Subjects and study design
The evaluation study was designed to include a representative sample of the Norwegian population. The participants were recruited via mail to 4500 randomly selected women and men with home addresses in the Norwegian capital or surrounding area. The random selection of subjects was administered by The National Tax Office/Population Registration Office, and an equal number of men and women were invited, aged 18 to 80 years. Five hundred and four invited men and women (11%) responded to the invitation. After interview screening 346 participants were enrolled in the study, of which 232 took part in the present part of the evaluation study. Exclusion criteria were pregnancy, weight loss of more than 5 kg during the last 6 months prior to the study and participation in other research projects. Data collection was carried out from September 2006 until October 2007. After recruitment the participants received the FFQ and written instructions by mail and were asked to fill in the FFQ at home. Within two weeks the participants attended a physical examination and returned the FFQ. Measurements of height and weight were performed by trained staff and the participants were lightly dressed with indoor clothes and without shoes when the anthropometric measures were taken. At the physical examination, the participants were randomly assigned to participate in either the assessment of EE (n = 64), or to conduct the 7-days WR (n = 168). The participants were given both oral and written instructions how to perform the tasks. The EE measurements and the 7-days WR were initiated 3 to 4 weeks after the participants filled in the FFQ. Social economic status of the participants was not assessed.
This study was conducted according to the guidelines laid down in the Declaration of Helsinki and all procedures involving human subjects were approved by the Regional Ethics Committee for Medical Research. Written informed consent was obtained from all subjects.
The semi-quantitative food-frequency questionnaire
The 14-page questionnaire was designed to capture the habitual food intake among Norwegian adults the preceding year. The FFQ was an extended and revised version of the semi-quantitative food-frequency questionnaire used in the Norwegian nation wide survey NORKOST 1997 (NFFQ). The original NFFQ was a validated, 180 items optical readable FFQ, developed to cover 100% of the total EI of the population [3–6]. Based on our extensive screening of antioxidant content in foods and beverages , the NFFQ was updated and revised with questions about food products and food categories assumed to be important sources of antioxidant intake in Norway. The new FFQ included 270 food items, grouped together according to the Norwegian meal pattern. Additional questions were added concerning intake of several food categories. In detail, 19 questions about berries, 4 questions about fruit, 6 questions about vegetables, 2 questions about chocolate, 3 questions about coffee and 2 questions about tea were added. Questions concerning the variable intake of berries due to seasonal variations were included. Furthermore, 10 questions were added about nuts and seeds and 27 questions about spices and herbs. The options of frequency of consumption of particular food items varied from several times a day to never/seldom, with portion sizes based on typical household units: slices, glasses, cups, pieces, spoons and teaspoons. When frequency was answered but not portion size, the food item was given the smallest portion size. When portion size was answered but not frequency, the food item was given the value zero. One participant was excluded from the study because a substantial amount of the questions in the FFQ were left unanswered. The dietary questions totaled 11 pages of the questionnaire whereas the last 3 pages were dedicated to questions concerning dietary supplements, smoking, physical activity and past and present illnesses and medication. The answered FFQs were scanned and the image files translated into data files using the Cardiff Teleform 2006 software.
The energy study
Sixty-four participants were randomly selected to carry out the EE measurements. The position-and-movements monitor ActiReg® (PreMed AS, Oslo, Norway) was used to measure EE for 7 consecutive days. Two participants were excluded from the data analyses due to failure in the ActiReg® recording system and three were excluded due to measurement periods of less than 7 days. Thus, EE data from 59 participants were available for comparison with the EI estimated from the FFQ.
The ActiReg® system uses a combined second-to-second recording of body position and motion to calculate EE . The monitor has two pairs of position-and-motion sensors connected by cables to a battery-operated storage unit fixed to a waist belt. Each pair of sensors is attached by medical tape to the chest and the front of the right thigh, respectively. When the participants were sleeping, the ActiReg® equipment was taken off and placed in a horizontal position. Collected data were transferred to a computer and processed by a dedicated program called ActiCalc® . The ActiReg® system has been validated against both the doubly labeled water method and whole-body indirect calorimetry among young adults . The validation studies demonstrated that there was no significant mean difference between EE measured by the ActiReg® system and EE measured with indirect calorimetry or the doubly labeled water method . The correlation coefficients between EE measured with the ActiReg® system and EE measured with indirect calorimetry or the doubly label water method were 0.86 and 0.76, respectively .
The 7-days weighed food records study
One hundred and sixty-eight participants were randomly selected to do the 7-days WR. Data from 21 participants were excluded from the analyses because less than 7 days had been recorded. The participants got written and oral instructions how to weigh and record all foods and beverages consumed in a food diary for 7 days, divided into 2 periods of 4 and 3 consecutive days, one week apart, including all days of a week. Each participant was provided with a food diary and a digital scale (Phillips HR 2393/95). The WRs were manually transcribed into data files which were imported into the KBS system. The WRs were distributed to the participants over a period of 7 months, from September to March, omitting the Christmas holidays.
Food, nutrient and antioxidant databases
Average daily intake of energy, nutrients and antioxidants from the FFQ and the WR were computed using the food database KBS AE-07 and KBS software system (KBS, version 4.9, 2008) developed at the Department of Nutrition, University of Oslo, Norway. The food database KBS AE-07 is based on the 2006 edition of the Norwegian food composition table http://www.norwegianfoodcomp.no. The antioxidant values in KBS AE-07 are based on our extensive analyses of antioxidants in more than 3100 food samples procured worldwide [7, 10, 11]. The total antioxidant content in foods were measured using the FRAP (ferric-reducing ability of plasma) method and is expressed as mmol/100 g sample [7, 10, 11].
Sample size calculation for the EI study was based on a SD of EI of 2 MJ and a significance level of 0.05 with 80 percent power [12, 13]. Thirty-two participants were required to detect a mean difference of 1 MJ between EE measured with ActiReg® and EI assessed with the FFQ. Height, weight, EE, EI, EI-EE and EI/EE were normally distributed and are presented as means and 95% confidence intervals (CI). Differences between EI and EE for all participants and between the genders were analyzed using paired and unpaired T-tests, respectively. All significance levels were two-sided. The visual agreement between the methods was analyzed as described by Bland and Altman [14, 15] using a plot of the differences between the two methods versus the average of the measurements. This type of plot shows the magnitude of disagreement, spot outliers and any trend. The relationship between EI and EE was investigated using Pearson product-moment correlation coefficient and by cross-classification of the participants into tertiles of EE and EI. The accuracy of the reported EI was calculated by expressing the ratio EI/EE, for which a value of 1 would mean complete agreement between EI and EE. Acceptable reporters (AR) were defined as having the ratio EI/EE in the range of 0.80 to 1.20, under-reporters (UR) as EI/EE < 0.80 and over-reporters (OR) as EI/EE > 1.20. These definitions are partly based on the 95% confidence limits of agreement between EI and EE measured by the double labeled water method .
Chi-square tests for independence were conducted to assess if there was any difference in the distribution of men, women, smokers and non-smokers in UR and AR.
Percent energy from protein, fat and carbohydrates were normally distributed and are presented as means and SD. All micronutrient intakes and most food intake estimates were not normally distributed and data are therefore presented as median values with 25 and 75 percentiles. The differences between food intakes estimated by the FFQ and the WR were tested using Wilcoxon signed rank test (paired data). The relationship between the methods was analyzed using the Spearman rank correlation coefficient or the Pearson product-moment correlation coefficient and by classifying participants into quartiles of intake. Correctly classified participants were defined as participants categorized in the same quartile as defined by the WR estimate, whereas 'grossly misclassified' participants were defined as participants classified into opposite quartile. The accuracy of the estimated intakes was explored by expressing the ratio of estimates (FFQ/WR) for all nutrients and foods. Energy adjustment of intakes was calculated as absolute intake per 10 MJ. Results were considered to be statistically significant at p < 0.05. Data were analyzed using SPSS for Windows release 16.0 (SPSS Inc., Chicago, IL, USA).