KATZMARZYK, P. T., L. PÉRUSSE, D. C. RAO and C. BOUCHARD. Familial risk ratios for high and low physical fitness levels in the Canadian population. Med. Sci. Sports Exerc., Vol. 32, No. 3, pp. 614–619, 2000.
Purpose: To estimate the familial risk of being physically fit or unfit in Canada.
Methods: The sample consisted of 11,680 participants from 4144 nuclear families of the 1981 Canada Fitness Survey. Indicators of physical fitness included estimated physical working capacity at a heart rate of 150 beats·min−1 derived from a step test (PWC150), hand grip strength, sit-ups, and trunk flexibility. Probands were defined as reference individuals who were physically fit (95th, 85th, and 75th percentiles) or physically unfit (25th, 15th, and 5th percentiles) for each fitness indicator, respectively.
Results: Standardized risk ratios, adjusted for age and sex (SRR), for spouses and first-degree relatives of probands exceeding the 95th percentile are 1.63 and 1.81 for PWC150, 2.38 and 3.16 for grip strength, 2.63 and 3.98 for sit-ups, and 2.59 and 3.56 for trunk flexibility, respectively, whereas the SRR for spouses and first-degree relatives of probands below the 5th percentile are 1.54 and 1.34 for PWC150, 1.83 and 1.85 for grip strength, 1.13 and 1.53 for sit-ups, and 1.42 and 1.84 for trunk flexibility, respectively. The familial risks tend to be greatest at the extremes (95th and 5th percentiles) and the risks for first-degree relatives of physically fit probands are generally greater than those for spouses for grip strength, sit-ups and trunk flexibility, whereas those for PWC150 show no clear pattern.
Conclusion: There is significant familial risk for being physically fit or unfit in the Canadian population. The pattern of SRR suggests that the familial risk for indicators of strength and flexibility may be due, in part, to genetic factors, whereas the shared family environment is largely contributing to the familial risk for PWC150.
Thereare several components to physical fitness, each of which can be measured with a variety of indicators (6). Each fitness indicator in turn is influenced to varying degrees by genes, the environment, and their interactions. Given that the genetic contribution to physical fitness varies from trait to trait and from population to population, the study of the inheritance of physical fitness is a complex and ambitious endeavor (5).
There is considerable evidence that indicators of physical fitness aggregate within families. Family studies have demonstrated significant familial resemblance for indicators of aerobic fitness (2,4,17,18,23–25), muscular strength and endurance (20,21,23–25,29), and flexibility (7,15,23). Estimates of generalized heritability vary among studies, and the respective roles of genes and the shared family environment and lifestyle on physical fitness have yet to be determined.
Heritability (h2) is a population parameter that provides the proportion of the observed variance in a trait that can be attributed to genetic factors. Sometimes genetic effects and familial environmental effects are combined into generalized heritability. However, population statistics such as h2 are difficult to interpret at the level of an individual. A related concept to heritability is that of familial risk. Familial risk estimates provide an indication of the risk to an individual of having a given characteristic (obesity, diabetes, being unfit) based on their relationship to a proband, or affected individual. For discrete traits, familial risk can be estimated by the lambda coefficient, λR=P(A‖R)]/[P(A)], where P(A) is the general population prevalence of a trait, and P(A‖R) is the prevalence among relatives of degree R of a proband who is affected (27). In addition, generalized relative risk ratios have been developed for quantitative traits (11). These are defined as λR(h,l)=[PR(l‖h)/P(l)], where P(l) is the probability that a randomly selected person in the general population has a trait value in the lth segment of the trait distribution, and PR(l‖h) is the probability that a person has a trait value in the lth segment given that a relative of type R has a trait value in the hth segment.
The concept of familial risk has been applied to the study of a variety of quantitative traits, including obesity in several populations (1,13,16,31); however, the familial risk for phenotypes related to physical fitness has yet to be determined. Knowledge of λR values for a given trait is not only useful for determining the risk profile at an individual level, but also for providing justification for complex and expensive molecular genetic studies. For example, Risch (27) has suggested that for traits with risks for first-degree relatives greater than five-fold, there exists the possibility of successfully testing for major gene effects. It may also be possible to maximize the efficiency of linkage studies by recruiting pairs of relatives with high λR values in addition to recruiting participants with extreme values of the trait (11,12,16,27) for such studies. Thus, the purpose of this study was to estimate the familial risks associated with being physically fit or unfit in the general Canadian population. To this end, data from the Canada Fitness Survey of 1981, a national representative sample of the Canadian population, were analyzed.