Eric B. Larson, MD, MPH; Li Wang, MS; James D. Bowen, MD; Wayne C. McCormick, MD, MPH; Linda Teri, PhD; Paul Crane, MD, MPH; Walter Kukull, PhD
Some studies suggest that people with high levels of physical activity are less likely to develop dementia.
All 1740 participants in this cohort study were 65 years of age or older and were cognitively intact at baseline. Over 6.2 years, the rate of dementia was 13.0 per 1000 person-years in those who exercised 3 or more times per week and 19.7 per 1000 person-years in those who exercised less than 3 times per week.
The only measure of exercise intensity was self-reported frequency. The cohort was largely white and well-educated.
This study adds to the evidence that regular exercise is associated with a lower risk for dementia. However, the existing evidence does not prove that regular exercise is associated with a lower dementia risk.
Persons who exercised 3 or more times per week were more likely to be dementia-free than those who exercised fewer than 3 times per week.
Persons who exercised 3 or more times per week had a higher probability of being dementia-free than those who exercised fewer than 3 times per week if their PPF score was less than 13. This relative risk reduction of dementia by exercise was greater among persons with lower PPF scores than among those with higher PPF scores.
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Video News Release - Regular Exercise Reduces Risk of Dementia and Alzheimer's Disease by 30%-40%
Hiroshima University Hospital
January 26, 2006
Exercise, dentition status, and risk for incident dementia in the elderly
Larson and colleagues recently reported that the exercise measured by self-reported frequency at baseline was associated with a delay in onset of dementia and Alzheimer disease during follow-up period in 1740 persons older than age 65 years without cognitive impairment1. In their study, several biological benefits such as increased oxygen delivery, improved circulation, induced fibroblast growth in the hippocampus, and reduced cell loss in sensitive area like the hippocampus, were considered to link regular excise at baseline with a delay in subsequent occurrence of dementia and Alzheimer disease.
In the viewpoint of oral health, Shimazaki and colleagues demonstrated in a six-year prospective cohort study that worse dentition status at baseline led to significantly worse physical and mental impairment, and higher mortality2. Onozuka and colleagues found a possible link between reduced mastication (worse dentition) and hippocampal neuron loss that may be one risk factor for senile impairment of spatial memory in aged SAMP8 mice3. Maintenance of normal chewing might prevent the brain from degenerating4. Since increased physical activity may reduce prevalence of periodontal disease that is a major risk factor for worse dentition5, it is likely that regular exercise at baseline may contribute to the maintenance of better dentition at baseline and during follow-up period in the study of Larson and colleagues, resulting in a delay in onset of dementia and Alzheimer disease. If dentition status is one of potential pathways linked regular exercise with a delay in onset of dementia and Alzheimer disease in their study, the improvement of dentition status might be a new strategy to prevent dementia and Alzheimer disease in the elderly. It would be beneficial for the dentists as well as the elderly if Larson and colleagues can clarify this hypothesis in the database of their cohort study.
1. Larson EB, Wang L, Bowen JD, McCormick WC, Teri L, et al. Exercise is associated with reduced risk for incident dementia among persons 65 years of age and older. Ann Intern Med. 2006;144:73-81.
2. Shimazaki Y, Soh I, Saito T, Yamashita Y, Koga T, Miyazaki H, et al. Influence of dentition status on physical disability, mental impairment, and mortality in institutionalized elderly people. J Dent Res. 2001;80:340- 5.
3. Onozuka M, Watanabe K, Mirbod SM, Ozono S, Nishiyama K, Karasawa N, et al. Reduced mastication stimulates impairment of spatial memory and degeneration of hippocampal neurons in aged SAMP8 mice. Brain Res. 1999;826:148-53.
4. Miyamoto I, Yoshida K, Tsuboi Y, Iizuka T. Rehabilitation with dental prosthesis can increase cerebral regional blood volume. Clin Oral Implants Res. 2005;16:723- 7.
5. Al-Zahrani MS, Borawski EA, Bissada NF. Increased physical activity reduces prevalence of periodontitis. J Dent. 2005;33:703-10.
University of Michigan at Ann Arbor
March 6, 2006
The Use of Measures of Impact in Clinical Trials
TO THE EDITOR:
While many studies examine causality through measures of association such as relative risks or hazard ratios, measures of impact are often ignored. The recent article by Dr. Larson and colleagues(1) is used as a case in point. Although physicians tout the benefits of exercise in preventing or delaying the onset of several chronic illnesses(2), prospective trials examining this relationship are rare. In this regard, Dr. Larson has made a significant contribution with his article. However, he failed to report his important findings in terms of the excess risk due to infrequent exercise. This measure of impact is a more relevant epidemiologic measure to relay to our patient population than incidence or hazard rates alone.
For purposes of simplifying his study terminology in the following calculations, we shall classify those patients aged 65 years and older who exercise less than 3 times per week as non-excercisers, and those who exercise three or more times per week as exercisers. Using this terminology, the incidence rate of dementia in the exercisers is 13 per 1000 person-years, and in the non-exercisers is 19.7 per 1000 person- years. The excess risk is therefore 6.7, i.e., around 7 new cases of dementia per 1000 person-years among non-exercisers are directly attributable to lack of exercise. Using the above, we can calculate that 34% of new dementia cases per 1000 person-years among non-exercisers are due to lack of exercise.
Although Dr. Larson did not mention the overall incidence of dementia in his population, we have calculated it from the data provided: given 158 new cases of dementia, 1740 patients at risk originally, and 397 censored observations (121 withdrew, 276 died), the population incidence rate over 6.2 years is 16.5 per 1000 person-years. The population excess risk is therefore 4, i.e., if we assume that 1000 new cases of dementia will be diagnosed in the population this year, 4 cases will be directly due to lack of exercise. This means that 21% of new cases of dementia in this population are due to lack of exercise (population attributable risk percent).
Studies like these are pivotal in identifying modifiable risk factors towards the development of chronic illnesses. The measures of impact shown above may help in explaining the findings to our patient population; such calculations should be included in all published clinical trials.
1. Larson EB, Wang L, Bowen JD, McCormick WC, Teri L, Crane P, Kukull W. Exercise is associated with reduced risk for incident dementia among persons 65 years of age and older. Ann Intern Med. 2006;144:73-81
2. Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, DC: National Academy Pr, 2002: 880- 932. Accessed at http://www.nap.edu/openbook/0309085373/html/
Larson EB, Wang L, Bowen JD, McCormick WC, Teri L, Crane P, et al. Exercise Is Associated with Reduced Risk for Incident Dementia among Persons 65 Years of Age and Older. Ann Intern Med. 2006;144:73-81. doi: 10.7326/0003-4819-144-2-200601170-00004
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Published: Ann Intern Med. 2006;144(2):73-81.
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