This cohort of young adults provides an unparalleled opportunity to investigate associations of obesity and adverse health outcomes including cancer. Its large population of young adults is relatively representative for the underlying population in California. The availability of clinically assessed height and weight data for nearly 2,000,000 California members of the KP health system provides an outstanding basis for investigating the effects of weight class on health outcomes. In addition, there is substantial racial and ethnic diversity in this cohort, with a large proportion of Hispanics – about one-third of the cohort – and Asians – about 10%. The ability to link to various clinical and administrative databases including high-quality tumor registries, prescription medications, laboratory results, and diagnoses and procedures related to outpatient encounters or hospitalizations facilitates investigation of numerous outcomes of interest.
Health consequences of obesity may vary markedly by sex, race/ethnicity, and socioeconomic status. Whether variations in obesity are a principal reason for disparities in disease occurrence in young adults is unclear. Variations in disease occurrence in young adults have also been attributed to the lack of health insurance in this particular age group, with 40% lacking health insurance
[5, 6]. Another problem may be underinsurance, which includes electing coverage in plans with relatively low premiums but substantial copayments and high deductibles. A lack of health insurance or underinsurance may result in underutilization of health care services, and consequently, delayed or underdiagnosis of disease. This may bias results from epidemiological studies that do not take insurance status into account and lead to an underestimation of obesity-related risks. Current knowledge gaps of the health risks associated with obesity in young adults include: 1) lack of robust risk estimates for young adults, because most studies investigate adults of all ages without further stratification; 2) limited information on disparities and risk in subpopulations; and 3) lack of information on health outcomes with lower prevalence.
For this ongoing cohort study of young adults, the understanding of potential bias introduced by migration is crucial. The membership in KP, and therefore in the cohort, is dynamic, with individuals continuously joining and leaving the health plan. Retention rates in young adults are generally lower than in other age groups, likely due to the high rate of change in employment and family status in this age group. The current three-year retention rates for young adults who entered the cohort in 2007 is 68.4%, ranging from 58.1% in the youngest adults between 20.0 and 24.9 years of age to 75.6% in young adults between 25.0 and 29.9 years of age. These data suggest the feasibility of our study for medium-term health outcomes with an adequate length of follow-up based on sufficient membership retention rates. Future retention rates cannot be predicted accurately because of ongoing changes in health policy such as the Affordable Care Act; the interpretability of our results is limited to medium-term retention. However, we speculate that the expansion of health care to a larger population as planned in the Affordable Care Act would increase retention rates in this age group known for high rates of uninsured individuals
[5, 6]. The estimated membership retention is expected to increase slightly by combining KPSC and KPNC as members relocating from one health plan to another will continue to be available for follow-up.
Individuals who were retained in the health plan and those lost to follow-up were remarkably similar with regard to neighborhood income and education. However, those participants who were lost to follow-up were more likely to be younger, male, and non-Hispanic White. They were also less likely to be of unknown race/ethnicity. Differences in race and ethnicity between these two groups can be partially attributed to the introduction of a mandatory assessment of race/ethnicity during outpatient visits for every patient in 2009. Therefore, individuals who left the health plan before 2009 were more likely to be of unknown race/ethnicity. Overall, our data do not suggest major bias by attrition regarding sociodemographic factors. However, factors not considered in this analysis may exist that could introduce bias such as systematic differences between individuals retained and lost to follow-up in health risk factors related to the outcomes of interest that need future consideration.
Although cancer in young adults is one of the leading causes of disease-related deaths in this age group, little attention has been given to risk factors for cancers in young adults 20 to 39 years of age. Obesity
[8–10], and metabolic syndrome
[11–13] have all been linked to an increased risk for several cancers, based mostly on cancer incidence in adulthood
[9, 13–15]. Although young adults in the US have a high prevalence of obesity
[17, 18], and metabolic syndrome
, few epidemiologic studies have focused on these chronic conditions and subsequent cancer risk in this age group. This is particularly important given increasing evidence that a substantial proportion of cancers in young adults likely have a different underlying biology, etiology, and pathogenesis than in older individuals
[20, 21]. Thus, while studies have reported associations between obesity, diabetes, metabolic syndrome, and many common cancers in older adults, such associations in young adults have yet to be established.
At cohort entry, all members by definition have had health insurance coverage, and this may be perceived as limiting the generalizability of the findings. Indeed, having health insurance is associated with potentially key covariates in disease risk, such as employment status or income. Despite this, there is a substantial variation in socioeconomic status, based on census-based estimates. In addition, uniform health coverage is a substantial advantage as it minimizes the risk of underdiagnosis or delayed diagnosis, which may in turn result in biased risk estimates. This is a potential limitation for cohort studies in which young adults are enrolled without regard to health insurance status, as 40% of young adults in the US do not carry health insurance.
Despite these outstanding strengths, there are some limitations. As a cohort based on electronic health records, the availability of data on covariates of interest will vary, as not all cohort members will take routine advantage of services such as screening for conditions of interest. Thus, there is the possibility that availability of screening data will be linked to disease status. This is mitigated to some extent by the large numbers of cohort members and the ability to define subsets of the cohort by the availability of data or frequency of encounters with the health care system. In addition, the implementation of clinical practice guidelines, such as systematic screening for cardiovascular disease risk factors for all members 20 years of age and older on the first clinical visit and every five years thereafter, will assure broad availability of relevant data without bias as to disease status. On the other hand, particularly for the occurrence of short-term events, the potential for confounding by indication or the prodromal effects of disease resulting in more frequent health encounters will need to be taken into account in the interpretation of findings.
As noted previously, there is disenrollment of individuals from the KP health plan, with overall about 68% of cohort members maintaining their KP health insurance after three years. Retention rates are lowest for the youngest adults and somewhat higher for those in their fourth decade of life. The loss to follow-up is mitigated somewhat for endpoints such as death or cancer due to the possibility of linkage to state and SEER tumor registries or state and national vital statistics registries. The attrition rate does indicate that the ongoing update of exposure or comorbid information may be limited. However, we will be able to explore differences in those who are retained in the cohort as members of KP and those who have disenrolled to determine if there may be systematic biases in important factors associated with exposures of interest, such as body size, or outcomes of interest, such as diabetes, cardiovascular disease, and cancer rates.
Nationwide 6% of young adults are extremely obese; yet we know little to adequately quantify the health burden that can be attributed to obesity, especially extreme obesity, and which population groups are most susceptible to early health consequences. This cohort of young adults and their electronic medical record data provide an unparalleled opportunity to investigate associations of obesity-related factors and risk of cancer and other diseases in a large multiethnic population. These data sources are unusually rich and support the development of nuanced longitudinal care quality indices for preventive and disease management services, such as the Prevention Indices and Disease Management Indices
[22, 23]. Unlike claims-based quality standards, these indices draw on the full range of clinical and administrative data to define both the population and the delivery of the service over time.
In addition to the aforementioned limitations, the cohort currently has a relatively short follow-up that enables us to draw conclusions on short- and medium term outcomes. Analyses have to be designed carefully to account for systematic differences beyond demographic factors such as body weight and obesity-related conditions to investigate long-term health risks such as cancer risk. Linkage to other databases can help decrease the attrition in this cohort, including internal linkage between Kaiser Permanente regions (restricted to those who stay with Kaiser Permanente), linkage with state death files (restricted to death cases), and linkage with state cancer registries (restricted to those who remain in the state and limited to cancer diagnosis).
Planned future research on the young adult cohort will develop these quality measures in order to identify person-level characteristics associated with both variations in services related to cardiovascular disease, diabetes, and cancer in young obese adults and variations in the care they receive and its consequences for incident morbidity, mortality, and health care utilization. This planned research will also examine differences in the effectiveness of that care in demographically and medically defined subpopulations.