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Relative Validity of 16-Year Recall of Calcium Intake by a Dairy Questionnaire in Young Dutch Adults^1,2,

EMGO Institute, Faculty of Medicine, Vrije Universiteit, Amsterdam, The Netherlands ^* Department of Human Nutrition, Agricultural University, Wageningen, The Netherlands

Assessing past calcium intake in studies of osteoporosis is of major importance because calcium intake during young adulthood is thought to influence bone mass in later life. However, uncertainty exists about the accuracy of retrospectively reported calcium intake. We therefore evaluated whether a dairy questionnaire estimating calcium intake only from dairy products is a valid method to assess the retrospectively reported calcium intake of 29-year old Dutch males and females 8 and 16 y previously. This was examined in 166 subjects by comparing their original calcium intake reported in 1977 and 1985 by a cross-check dietary history method, with their 1977 and 1985 calcium intake values recalled in 1993 by the dairy questionnaire. The overestimation of the retrospectively reported calcium intake compared with the original intake (11–33%) suggests that the dairy questionnaire is a relatively poor tool to assess the absolute calcium intake of young adults 8 and 16 y ago. Based on the agreement statistics, the dairy questionnaire is a reasonably valid instrument to rank subjects according to their retrospective calcium intake. However, the reporting of the retrospective calcium intake appears to have been influenced by the current calcium intakes reported at the age of 29 y. Nevertheless, the retrospective calcium intake appears to be a better indicator of the original intake than the current intake because the subjects markedly increased their calcium intake over the 16-y period.

KEY WORDS: • calcium intake • dairy questionnaire • relative validity • retrospective study • humans

¹ Supported by grants from Stichting Zuivel, Voeding en Gezondheid (Dairy Foundation on Nutrition and Health), Het Praeventiefonds (Dutch Prevention Fund; project nos. 28-189a, 28-1106, 28-1106-1), De Nederlandse Hartstichting (Dutch Heart Foundation, project nos. 76051-79051, 90-312), and Het Ministerie van Volksgezondheid, Welzijn en Sport (Dutch Ministry of Health, Well Being and Sport, no. 90-170).

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence and reprint requests should be addressed.

Manuscript received 11 January 1996. Revision accepted 2 August 1996.