Statistics in the News: Chapter 17 Multiple
Regression and Correlation
Assessing the Link Between Diet and Dementia in the Elderly
Medical researchers seek to learn all they can about the
causes of diseases and how these causes might respond to various
forms of intervention. One recent study sought to shed light
on the hypothesis that elevated plasma homocysteine levels
may be associated with poor cognition and various forms of
dementia among the elderly. Thus, a reduction in these levels
might prevent those undesirable results.
The study followed the lives of 1,092 subjects, aged 68-97,
who were initially free of dementia. After a median period
of 8 years, some 111 of these individuals were diagnosed with
dementia (83 of these with Alzheimer's disease). Consider
the researchers' thoughts:
In the past, high plasma homocysteine levels, which rise
in people whose diets are heavy with animal protein, have
been shown to damage blood vessels and nerves. They have also
been linked to strokes and heart attacks. Accordingly, the
researchers suspected that low plasma homocysteine levels
(which can be achieved by a diet favoring fruits and leafy
vegetables containing plenty of folic acid and vitamins B6
and B12), might be associated with less damage to blood vessels
and nerves and, thus, a lower incidence of dementia.
A multiple regression model related the appearance
of dementia to a number of possible factors, including age,
plasma levels of folate and vitamins B6 and B12, educational
status, history of stroke, smoking status, alcohol intake,
presence of diabetes, body mass, blood pressure, and more.
The results: High plasma homocysteine levels raised the risk
of any type of dementia by 40 percent. They raised the risk
of Alzheimer's disease by 80 percent. (95% confidence intervals
for the associated relative risk ratios of 1.4 and 1.8 reached
from 1.1 to 1.9 in the former case and from 1.3 to 2.5 in
the latter case.) Interestingly, people whose homocysteine
levels initially exceeded 14 micromoles per liter of blood,
one quarter of the group, were found to have nearly twice
the risk of getting Alzheimer's as others with much lower
levels. The researchers concluded that the risk of dementia
could probably be reduced substantially by adding large doses
of folate and vitamins B6 and B12 to the diets of most people.
Caution: It is not clear that high plasma homocysteine
levels that precede the onset of dementia also cause it. It
is also possible that a third factor is at work, simultaneously
causing high levels of plasma homocysteine and dementia.
Sources: Sources: Adapted from Joseph Loscalzo, "Homocysteine
and Dementias," The New England Journal of Medicine,
February 14, 2002, pp. 466-468; Sudha Seshadri et al., "Plasma
Homocysteine as a Risk Factor for Dementia and Alzheimer's
Disease," ibid., pp. 476-483; and Denise Grady, "Alzheimer's
May Be Linked to Normal Diet Byproduct," The New York Times,
February 14, 2002, p. A25.
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