Chapter 3 Measuring Diet PDF

Chapter 3 Measuring Diet © 2013 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sal...

Chapter 3 Measuring Diet © 2013 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part. “… dietary measurement is an activity that is fraught with difficulties. There is no one method of measurement that can be regarded as a gold standard.” RJ Carroll, LS Freedman, AM Hartman. Am J Epidemiol 1996;143:392-404 2  Although collection of dietary data is critical to our understanding of nutrition, dietary intake data must be interpreted with considerable caution. 3 Most Common Techniques Used to Measure Diet 24-hour recall Food record or diary Food frequency questionnaire 4 Factors Influencing Choice of Techniques for Measuring Diet Available resources Timing Subject characteristics Research design 5 Collecting a 24-Hour Recall Interviewer asks respondent to recall all foods and beverages consumed in the past 24 hours. Interviewer records this information. Nutrient intake is determined using computerized diet analysis software. 6 Strengths of the 24-Hour Recall Relatively quick: < 20 minutes to administer Inexpensive Relatively easy to administer Low respondent burden Does not alter usual diet 7 Limitations of the 24-Hour Recall Relies on memory One recall is seldom representative of an individual’s usual intake Data entry and analysis can be labor intensive 8 USDA Automated Multiple Pass Method A computer-assisted, five-step, multiple-pass 24-hour recall system Primary method of collecting dietary intake data by the National Health and Nutrition Examination Survey 9 ASA24 Internet-based automated, self- administered 24-hour dietary recall Developed by the National Cancer Institute Uses technology drawn from the USDA’s Automated Multiple-Pass Method Available in English and Spanish 10 Food Record or Food Diary Respondent records the identity and amounts of all foods and beverages consumed for a certain period of time ranging from 1 to 7 days. The recording should be done at the time the food is consumed. 11 Estimated Food Record The respondent estimates portion sizes (bowls, glasses, coffee cups, etc.) or uses household measures. Approach more often used by North American researchers. May be less accurate than weighed food record, but the respondent burden is less. 12 Weighed Food Record Respondent weighs all foods and beverages consumed using a scale. Approach more often used by European researchers. May be more accurate than estimated food record, but the respondent burden is greater. 13 Strengths of Food Record Does not depend on memory. Can provide detailed data on food and beverage intake, time and place of meals, etc. Reasonably valid up to 5 days. 14 Limitations of Food Record Respondent must be literate Requires high degree of cooperation Act of recording may alter diet Response burden can result in low response rates when used in large national surveys Data entry and analysis can be labor intensive 15 Food Frequency Questionnaire Measures energy and/or nutrient intake by determining how frequently a person consumes a limited number of foods and beverages that are major sources of nutrients or of a particular dietary component of interest. 16 Food Frequency Questionnaire FFQs consist of a printed list of individual foods, food groups, and beverages. Respondent indicates how many times a day, week, month, or year an item on the list is usually eaten. Some FFQs ask about portion size. 17 “Screeners” FFQs developed to assess intake of specific nutrients or food components, e.g. calcium, dietary fiber, fruits, vegetables, LDL-raising food components, and total fat. Commonly used in epidemiologic research investigating diet-disease relationships. 18 Strengths of Food Frequency Questionnaires Modest demand on respondents. Can be self-administered and machine- readable. Can be completed on-line. Practical for studies having large numbers of subjects. 19 Limitations of Food Frequency Questionnaires The foods included in the FFQ may not be those usually eaten by respondents. Depends on ability of subject to describe diet. Respondent must be literate. 20 Food Availability Also known as “food disappearance.” Measures the amounts of major commodities that “disappear” from or leave the food distribution system or that are “available” for consumption. Different from data collected by surveys asking subjects to report their food intake. 21 Food Availability Begun in mid-1940s by USDA using data going back as far as 1909. Estimates mean per capita or per person availability of food for consumption. Good for showing trends over time. Unless adjusted for losses actual consumption is overestimated. 22 How Food Availability is Calculated 23 Surrogate Sources Persons providing information about another. Important when respondent has hearing, speech, memory problems. Examples include the respondent's spouse or partner, children, close relative or friend. Data quality proportional to the number of shared meals. 24 Validity The ability of an instrument to actually measure what it is intended to measure. For example, how well does a food frequency questionnaire estimate usual nutrient intake? 25 Accuracy The quantitative measure of validity. The difference between the measured value and the true value. Bias is a divergence from or lack of accuracy. When accuracy is low, bias is high. 26 Reliability The ability of a measurement method to produce consistent results when repeatedly applied to the same sample. 27 Precision The quantitative measure of reliability. The difference among numerous values arrived at by repeatedly measuring the same sample. 28 Missing & Phantom Foods Missing foods: foods that are eaten but not reported Phantom foods: foods that are reported but not eaten 29 Biological Markers Biological measurements associated with dietary intake. Examples – Urinary nitrogen excretion – Urinary sodium excretion – Plasma levels of carotenoids & vitamins – Fatty acids in adipose tissue or RBCs – Energy expenditure compared to reported energy intake 30 Biological Markers--Advantages Easily accessible and objective. May be closely associated with actual dietary intake. May be the best measurement method available. 31 Biological Markers-- Disadvantages Biological measurements may not always reflect dietary intake. May be affected by factors other than dietary intake. 32 Urinary Nitrogen Nitrogen in multiple 24-hr urine samples is measured and compared with reported protein intake. If urinary nitrogen compares favorably with reported protein intake, reported intake of other nutrients is likely accurately reported. 33 Energy Expenditure vs. Reported Energy Intake Energy expenditure is estimated or measured and then compared with reported energy intake. In a weight-stable person, energy expenditure should roughly equal reported energy intake. 34 Energy Expenditure vs. Reported Energy Intake If there is a marked discrepancy between energy expenditure and reported energy intake in a weight-stable subject, energy intake was not accurately reported. 35 Estimating Portion Size Accurate estimation of portion size is critical to obtaining accurate dietary intake data. Most people have difficulty accurately estimating portion size without training. 36 Methods for Helping Subjects Estimate Portion Size Two-dimensional and three-dimensional geometric shapes of various sizes. Bowls, plates, measuring cups and spoons, and drinking cups of various sizes. Three-dimensional food models. Photographs or digital images of foods. 37 The USDA Food Model Booklet A collection of life-size, two-dimensional images representing different shapes and sizes of foods. 38 Image from USDA Food Model Booklet 39 Image from USDA Food Model Booklet 40