Critical Analysis of a Published Research Article
Critical Analysis of a Published Research Article
Winkelmayer, W. C., Stampfer, M. J., Willett, W. C., & Curhan, G. C. (2005). Habitual caffeine intake and the risk of hypertension in women. JAMA, 294(18), 2330-5.
Title & Abstract Evaluation
“Habitual caffeine intake and the risk of hypertension in women” is a sufficiently specific title that indicates the nature of the research without revealing any results. The authors have appropriately avoided using jargon and acronyms that might be unknown to the audience. The title has not been formulated as a yes-no question. The primary variables (habitual caffeine intake and risk of hypertension) are mentioned in the title and the targeted population is identified as being represented by women. The title does not imply causality because the study merely examines the association between caffeine intake and incident hypertension in women. Overall, this is an effective and appropriate title.
The purpose of the study is clearly identified as examining the association between the two aforementioned variables. The abstract contains a “Methods” section that describes the design, settings and participants in the study. Titles of measures have been appropriately excluded from the abstract. The highlighted results are sufficiently detailed, but without providing an excessive amount of information. The authors suggest future research should elucidate the role of cola beverages in hypertension since the present study incidentally found a possible association between the two variables. These references to future research are specific and pertinent, and directly contingent upon the results of the study. Based on this analysis, the abstract of the study is overall effective and appropriate.
Introduction and Literature Review (this section has to be or almost 1 page)
The authors start by identifying a specific problem area: the lack of long-term studies on the effects of caffeine intake on the risk of developing hypertension in women. Several short-term studies and a recent longitudinal study in men are cited to support the above stated problem area. The research question therefore logically flows from the presented material. The “factual statements” are drawn from current scientific evidence which the authors appropriately cite in their introduction. The importance of the problem area is identified in the context of increased prevalence of hypertension in the U.S. population and the widespread consumption of beverages that contain caffeine. No conceptual definitions of key terms are provided in the introductory material. Overall, the introduction section is concise, but effective.
The study uses the two large cohorts of registered nurses of the Nurses’ Health Studies (NHS) I and II. The secondary nature of the data and the fact that the sample population is limited to registered nurses does not make it possible for the results to be generalized. The process used in selecting the participants for the NHSs is not specified, but absence of hypertension and participation in the follow-up were identified by the authors as the basis for selecting participants from the two cohorts. Therefore, from a total of 238,371 nurses originally enrolled in the NHSs a sample of 155,594 women free from physician-diagnosed hypertension were followed up over 12 years. The study mentions the follow-up for both cohorts exceeded 90%. The secondary nature of the data allowed for the selection of an adequate sample size. The demographics used to describe the sample population were age, sex, and occupation. Sampling bias is difficult to establish given the fact that the authors do not mention the criteria used to include nurses in the NHSs. The study was approved by the institutional review board at Brigham and Women’s Hospital, Boston, and informed consent was obtained from each participant at the time of enrollment in the NHSs.
The instrument employed by the study was the questionnaire used by the NHSs to measure dietary intake, physician diagnosed hypertension, age, height, family history of hypertension, oral contraceptive use, smoking status, physical activity, and frequency of analgesic drugs used. The response format for one of the most important questionnaire items (dietary intake) was described in detail as was the actual content of the question. Additionally, sources where additional information regarding the survey items can be obtained were appropriately cited. No information regarding the reliability and validity of the retrieved data was provided. Provided it is adequately constructed, a questionnaire seems to be an appropriate instrument in light of the research purpose.
The authors hypothesize the existence of an association between habitual consumption of caffeinated beverages and incident hypertension. The null hypothesis would be that no association exists between the two variables. The study design and methods, settings, and participants are described in sufficient detail so as to facilitate any further replication of the research. Data from the two NHSs were collected, but as previously mentioned, the validity and reliability of the employed instrument was not addressed. The independent variable was habitual caffeine intake, while the dependent variable was the development of hypertension. The statistical tests used to estimate relative risks and 95% confidence intervals were age-adjusted Cox proportional hazards regression models. Additionally, multivariate models were used to adjust for other known risk factors of the study outcome: age (continuous variable), body mass index (continuous variable), alcohol use (ordinal variable), physical activity (ordinal variable), smoking status (ordinal variable), family history of hypertension (dichotomous variable), current oral contraceptive use (dichotomous variable), and nutritional supplement intake (ordinal variable). The SAS software package was used to perform the statistical analyses. Five self-explanatory tables report the age-adjusted and multivariate relative risks for incident hypertension according to quintile of caffeine intake, frequency of coffee intake, frequency of caffeinated tea intake, frequency of sugared cola intake, and frequency of diet cola intake. The tables are well-structured and necessary, and the significance of their content is briefly described in the text. Trend P values for each of the exposures of interest were determined by using the median for each category. Level of significance for trend P values was <0.05. The statistical procedures employed successfully answered the research question. Evaluating Results The research purpose stated in the introductory section is readdressed when the authors present their results indicating that no linear association was found between caffeine consumption and incident hypertension. The results section is clear and cohesive, and the most significant of the statistical outcomes presented comprehensively in table format are also highlighted in the narrative of the section. Baseline descriptive characteristics of the cohort are presented before the results to inferential tests are revealed. Results that were only marginally significant were identified as such (e.g. the inverse association between coffee intake and the risk of hypertension in both cohorts). Overall, the results section of the study successfully describes the association (or lack thereof) between the different types of caffeinated beverages and the incidence of hypertension. Evaluating Discussion The section starts with a brief summary of the purpose and results of the study. The latter are discussed in light of the previously cited literature which studied the same phenomenon but was based primarily on short periods of observation. References not previously cited in the introductory section are added in the discussion section. While pertinent, they should have also been included in the introduction. The authors fully acknowledge the limitations of their study. The study found strong evidence that coffee consumption is not associated with an increased risk of hypertension in women. Given the high consumption of coffee among Americans and people's increased awareness of hypertension and its deleterious effects, the authors acknowledge the importance of their finings. The research also revealed a positive association between frequency of caffeinated soft drink consumption and the risk of hypertension. The authors speculate (and they appropriately acknowledge doing so) that it is not caffeine, but maybe some other substance contained in soft drinks that may be responsible for the increased risk of hypertension. Therefore, their suggestions for future research are very specific, stemming from their finding that caffeinated soft drinks are related to the risk of hypertension and encouraging researchers to tackle this problem.