This paper is about the important public health problem of understanding the distribution of episodically consumed dietary component intakes in terms of their energy-adjusted amounts, and relating this to diet-disease relationships. Before commenting in more detail, we first discuss the literature for simpler problems that are also of interest.
In nutritional surveillance and nutritional epidemiology, there is considerable interest in understanding the distribution of usual dietary intake, which is defined as long-term daily average intake. In addition, of interest is the regression of this intake on measured covariates, which is needed to correct diet-disease relationships for measurement error in assessing diet. If the dietary component of interest is ubiquitously consumed, as most nutrients are, the data are continuously distributed and methods are well-established for solving both problems. See for example Nusser, et al. (1997)
for surveillance and Carroll, et al. (2006)
for measurement error modeling.
Another class of dietary components is those which are episodically consumed, as is true of most foods, e.g., fish, red meat, dark green vegetables, whole grains. When consumption is measured by a short-term instrument such as a 24 hour recall, hereafter denoted by 24hr, the episodic nature of these dietary components means that their reported intake may either equal zero on a non-consumption day, or is positive on a day the component is consumed. In many studies, non-consumption days predominate for several episodically consumed foods of interest. For example, in our data example, for fish and whole grains, 65% and 12% reported no consumption on both of two administrations of the 24hr, respectively. Thus, data on episodically consumed dietary components are zero-inflated data with measurement error. Recently, Tooze, et al. (2006)
for nutritional surveillance and Kipnis, et al. (2009)
for nutritional epidemiology have reported so-called two-part methods, which are actually nonlinear mixed effects models, for analyzing episodically consumed dietary components in the univariate case. These methods are known commonly as the “NCI method” because many of the co-authors of these papers are members of the National Cancer Institute (NCI), and because SAS routines based upon the NLMIXED procedure are available at http://riskfactor.cancer.gov/diet/usualintakes/
, an NCI web site. Other two-part models in different contexts are described for example in Olsen and Schafer (2001)
, Tooze, et al. (2002)
and Li, et al. (2005)
We are interested in the more complex public health problem of understanding the usual intake of an episodically consumed dietary component adjusted for energy intake (caloric intake), along with the distribution of usual intake of energy. This is critical because it addresses the issue of dietary component composition, and makes comparable diets of individuals whose usual intakes of energy are very different. As an example, the U.S. Department of Agriculture’s Healthy Eating Index-2005 (www.cnpp.usda.gov/HealthyEatingIndex.htm
) is a measure of diet quality that assesses conformance to Federal dietary guidance. One component of that index is the number of ounces of whole grains consumed per 1000 kilocalories: there are other items in the HEI-2005 that deal with episodically consumed dietary components, and all of them are adjusted for energy intake. The data needed to compute such variables are thus the usual intake of the dietary component consumed and the usual amount of calories consumed, and (possibly normalized) ratios of them.
Recently, Kipnis, et al. (2010)
have developed a model for an episodically consumed dietary component and energy, see Section 2. They fit this model using nonlinear mixed effects models with likelihoods computed by adaptive Gaussian quadrature using the SAS procedure NLMIXED. However, as described in Section 2 and documented in Section 4, this form of computation can be slow, and can have serious convergence issues. This is extremely problematic, because of the importance of the problem and the fact that solutions will find wide use in the nutrition community, but only if they are numerically stable.
In this paper, we take an alternative Markov Chain Monte Carlo (MCMC) approach to computation, which is faster and numerically more stable. There are many good introductory papers reviewing MCMC, such as Casella, et al. (1992)
, Chib, et al. (1995)
and Kass, et al. (1998)
. Effectively, we exploit the well-known fact (Lehmann and Casella, 1998
, Chapter 6.8) that in fully parametric regular models of the type we study, Bayesian posterior means of parameters are asymptotically equivalent to their corresponding maximum likelihood estimators. To implement an MCMC approach in our problem, there are technical issues that have to be overcome, including the fact that one of the covariance matrices in the model of Kipnis, et al. (2010)
is patterned. Besides fitting the model, our main focus in this paper is to discuss how to use the parameter estimates to then estimate the distributions of the usual intake of energy and energy-adjusted usual intake of dietary components.
In Section 2, we describe the model of Kipnis, et al. (2010)
. In Section 2, we also briefly outline some of the details of our implementation, although the technical details are given in the Appendix
. In Sections 3 and 4, we take up the analysis of the NIH-AARP Study of Diet and Health (http://dietandhealth.cancer.gov/
) as an illustration of our model and method. Section 5 gives concluding remarks.