Sarcopenia is a progressive skeletal muscle disease characterized by a gradual loss of muscle mass and function and related to a high risk for falls, frailty, hospitalization, and mortality.[1] Epidemiological studies showed that sufficient protein intake was positively associated with muscle protein synthesis in the elders.[2,3] The European Society for Clinical Nutrition and Metabolism Expert Group suggested protein intakes of 1.0 to 1.2 g/kg/day for healthy adults in the age of more than 65 years old, and 1.2 to 1.5 g/kg/day for older adults with the disease at risk of malnutrition.[4] Our previous double-blind randomized controlled trial reported that protein supplementation of 1.5 g/kg/day improved muscle mass and physical performance compared to protein supplementation of 0.8 g/kg/day or 1.2 g/kg/day in undernourished pre-frail and frail older Koreans.[5] However, the Korean National Health and Nutrition Examination Survey (KNHANES) showed that 48% of elderly men and 60% of elderly women consumed less than the amount of dietary reference intakes for Koreans (KDRI), the recommended amount of nutrients for Korean population, 0.91 g/kg/day.[6]

Identifying appropriate methods is important for evaluating protein intake. Protein intake cannot be measured using serum or urine samples, since serum level of protein is always maintained at a certain level, and protein is not appeared in urine unless patients have kidney disease.[7] Thus, protein intake can be evaluated by various dietary surveys including 24-hr recall, dietary records, and the food frequency questionnaire (FFQ), which measure all nutrients including protein.[8] Dietary records and 24-hr recall are not good for the measurement of habitual intake, while FFQ is reflect habitual intake but takes a long time to complete because of containing 112 food items to measure all nutrient intake.[9] There was a rapid self-management FFQ to evaluate dietary protein intake which included food items frequently consumed by the French population.[10] However, since the major protein foods for the Korean and French population are different according to the KNHANES, the French FFQ could not be used in the Korean population.[11] Thus, a new rapid questionnaire to measure only protein intake is needed to develop based on major protein foods consumed by Korean using the KNHANES data. To validate the newly developed questionnaire called Korean Protein Assessment Tool (KPAT), protein intake was measured by KPAT and FFQ using KNHANES was compared. Therefore, the purpose of the present study was to investigate the hypothesis that protein intakes measured by the newly developed simplified method called KPAT and FFQ using KNHANES were not significantly different.

One hundred twenty participants completed both the FFQ and the KPAT (Table 2). There were 33% males and 67% females in total participants, 31 (54.4%) of participants aged <65 years, and 49 (77.8%) of participants aged ≥65 years were women. Participants were classified based on the age of 65 due to the difference in daily protein intake in accordance with KDRI.[11] Participants aged <65 years were significantly taller and heavier and consumed higher energy, carbohydrate, fat, and protein compared to participants aged ≥65 years. There was no significant difference in body mass index between the participants aged <65 years and aged ≥65 years.

Protein intakes investigated by the FFQ and the KPAT were not significantly different in both participants aged <65 years and aged ≥65 years (Table 3). The intraclass correlation coefficient of 0.979 (95% CI, 0.970-0.986) indicated excellent reliability in protein intakes (g/day) estimated by both the FFQ and the KPAT (Table 4). Regarding protein intake as g/kg/day, the intraclass correlation coefficient of 0.972 (95% CI, 0.960-0.980) also indicated excellent reliability in protein intakes assessed by FFQ and KPAT (Supplementary Appendix 1). Pearson’s correlation coefficient showed a significant positive correlation between the FFQ and the KPAT in total participants and participants aged <65 years and aged ≥65 years (Fig. 1). With the Bland-Altman plot, there were no significant mean differences of estimated protein intake measured by 2 methods within 95% limits of agreement (Fig. 2).

The present study showed that protein intake estimated by both the FFQ and the KPAT were not significantly different, and the measurement values of protein intake between the FFQ and the KPAT were positively correlated.

According to 2013 to 2017 the KNHANES, the average protein intake was 83 to 88 g/day for men aged <65 years, 58 to 64 g/day for women aged <65 years.[13-17] In this study, the average protein intake estimated by FFQ and KPAT was 83 and 81 g/day for men aged <65 years, 64 and 63 g/day for women aged <65 years, respectively. Protein intake of participants aged <65 years in this study showed similar results to the KNHANES data. In addition, according to the 2013 to 2017 KNHANES, the average protein intake was 58 to 69 g/day for men aged ≥65 years, 38 to 50 g/day for women aged ≥65 years.[13-17] In this study, the average protein intake estimated by FFQ and KPAT was 62 and 60 g/day for men aged ≥65 years, 50 and 50 g/day for women aged ≥65 years, respectively. Thus, the protein intake of participants aged ≥65 years in this study showed similar results to the KNHANES data. Previous studies reported that Korean elders over 65 years old consumed about 20 g less protein per day than adults aged 30 to 64 years old, which showed similar to the results of the present study.[6,19] The KDRI recommends a daily protein intake of 60 to 65 g/day for men aged <65 years and 50 to 55 g/day for women aged <65 years, and 60 g/day for men aged ≥65 years and 50 g/day for women aged ≥65 years.[11] The study showed that 21% of the Koreans consumed less than the recommended amount of intake based on the KDRI for protein. Our result was similar to 23%, which was reported by the KNHANES.[20]

Morin et al. [10] previously developed a rapid the FFQ for protein intake of the French population and showed that protein intakes were not significantly different using paired t-test between their new FFQ and dietary record (P=0.075), and between the new FFQ and 24-hr recall (P=0.520), which were consistent with our study (P=0.144). The French study showed that protein intakes measured by the new FFQ, dietary record, and 24-hr recall had no difference in variability using the Bland-Altman plot.[10] In the present study, protein intakes measured by the FFQ and KPAT had no difference in variability using the Bland-Altman plot with the narrow range compared to the previous French study.

The French FFQ comprised 20 food items, frequently consumed by the French population, such as meat and dairy products.[10] However, KPAT included not only meat and dairy products but also grains which were major protein foods in Korea. According to the KNHANES, grains were the top protein source, followed by meat, fish and shellfish, vegetables, beans, and legumes.[11] In this study, grains were also the top protein source, followed by meat, egg, vegetables, pizza. The ranking of protein-based foods in our study was similar to that of protein-based foods in the KNHANES. Animal-based proteins with a high protein content of a higher quality contained all the essential amino acids required by the human body and were more digestible.[21] However, the main sources of dietary protein in the Korean population were rice and other grains. The Korean diet is a plant-based protein, contributing to nearly 2-thirds of total protein intake and the amount of protein in these foods could not be ignored.[22]

In the present study, the Pearson correlation coefficient of protein intake estimated by the FFQ and the KPAT was 0.92 to 0.96, suggesting a very high correlation as compared with other nutrient validation. Previous studies showed that Pearson correlation coefficients obtained from calcium intakes to validate questionnaires were in the range of 0.56 to 0.84 in Asian,[23,24] and 0.64 to 0.90 in westerners.[25,26] The correlation coefficients of calcium questionnaires were narrower ranges in studies done in Americans and Europeans than in studies done in Asian. The sources of calcium intake for Americans and Europeans were simple due to their high large dependence on milk and milk products, which were rich in calcium content and had high availability and good absorption in the intestine.[27] The sources of calcium in the Asian diet were more diverse than the American and European diet, and their source of calcium included dark green, beans, seaweed, and seafood.[28]

Gross classification according to the quartiles of intake indicated that 74% of participants were classified into the same or adjacent quartile. Intraclass correlation coefficient showed excellent reliability of 0.979, a 97.9% agreement, between the FFQ and the KPAT. This result was better than 0.79 to 0.86 of the intraclass correlation coefficient obtained from calcium intake in a previous study.[29] In addition, the Bland-Altman plot showed good agreement between the FFQ and the KPAT.

The present study had a few limitations. First, basic dietary survey methods might have some reporting bias using self-reported dietary intake and recall bias using intake memories over the past year. Second, the intake of protein could be affected by seasonal variation, and the developed KPAT was not validated in all 4 seasons. Third, Participants were recruited only in a hospital, not the entire population, and selection bias might have existed.

The KPAT has a few strengths. First, the newly developed KPAT is a simple questionnaire tool to assess protein intake. Second, the KPAT is convenient and simple to estimate protein intake, since KPAT consists of only 39 questions but FFQ has 112 questions. Third, the dietary protein intake can be assessed by good validity and reproducibility. The present study suggests that the developed KPAT can be a useful and simple evaluation tool to assess dietary protein intake for the Korean population. The KPAT can be useful for the rapid evaluation of protein intake at the clinical setting, and for prescribing appropriated amount of protein for patients whose protein intake is low. The KPAT will be released as an online version to use at the clinical setting.