The study sample consisted of 10,977 adults (age ≥ 19 years) who completed a 24-h dietary recall in, the dietary interview component of the NHANES, 2007–2010. Analyses included only individuals with complete and reliable dietary records using the USDA automated multiple-pass method. Pregnant or lactating women were excluded. All participants or proxies provided written informed consent and the Research Ethics Review Board at the National Center for Health Statistics approved the survey protocol. Detailed description of the survey design and the data collection procedures are reported elsewhere [ 8 ].

2.2. Estimating Level and Source of Protein Intake

USDA food composition databases were used to determine protein gram intake and protein type from foods consumed by NHANES participants. This process estimates nutrient content of reported foods, by linking the Food and Nutrient Database for Dietary Studies (FNDDS) with food composition data provided by the USDA Nutrient Database for Standard Reference (SR). The ingredients of disaggregated survey food recipes (coded using the SR food codes) were linked to the appropriate food composition databases using the SR-Link file of the FNDDS (versions 4.1 and 5.0 link SR releases 22 and 24 respectively) [ 9 10 ].

Protein gram amounts by type associated with an intake in the NHANES individual foods file was obtained via the FNDDS SR Links and SR nutrients files. Every SR code with protein was assigned via the SR code description to a source; animal, dairy, plant or mixed protein. Mixed protein was used to denote that the source for the SR code was from more than one of animal, dairy or plant proteins. For each food code, the SR weights and SR links were used to determine the percentage of protein of each of the types (animal, dairy, plant, mixed) that made up the protein in the food code. These percentages were then applied to the total protein for the food code if each food consumed by each subject. The calculations were done separately for each NHANES data release using the individual food files, FNDDS and SR files corresponding to that NHANES release.

The nutrient profiles for some missing SR codes were obtained from addendums to FNDDS files for missing SR codes. There were only 13 remaining SR codes with missing nutrient profiles. These were obtained from the nearest SR version where it was available or from SR codes with similar descriptions.

An analysis of all the protein intake in the NHANES files indicated these methods result in more than 90% of all protein gram intake categorized into animal, dairy or plant with only less than 10% in the mixed category. An example of protein in the mixed category was pizza. For example, the food code 58106220 “Pizza, cheese, from restaurant or fast food, thin crust” links to the single SR code 21301 denoting pizza. Using the SR description the protein is assigned to the mixed category since its source contains both dairy and plant-based protein and the individual amounts in the dairy and plant categories cannot be calculated from the SR data. Several food categories (such as mixed dishes, burritos and tacos, soups, cakes and pies, and eggs and omelets) were common for more than one source of protein.