To assess the phytate intake and molar ratios of phytate to calcium, iron and zinc in the diets of people in China.
2002 China Nationwide Nutrition and Health Survey is a cross-sectional nationwide representative survey on nutrition and health. The information on dietary intakes was collected using consecutive 3 days 24 h recall by trained interviewers.
The data of 68 962 residents aged 2–101 years old from 132 counties were analyzed.
The median daily dietary intake of phytate, calcium, iron and zinc were 1186, 338.1, 21.2 and 10.6 mg, respectively. Urban residents consumed less phytate (781 vs 1342 mg/day), more calcium (374.5 vs 324.1 mg/day) and comparable amounts of iron (21.1 vs 21.2 mg/day) and zinc (10.6 vs 10.6 mg/day) than their rural counterparts. A wide variation in phytate intake among residents from six areas was found, ranging from 648 to 1433 mg/day. The median molar ratios of phytate to calcium, iron, zinc and phytate × calcium/zinc were 0.22, 4.88, 11.1 and 89.0, respectively, with a large variation between urban and rural areas. The phytate:zinc molar ratios ranged from 6.2 to 14.2, whereas the phytate × calcium/zinc molar ratios were from 63.7 to 107.2. The proportion of subjects with ratios above the critical values of phytate to iron, phytate to calcium, phytate to zinc and phytate × calcium/zinc were 95.4, 43.7, 23.1 and 8.7%, respectively. All the phytate/mineral ratios of rural residents were higher than that of their urban counterparts.
The dietary phytate intake of people in China was higher than those in Western developed countries and lower than those in developing countries. Phytate may impair the bioavailability of iron, calcium and zinc in the diets of people in China.
Although the dietary intakes and nutritional status of people in China have been improved apparently along with the rapid economic development, micronutrient deficiencies are still the major nutritional problems among Chinese people. The diets of Chinese people are plant foods based. The average daily intake of cereal grains was 402 g, which accounted for 57.9% of the total energy intake (Wang, 2005).
Plant foods-based diets are rich in bioactive compounds that may prevent some types of non-communicable chronic diseases, such as cancer, diabetes mellitus, etc. (Katayama, 1997; Reddy et al., 2000). Plant foods-based diets also have high phytate content. Although studies revealed that phytate may have beneficial roles as an antioxidant and anticarcinogen (Jeanb and Thompsonm, 2002), owing to its ability to chelate and precipitate minerals, phytate can decrease the bioavailability of critical nutrients such as zinc, iron, calcium (Weaver and Kannan, 2002) and magnesium (Pallauf et al., 1998).
There have been many studies on the phytate contents of different foods; however, data on phytate intake are scarce. Studies on phytate intakes can be found in USA (Harland and Peterson, 1978; Ellis et al., 1982), UK (Wise et al., 1987), Sweden (Torelm and Bruce, 1982), Italy (Carnovale et al., 1987), Napal (Ellis et al., 1987), Turkey (Ersöz et al., 1990), Taiwan (Wang et al., 1992), India (Khokhar et al., 1994) and Korea (Kwun and Kwon, 2000; Joung et al., 2004). In China, with a wide variation in the diets (Wang, 2005), data on phytate intake are lacking (Yang et al., 2002).
Based on the 2002 China Nationwide Nutrition and Health Survey, we studied the following research questions: What are the levels of phytate intake in China? Are there differences between different geographical areas? What differences in phytate intake are observed compared to other countries? What is the possible effect of phytate on the bioavailability of zinc, iron and calcium?
Subjects and methods
The 2002 China Nationwide Nutrition and Health Survey is a nationally representative cross-sectional survey that covered 31 provinces, autonomous regions and the municipalities directly affiliated to the Central Government (Hong Kong, Macao and Taiwan were not included). Multistage cluster sampling method was used for subject selection. Stage 1: all the 2860 counties/districts/cities of China were divided into six areas (big cities, medium and small cities, rural 1, 2, 3 and 4) based on its type and the level of economic development (from high to low). Twenty-two counties/districts/cities from each area were randomly selected. A total of 132 counties/districts/cities were randomly selected at this stage. Stage 2: three townships/sub-districts were randomly selected from each selected counties/districts/cities. A total of 396 townships/sub-districts were randomly selected at this stage. Stage 3: two villages/neighborhood committees were randomly selected from the selected townships/sub-districts. A total of 792 villages/neighborhood committees were randomly selected at this stage. Stage 4: 90 households were randomly selected from each selected villages/neighborhoods, and finally, a total of 71 971 households were randomly selected to represent the national data.
Dietary intake assessment
The dietary survey was conducted among all members of 30 households that randomly selected from the pre-selected 90 households. All family members above 2 years old of the selected households were invited for the dietary intake assessment. A total of 23 470 households participated in the dietary survey (Wang, 2005).
The information on food intake was collected using a 24-h dietary recall method for 3 consecutive days (two weekdays and one weekend day) by trained interviewers. The intakes of calcium, iron and zinc were calculated using the data of dietary recall and 2000 China Food Composition Table (Yang et al., 2002). The contents of calcium, iron and zinc in foods were determined by atomic absorption spectrophotometry.
The phytate intake was calculated using the phytate content of foods we measured (Ma et al., 2005) owing to the lack of data in China Food Composition Table. The anion-exchange method (Harland and Oberleas, 1986) was used for the determination of phytate content and the information on food consumption from the 2002 China Nationwide Nutrition and Health Survey was used for food sample selection. The average daily dietary intake of calcium, iron, zinc and phytate were calculated using the mean value of the 3 days intakes.
The molar ratios of phytate to zinc, calcium or iron are calculated as the millimoles of phytate intake per day divided by the millimoles of zinc, calcium or iron intake per day, respectively. The calcium × phytate/zinc molar ratio is expressed as millimoles per day.
The proportion of subjects with ratios above the suggested critical values was calculated: phytate:calcium >0.24 (Morris and Ellis, 1985), phytate:iron >1 (Hallberg et al., 1989), phytate:zinc >15 (Turnlund et al., 1984; Sandberg et al., 1987; Morris and Ellis, 1989), phytate × calcium /zinc>200 (Davies et al., 1985; Bindra et al., 1986).
Median and quartiles range were used to express the dietary intake of calcium, zinc, iron, phytate, the molar ratios of phytate to calcium, iron, zinc and phytate × calcium/zinc as the values for the above-mentioned variables were not normally distributed.
The dietary phytate, calcium, iron and zinc intake of each individual was calculated. In order to eliminate the differences owing to energy requirement in different age, sex, physical activity level and physiological conditions, the reference man is used to adjust the dietary intakes of each individual. The reference man is defined as male adult aged 18 years and over, with light physical activity level, whose daily reference energy intake is 2400 kcal (China Nutrition Society, 2001). The use of reference man allows the comparison with the results of 1982 (Institute of Health, 1985) and 1992 China National Nutrition Survey (Ge, 1996). The dietary phytate, calcium, iron and zinc intakes of each individual were calculated as phytate/calcium/iron/zinc intake (mg/day) times 2400 kcal divided by his/her RNI of energy (kcal) (China Nutrition Society, 2001). The dietary intakes of phytate, calcium, iron and zinc were expressed as mg/day per reference man.
Considering the sampling method of equal-sample-size of the six areas and the proportion difference between the sampling and whole population, the data of 2002 China Nationwide Population Census (National Bureau of Statistics of China, 2002) were used for the adjustment of areas in data analysis.
A general linear model factorial analysis was applied with Tukey's post hoc comparisons to compare the differences of daily phytate and mineral intakes and the ratios between different areas, age and sex were included as co-variables in the model to reduce the potential difference owing to age and sex proportion in six areas. The non-parameter one-way comparison with Wilcoxon's test was used to compare the difference between urban and rural areas. Multiple logistic regression analysis was performed to compare the percentage of people with ratios above the suggested critical level, whereas age and sex were also included in the models. All statistical analyses were carried out with the SAS Statistical Package (SAS 8.2e for Windows, SAS Institute Inc., Cary, NC, USA), and statistical significance was set at 0.05.
Characteristics of the subjects
Table 1 summarizes the characteristics of study population of 2002 China Nationwide Nutrition and Health Survey. A total of 68 962 subjects (33 551 male and 35 411 female) were included in this study. In all, 21 103 subjects were from urban areas, whereas 47 859 from rural areas.
Phytate intakes of people in six areas
Table 2 shows the dietary intakes of phytate, calcium, iron and zinc of people in China. The median dietary intake of phytate, calcium, iron and zinc were 1186, 338.1, 21.2 and 10.6 mg/day per reference man, respectively. Significant differences in phytate and calcium intakes were found between urban and rural areas. Rural residents consumed significantly higher phytate than their urban counterparts (1342 vs 781 mg/day). The calcium intake of rural residents was 324.1 mg/day per reference man, which was 50 mg less than that of their urban counterparts. The iron and zinc intakes of urban and rural residents were comparable (21.1 vs 21.2 mg/day; 10.6 vs 10.6 mg/day).
Variations were found in the intakes of phytate, calcium, iron and zinc between each two of six areas. The daily phytate intake ranged from 1433 mg for rural 3 to 648 mg for large cities. Calcium intakes were between 451.8 mg/day for large cities and 292.9 mg/day for rural 4. The range of iron intakes was from 22.7 mg/day for rural 3 to 20.7 mg/day for rural 4, whereas that of zinc was 11.2 mg/day for rural 1 and 9.9 mg/day for rural 3.
The molar ratios of phyate:calcium, phytate:zinc, phyate:iron and phytate × calcium/zinc
Table 3 presents the molar ratios of phytate to calcium, zinc, iron, phytate × calcium/zinc and the proportion of subjects with ratios above the suggested critical values. The median molar ratios of phytate to calcium, iron, zinc and phytate × calcium/zinc were 0.22, 4.88, 11.1 and 89.0, respectively. All the four ratios of rural residents were significantly greater than that of their urban counterparts. A wide variation was found in the four ratios among six areas. The phytate:calcium molar ratios were between 0.09 and 0.28. The phytate:calcium molar ratio of 43.7% subjects were above the proposed critical value. The phytate:iron molar ratios ranged from 2.55 to 5.72. The phytate:iron molar ratio of 95.4% subjects were greater than 1. The phytate:zinc molar ratios were between 6.2 and 14.2. 23.1% subjects had molar ratios above the proposed critical level. The phytate × calcium/zinc molar ratios varied from 63.7 to 107.2. The phytate × calcium/zinc molar ratios of 8.7% subjects were higher than 200.
Phytic acid is myo-inositol 1,2,3,4,5,6 hexakis phosphate (IP6), and it accumulates in cereal grains, nuts and legume seeds. Phytic acid is a strong chelator of divalent minerals such as copper, calcium, magnesium, zinc and iron. As phosphate groups are progressively removed from the IP6, the mineral binding strength decreases and solubility increases (Jackman and Black, 1951). At phosphorylations 5, iron solubility was decreased (Sandberg et al., 1989), zinc (SandstrÖm and Sandberg, 1992) and calcium (Lonnerdal et al., 1989) absorption was inhibited. The phytate contents in food samples are determined using anion-exchange method (Harland and Oberleas, 1986) in the present study. Rice and wheat products are the staple foods in China. The phytate contents of wheat products ranged from 3 mg/100 g for fresh noodle to 420 mg/100 g for wheat flour (85% extraction rate), whereas that of rice ranged from 55 mg/100 g for Thailand rice to 183 mg/100 g for Heilongjian rice (Ma et al., 2005). The disadvantage of anion-exchange method is the lack of specificity in distinguishing between IP6 and its hydrolysis products. IP3, IP4 and IP5 were included in this method. Another disadvantage is the difficulty of determining low IP6 levels. Therefore, the IP6 contents in foods in the present study were overestimated to some extent.
A wide variation in phytate intakes was calculated in the diets of people in China. The average phytate intakes of people in China are higher than those in developed countries, and lower than those in Africa and Asia. It is reported that the average American consumes about 750 mg phytate per day (Harland and Peterson, 1978). The estimates of daily phytate intakes in the United Kingdom range from 600 to 800 mg (Davies, 1982). Average phytate intake in Finland has been estimated to be 370 mg/day (Plaami and Kumpulainen, 1995). The average national phytate intake in Italy was 219 mg/day (Carnovale et al., 1987). Swedish people appear to consume very low levels of phytate (180 mg/day) (Torelm and Bruce, 1982). Nigerians consume as much as 2000–2200 mg/day (Harland et al., 1988a) phytate, which is about three times more than the North Americans. Middle Eastern inhabitants also have very high amounts of phytate in their diets (Davies, 1982). A few studies in Asia indicate that phytate intake is higher compared to Western countries. Indian people consume as much as 1560–2500 mg phytate per day (Khokhar et al., 1994). Kwun and Kwon (2000) reported that the phytate intake of South Koreans was 1676.6 mg/day. The daily average phytate intake of people in China was higher than that in Western countries (Harland and Peterson, 1978; Davies, 1982; Torelm and Bruce, 1982; Carnovale et al., 1987; Plaami and Kumpulainen, 1995), and less than that of Korean (Kwun and Kwon, 2000), Indian (Khokhar et al., 1994) and Nigerians people (Harland et al., 1988a). A large variation was found in phytate intakes between people from different areas of China. It ranged from 781 mg/day for urban residents to 1433 mg/day for rural residents.
The variation in dietary pattern may be responsible for the discrepancy in intakes of phytate and minerals between urban and rural areas. The report of the 2002 China Nationwide Nutrition and Health Survey (Wang, 2005) indicated that the daily consumptions of cereal grains of urban and rural residents were 366 and 416 g per reference man. Plant foods including cereal grains, legumes and tubers accounted for 52.6 and 66.3% to the energy intakes for urban and rural residents, whereas cereals and legumes provided 48.0 and 64.1% of protein for urban and rural residents, respectively. Although variation was found between urban and rural areas, the diets of people in China are still plant food based. Plant foods are also the major resource of minerals, which provided most of the dietary intakes of calcium (54.6 vs 71.7%), iron (76.9 vs 86.1%) and zinc (61.2 vs 76.9%) for urban and rural residents in China. Differences in age proportion between urban and rural areas may not explain the phytate and minerals differences, as the differences were still significant after including the age and sex in the models as co-variables.
The influence of phytate on the bioavailability of minerals depends not only on the phytate contents in the diet but also on the interaction between phytate and minerals. The phytate to minerals molar ratios are used to predict the inhibitory effect of phytate on the bioavailability of minerals. Phytate:calcium molar ratio >0.24 will impair calcium absorption (Morris and Ellis, 1985). Phytate:iron molar ratio >1 will significantly decrease the iron absorption (Hallberg et al., 1989). Turnlund et al. (1984) indicated that zinc absorption is greatly reduced and results in negative zinc balance when phytate:zinc molar ratio is 15.
Iron, zinc and calcium are essential minerals that are often lacking in human diets, either due to insufficient intake or due to poor absorption. There are two types of food iron, haem iron from animal foods and non-haem iron from both animal and plant foods. The absorption of haem iron is little influenced by dietary pattern. The absorption of non-haem iron is influenced by both enhancing and inhibitory factors in the diets. Ascorbic acid from fruits and vegetables and meat/fish/poultry are the main enhancing substances for iron absorption (Taylor et al., 1986; Ballot et al., 1987). Phytic acid from cereal grains and legumes (Hallberg et al., 1987; Hurrell et al., 1992), and polyphenol compounds from tea and coffee (Hurrell et al., 1999) are the major inhibitory substances. It is reported that 85–95% anemia in China is caused by iron deficiency (Zhang, 1987; Cai and Yan, 1990; Wang et al., 1990; He et al., 1994). As the iron intakes were high (Wang, 2005), low iron bioavailability is considered a major factor in the etiology of iron deficiency anemia (Taylor et al., 1995). When phytate:iron molar ratio >1 is used as the critical value (Hallberg et al., 1989), the bioavailability of iron in most subjects (95.4%) was inhibited. Phytate may play an important role in the anemia problem in China.
Milk and milk products are the most important sources of calcium for people living in developed countries, whereas plant foods are the main source of calcium for people in China. Oxalate is a potent dietary inhibitory of calcium absorption (Heaney and Weaver, 1989) with phytic acid possessing a much smaller inhibitory effect (Heaney et al., 1991). It is considered that the major factor resulting in an inadequate supply of calcium in the diets of people in China is low calcium intake from a low consumption of milk products, rather than low bioavailability. In the present study, we found that one-fifth of the urban residents and one-half of the rural residents have phytate:calcium molar ratio above the critical level, which implies the calcium bioavailability of this portion of population was affected by phytate.
Meat and seafoods are good sources of zinc. However, meat and seafoods only provided 17.5% of zinc, whereas cereals and legumes contributed 56.8% zinc for people in China (unpublished data). These plant foods are high in phytic acid, which is a potent inhibitor of zinc absorption (Nävert et al., 1985). The median phytate:zinc ratio was 11.1, which is similar to that in the diets of Taiwanese (Wang et al., 1992) and Korean (Joung et al., 2004), but lower than those of American lacto-ovo vegetarians (Harland et al., 1988b), Middle Easterners (Davies, 1982) and Indian (Khokhar et al., 1994); and higher than those of a typical American hospital diet (Oberleas and Harland, 1981) and omnivorous diets. Our data suggest that phytate has little influence on zinc bioavailability of most residents in large cities of China. As 19–45% of rural residents had phytate:zinc molar ratios above the critical level, suggesting that phytate might increase the risk of impaired zinc bioavailability for rural residents in China.
It is suggested that the effect of other factors such as calcium on zinc bioavailability should be taken into consideration in diets that are both high in phytate and calcium but low in zinc (Davies et al., 1985). Considering the low calcium intake, phytate × calcium/zinc molar ratio might not be suitable for predicting the interaction effect of phytate and calcium on the absorption on zinc for people in China.
In conclusion, people in China consume more phytate in their diets than those in developed countries, and less than those in developing countries. A wide variation was found in phytate intake of people in different areas of China. The inhibitory effect of phytate on iron bioavailability for both urban and rural residents, and zinc bioavailability for rural population should be addressed.