Factors of importance for self-related health

Abstract

Even though self-rated health is increasingly accepted as an important measure of health status, there are several uncertainties as to why people differ in their health perception. The extent of age differences across the life span and whether there are different determinants in men than in women is unclear.

The overall objective of this thesis was to increase understanding as to why individuals differ in their health perception through quantitative genetic and epidemiological approaches using sub-samples from the Swedish Twin Registry. By studying twins, it is possible to estimate the relative importance of genetic and environmental factors for self-rated health.

In Paper I we included both like- and opposite sexed twins in adult ages in order to evaluate crosssectional age group and sex differences in the relative importance of genetic and environmental factors for self-rated health. Individual specific environmental variance is the most important component in adults under 45 years whereas the increase in total variance in the older age groups (45-74) is primarily due to genetic influences. The individual specific environment becomes more important again in the oldest age group (>74). No significant sex differences were found in variance components. Similarly, the same genetic effects were of importance in men and women.

Paper II investigated decreases in means and increases in individual differences with age longitudinally. Results indicate that previously reported changes in self-rated health over the life span primarily reflect cohort differences rather than age changes. Stability between time points reflects both environmental and genetic factors.

Intact cognitive functioning is an important aspect of health with increasing age, therefore, Paper III focused on the associations between self-rated health and cognitive abilities in normal aging. There was only slight evidence that associations between self-rated health and cognitive test scores were mediated by chronic disease conditions. In the age group younger than 67 years, associations between self-rated health and spatial reasoning and perceptual speed were found, mediated by both genetic and environmental factors. In the older age group (> 67 years), associations between self-rated health and verbal ability, spatial reasoning, perceptual speed and visual memory were entirely due to genetic factors.

Paper IV investigated the importance of health behavior and risk factors for future self-ratings of health. We found that recurrent headache, back- and neck pain, lack of exercise, smoking, obesity, perceived stress, unemployment and personality were associated with poor self-rated health, some 25+ years later. Genetic and familial factors only slightly influenced the relationships between recurrent headache, exercise, obesity, and poor self-rated health.

In conclusion, both genetic and environmental factors are of importance for individual differences in self-rated health and the effect is equal for men and women. Genetic effects for self-rated health can probably be explained by genetic influences on disease status. Childhood socioeconomic status did not explain the finding of cohort differences in self-rated health. Societal changes not tapped by our measure more likely explain these differences. We found weak associations between self-rated health and cognitive abilities, indicating that cognition is not substantially influencing self-rated health in a normally aging population. Finally, health behavior and risk factors are of importance for self-rated health. Life-style changes such as reduced weight and more exercise might help prevent people from experiencing their health as poor in the future. This in turn might result in a decrease in morbidity and increase in survival.