsychiatr Genet 171: 250-256.
33. Devi G, Ottman R, Tang MX, Marder K, Stern Y, et al. (2000) Familial aggregation of Alzheimer disease among whites African Americans and Caribbean Hispanics in northern Manhattan. Arch Neurol 57: 72-77.
34. Rossor MN, Fox NC, Mummery CJ, Schott JM, Warren JD (2010) The diagnosis of young-onset dementia. Lancet Neurol 9: 793-806.
35. Van Broeckhoven C, Backhovens H, Cruts M, De Winter G, Bruyland M, et al. (1992) Mapping of a gene predisposing to early-onset Alzheimer's disease to chromosome 14q24.3. Nat Genet 2: 335-339.
36. Sherrington R, Froelich S, Sorbi S, Campion D, Chi H, et al. (1996) Alzheimer's disease associated with mutations in presenilin 2 is rare and variably penetrant. Hum Mol Genet 5: 985-988.
37. Callahan CM, Hall KS, Hui SL, Musick BS, Unverzagt FW, et al. (1996) Relationship of age, education and occupation with dementia among a community-based sample of African Americans. Arch Neurol 53: 134-140.
38. Costello EJ, Keeler GP, Angold A (2001) Poverty, race/ethnicity and psychiatric disorder: A study of rural children. Am J Public Health 91: 1494-1498.
39. Langa K, Kabeto M, Weir D (2010) Report on race and cognitive impairment using HRS. In Alzheimer's Association, 2010 Alzheimer's disease facts and figures. Alzheimers Dement 6: 158-194.
40. Mehta KM, Simonsick EM, Rooks R, Newman AB, Pope SK, et al. (2004) Black and white differences in cognitive function test scores: What explains the difference? J Am Geriatr Soc 52: 2120-2127.
41. Rodgers W (2008) Understanding the Black and White earnings gap: Why do African Americans continue to earn less despite dramatic gains in education? The American Prospect.
42. Glymour MM, Manly JJ (2008) Life course social conditions and racial and ethnic patterns of cognitive aging. Neuropsychol Rev 18: 223-254.
43. Hall KS, Gao S, Unverzagt FW, Hendrie HC (2000) Low education and childhood rural residence: Risk for Alzheimer's disease in African Americans.  Neurology 54: 95-99.
44. Gross AL, Mungas DM, Crane PK, Gibbons LE, MacKay-Brandt A, et al. (2015) Effects of education and race on cognitive decline: An integrative study of generalizability versus study-specific results. Psychol Aging 30: 863-880.
45. Albert MS (1995) How does education affect cognitive function? Ann Epidemiol 5: 76-78.
46. Jones RN (2003) Racial bias in the assessment of cognitive functioning of older adults. Aging Ment Health 7: 83-102.
47. Sloan FA, Wang J (2005) Disparities among older adults in measures of cognitive function by race or ethnicity. J Gerontol B Psychol Sci Soc Sci 60: 242-250.
48. Zsembik BA, Peek MK (2001) Race differences in cognitive functioning among older adults. J Gerontol B Psychol Sci Soc Sci 56: S266-S274.
49. Braveman PA, Cubbin C, Egerter S, Chideya S, Marchi KS, et al. (2005) Socio-economic status in health research: One size does not fit all. JAMA 294: 2879-2888.
50. Williams DR (1999) Race, socioeconomic status, and health. The added effects of racism and discrimination. Ann N Y Acad Sci 896: 173-188.
51. Williams DR, Mohammed SA, Leavell J, Collins C (2010) Race, socioeconomic status and health: Complexities, on-going challenges, and research opportunities. Ann N Y Acad Sci 1186: 69-101.
52. Crowe M, Clay OJ, Martin RC, Howard VJ, Wadley VG, et al. (2013) Indicators of childhood quality of education in relation to cognitive function in older adulthood. J Gerontol A Biol Sci Med Sci 68: 198-204.
53. Early DR, Widaman KF, Harvey D, Beckett L, Park LQ, et al. (2013) Demographic predictors of cognitive change in ethnically diverse older persons. Psychol Aging 28: 633-645.
54. Sisco S, Gross AL, Shih RA, Sachs BC, Glymour MM, et al. (2015) The role of early-life educational quality and literacy in explaining racial disparities in cognition in late life. J Gerontol B Psychol Sci Soc Sci 70: 557-567.
55. Arvanitakis Z, Wilson RS, Bienias JL, Evans DA, Bennett DA (2004) Diabetes mellitus and risk of Alzheimer disease and decline in cognitive function. Arch Neurol 61: 661-666.
56. Breteler MM (2000) Vascular risk factors for Alzheimer's disease: An epidemiologic perspective. Neurobiol Aging 21: 153-160.
57. Craft S (2009) The role of metabolic disorders in Alzheimer disease and vascular dementia: Two roads converged. Arch Neurol 66: 300-305.
58. Honig LS, Tang MX, Albert S, Costa R, Luchsinger J, et al. (2003) Stroke and the risk of Alzheimer disease. Arch Neurol 60: 1707-1712.
59. Kennelly SP, Lawlor BA, Kenny RA (2009) Review: Blood pressure and dementia-A comprehensive review. Ther Adv Neurol Disord 2: 241-260.
60. Kuller L, Lopez O, Jagust W, Becker J, DeKosky S, et al. (2005) Determinants of vascular dementia in the cardiovascular health cognition study. Neurology 64: 1548-1552.
61. Newman AB, Fitzpatrick AL, Lopez O, Jackson S, Lyketsos C, et al. (2005) Dementia and Alzheimer's disease incidence in relationship to cardiovascular disease in the Cardiovascular Health Study cohort. J Am Geriatr Soc 53: 1101-1107.
62. Patterson C, Feightner J, Garcia A, MacKnight C (2007) General risk factors for dementia: A systematic evidence review. Alzheimers Dement 3: 341-347.
63. Atkins ER, Bulsara MK, Panegyres PK (2012) Cerebrovascular risk factors in early-onset dementia. J Neurol Neurosurg Psychiatry 83: 666-667.
64. Luchsinger J, Reitz C, Honig LS, Tang MX, Shea S, et al. (2005) Aggregation of vascular risk factors and risk of incident Alzheimer disease. Neurology 65: 545-551.
65. Purnell C, Gao S, Callahan CM, Hendrie HC (2009) Cardiovascular risk factors and incident Alzheimer disease: A systematic review of the literature. Alzheimer Dis Assoc Disord 23: 1-10.
66. Zhang J, Chen C, Hua S, Liao H, Wang M, et al. (2017) An updated meta-analysis of cohort studies: Diabetes and risk of Alzheimer's disease. Diabetes Res Clin Pract 124: 41-47.
67. Anstey KJ, Lipnicki DM, Low LF (2008) Cholesterol as a risk factor for dementia and cognitive decline: A systematic review of prospective studies with meta-analysis. Am J Geriatr Psychiatry 16: 343-354.
68. Hall K, Murrell J, Ogunniyi A, Deeg M, Baiyewu O, et al. (2006) Cholesterol, APOE genotype and Alzheimer disease: An epidemiologic study of Nigerian Yoruba. Neurology 66: 223-227.
69. Noble JM, Manly JJ, Schupf N, Tang MX, Luchsinger JA (2012) Type 2 diabetes and ethnic disparities in cognitive impairment. Ethn Dis 22: 38-44.
70. Moroney J, Bagiella E, Hachinski V, Mölsá C, Gustafson P, et al. (1997) Misclassification of dementia subtype using the Hachinski Ischemic score: Results of a meta-analysis of patients with pathologically verified dementias. Ann N Y Acad Sci 826: 490-492.
71. Sacco RL, Boden-Albala B, Gan R, Chen X, Kargman D, et al. (1998) Stroke incidence among white black and Hispanic residents of an urban community the Northern Manhattan Stroke Study. Am J Epidemiol 147: 259-268.
72. Snowdon DA, Greiner LH, Mortimer JA, Riley KP, Greiner PA, et al. (1997) Brain infarction and the clinical expression of Alzheimer disease: The nun study. JAMA 277: 813-817.