|Sentinel Foods: Sampling and Analysis|
The Nutrient Data Laboratory (NDL) is monitoring Sentinel Foods periodically by sampling Sentinel Foods at stores and restaurants around the country, followed by chemical analysis in laboratories to determine nutrient levels, using standardized procedures developed under the National Food and Nutrient Analysis Program (1). This program administered by NDL since 1997, in cooperation with other government agencies, generates original analytical data on foods.
The major steps in sampling and analyses of Sentinel Foods are described below.
I. Developing a Sampling Plan
NDL created sampling plans for purchase of each Sentinel Food. To develop these plans, NDL used a three-stage, probability-proportional-to-size sampling design (2). The three stages were to choose:
1. Twelve counties and cities in different geographical regions around the country based on their
population densities (number of people per square mile) according to the most recent US Census.
2. Grocery stores and restaurants in the 12 counties where NDL planned to buy the Sentinel Food
samples based on annual sales data.
3. Specific food brands to buy based on the market share of those brands sold according to Nielsen data, aiming to represent upto 70-75% of brands sold. The plans call for buying samples of national and private (store) brands at retail or wholesale stores. The restaurant food samples are mostly from fast food and family-style restaurant chains.
II. Acquiring and Analyzing Food Samples
NDL prepares instructions for professional purchasing agents to buy samples of the Sentinel Foods. Agents carefully packaged and shipped the food samples they bought to laboratories at Virginia Tech University or Texas Tech University. Scientists at these laboratories prepared the samples and shipped them to commercial laboratories, where they measured sodium levels by inductively coupled plasma-atomic emission spectroscopy [a]. Details on the methods used to measure levels of other nutrients are available in the USDA National Nutrient Database for Standard Reference documentation. NDL reviewed the data from the laboratories for accuracy and used statistical techniques to generate nationally representative values for nutrients in these foods. The figure below illustrates the steps in the sampling and analysis of a Sentinel Food "meat and poultry hot dog".
III. Re-sampling and Analyzing Sentinel Foods
NDL will buy new samples of Sentinel Foods and measure nutrient levels in these samples every 4-8 years. These analyses will use the same methods as those described above. NDL has organized the Sentinel Foods into four groups to determine how often to sample and analyze the foods in the future based on how popular the foods are, how much sodium in these foods could be reduced, and how often sodium levels in these foods have changed in the past. NDL will review changes in the levels on the Nutrition Facts Panel and consumption of these foods in the latest What We Eat In America, National Health And Nutrition Examination Survey before deciding whether to resample.
More details on the methods that NDL uses to track Sentinel Foods are available in the March 2015 issue of The American Journal of Clinical Nutrition.
1. Haytowitz DB, Pehrsson PR, Holden JM. (2008). The National Food and Nutrient Analysis Program: A decade of progress. Journal of Food Composition and Analysis 21, S94-S102. 2. Pehrsson PR, Perry C, Daniel M. ARS, USDA updates food-sampling strategies to keep pace with demographic shifts. 36th Annual National Nutrient Databank Proceedings. Procedia Food Science 2013; 2:52-9.
1. Haytowitz DB, Pehrsson PR, Holden JM. (2008). The National Food and Nutrient Analysis Program: A decade of progress. Journal of Food Composition and Analysis 21, S94-S102.
2. Pehrsson PR, Perry C, Daniel M. ARS, USDA updates food-sampling strategies to keep pace with demographic shifts. 36th Annual National Nutrient Databank Proceedings. Procedia Food Science 2013; 2:52-9.
[a] AOAC method 985.01 (3.2.06) + 984.27 (50.1.15)