Introduction

The food-derived heterocyclic amines (HCAs) comprise a family of mutagenic/carcinogenic compounds formed from creatin(in)e, amino acids and sugars in muscle meats by cooking meats at high temperatures using ordinary household methods (Jagerstad et al, 1984). Today approximately 20 HCAs have been identified and several, including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), are recognized as potent experimental mutagens and carcinogens capable of inducing tumors in multiple tissue sites in several animal species (Hasegawa et al, 1994; Yoshimoto et al, 1999; Archer et al, 2000; Nagao et al, 2002). Recently, there has been a growing body of epidemiological evidence supporting the relationship between intake of dietary HCAs and increased risk of certain cancers of major importance in many countries, such as the colo-rectum (Gerhardsson de Verdier et al, 1991; Lang et al, 1994; Kampman et al, 1999; Wu et al, 2001; Murtaugh et al, 2004), lung (Deneo-Pellegrini et al, 1996; Sinha et al, 1998a), pancreas (Anderson et al, 2002) and breast (Knekt et al, 1990; De Stefani et al, 1997; Zheng et al, 1998; Sinha et al, 2000).

HCAs have been detected at parts per billion (ng/g) concentrations in cooked meat and the most important factors influencing their formation in cooking are meat type, cooking method, temperature and time (Sinha et al, 1995; 1998b; Skog et al, 1998; Keating et al, 1999). Differences in methods of preparing meat before cooking, the use of marinade or type of cooking oil may also affect the formation of HCAs in cooked meat (Knize et al, 1995; Layton et al, 1995; Monti et al, 2001). Exposure to HCAs therefore varies greatly between populations, depending on dietary preferences and cooking methods. An understanding of the potential effect of HCAs on cancer risk in diverse populations requires an instrument capable of measuring exposure to these compounds in diets specific to that population. Such an instrument would comprise a questionnaire which addresses frequency of intake, type of meat and cooking methods, and reliable estimates of HCA concentrations in meats cooked according to representative cooking practices used in that population (Sinha & Rothman, 1997; Keating et al, 1999). Such questionnaires have been developed and applied in various populations in the West (Layton et al, 1995; Augustsson et al, 1997; Byrne et al, 1998; Norrish et al, 1999) and in Japan (Kobayashi et al, 2002), as a means of estimating exposure of HCA in the usual diet of these cultures, as well as in epidemiologic studies of the relationship between HCA intake and cancer risk (Sinha et al, 1998a; Zheng et al, 1998; Norrish et al, 1999). To our knowledge, such an instrument has yet to be developed for the Chinese diet, and analytical data on HCA concentrations in meat, in the form commonly consumed by these populations, is lacking.

In this study, we describe the pattern of meat consumption and specific cooking methods among middle-aged and older Chinese men and women in Singapore.

Methods

Study population

Data were obtained from 24-h dietary recall interviews conducted among a random sample of participants of the Singapore Chinese Health Study. The latter is an ongoing prospective cohort of 63 257 healthy Chinese men and women (Hokkien and Cantonese in dialectal origin) who are citizens or permanent residents living in public housing estates (these provide housing for close to 90% of the population). The cohort was recruited between 1993 and 1998, and all members completed at baseline a food frequency questionnaire. As part of the validation study, a random sample of participants was selected to complete two 24-h recall interviews covering a weekday and a weekend. The current data is based on the first of these 24-h recall interviews from the 986 cohort members who represent a randomly selected 3% of the study population recruited between February 1993 and August 1996 (Hankin et al, 2001).

Data collection

All interviews were conducted at participants' homes between April 1994 and March 1997, by trained interviewers. Each interview lasted about 25–30 min and the interviewers followed a written protocol to ensure uniformity in eliciting and recording all food and beverage items consumed within and outside the home over the last 24 h. A total of 986-person days were recorded with 293 (29.7%) being weekends (Saturday and Sunday).

For each food item consumed, the name of the dish, type of cooking method used (stir-fried, boiled/steamed, deep-fried, pan-fried, roasted/grilled, stewed), ingredients and portion size of each ingredient were elicited. Most Chinese dishes are mixed dishes with reasonably standard recipes. The cooking methods are also easily distinguishable, and participants did not encounter any difficulties in describing these methods for food consumed either inside or outside the home. Portion sizes were recorded as multiples of common household measurements, and the interviewers carried a set of standard household utensils to help the subjects indicate the actual amount eaten (eg half a rice-bowl of sliced pork, one Chinese spoonful of dried mushrooms). After the interview, all items in the recall were coded and gram weights assigned according to the Singapore Chinese Food Composition Database (Hankin et al, 2001). Based on these data, we selected all meat items (including fresh meat, processed meat and meat products, see definitions below) consumed by the respondents and categorized them according to type of meat (pork, chicken, etc) and cooking method (boiling, pan-frying, stir-frying, etc). Where cooking methods were specific to the form of meat (eg pork ribs and minced pork are commonly cooked using different methods), we distinguished between specific forms of meat for clarity. In this way, the gram weight equivalents of each type of meat by different cooking methods consumed in the past 24 h were obtained for each subject.

Definition of cooking method and meat type

The definitions of the various cooking methods are listed in Table 1. In the analysis, we also regrouped these methods according to the potential to generate HCAs based on cooking temperatures cited in the literature. Hence, steaming and boiling were grouped as 'low' temperature methods; stewing and deep-frying as 'medium'; pan-frying, barbecuing/open roasting and grilling as 'high' temperature methods (Skog, 1993; Robbana-Barnat et al, 1996; Knize et al, 1999). Stir-frying was analyzed separately as there is insufficient data in the literature on the range of temperature achieved during cooking with this method. We also examined fresh and processed meat intake separately. In summing up intake of fresh meat, we excluded organ meats, for example, liver and kidneys, since these foods contain little creatine and therefore have relatively low concentrations of HCA when cooked (Robbana-Barnat et al, 1996). We also excluded meat products such as prepackaged meat-balls and fish-balls, which contain a high proportion, by weight, of flour. Processed meats in this analysis refers to foods that have been precooked, preserved or mixed with other foods prior to the packaging by the manufacturer. Some examples are Chinese sausages, luncheon meat, bacon slices, Western sausages, canned sardines and tuna, salted fish, dried shrimps, dried oysters or mussels, dried scallops and dried cuttlefish.

Table 1 - Definition of terms used to describe cooking methods in the study.

Full table

Statistical methods

Differences in meat intake between gender and dialect groups were tested using the Mann–Whitney U-test. All P-values reported are two-sided, and P-values of <0.05 were considered statistically significant. All analyses were performed using the SPSS 11.5 statistical package (SPSS Inc., Chicago, IL, USA).

Results

This study population represents a randomly generated subpopulation of the cohort of men and women in the Singapore Chinese Health Study. From the list of 1421 subjects, 986 agreed to participate, representing a response rate of 69%. There were a total of 400 men (40.6%) and 586 women (59.4%), with a dialect distribution of 609 (61.8%) Hokkiens and 377 (38.2%) Cantonese. The age range was 45–74 y, and the mean age was 58.0 (standard deviation s.d. 8.2) y. These distributions were very similar to the age and gender distribution in the original cohort (mean age 56.5 y, s.d. 8.0 y, 44.2% men and 55.8% women), but there was a higher proportion of Hokkiens (who comprised 53.7% in the parent population). The study population in this paper was also very similar to the parent population in the distribution of educational level (27% in both study populations had no formal education), marital status (85% in the current study vs 83% in the parent population were married and living with their spouses), birth place (71% in the current study vs 73% in parent study were born in Singapore) and length of residence in Singapore (98% in both populations had lived in Singapore for 25 y or more).

The mean intake of meat among the 986 subjects was 103.0 (s.d. 74.2) g/day. Fresh or nonprocessed meat constituted 97.2% by weight of all meats consumed. Table 2 shows the mean daily intake per person (g/day), standard error of the mean and the percentage of total meat intake of each type of meat by gender and dialect group among the 986 subjects. Fish, pork, poultry (chicken and duck) and crustaceans (primarily prawns/shrimp and cuttlefish) accounted for 98.7% of all meats consumed. Among these, fish and pork were the two most common types of meat in both dialect groups and genders, constituting 38.0 and 30.6%, respectively, among all meats in this study population. Poultry constituted 21.0%, the majority of the poultry being chicken (78% chicken vs 22% duck). Conversely, other meats like beef, lamb and mutton, which are commonly consumed in the Western populations, only constituted about 1% of all meats among the subjects.

Table 2 - Intake of fresh and processed meat by gender and dialect group among 986 Singapore Chinese men and women.

Full table

The mean intake of meat for men (121.8 g/day) was significantly higher than that of women (90.1 g/day, P<0.001). This difference extended to all meat types, particularly pork (P<0.001), fish (P=0.010) and poultry (P=0.053). In general, meat intake among Cantonese was higher than among Hokkiens in both genders (P=0.096), the main difference being in the intake of poultry (chicken and duck), which was markedly higher among Cantonese (P<0.001).

Table 3 describes the mean daily intake per person (g/day), standard error of the mean and percentage of total meat intake of the various meats by cooking method, and their relative contribution within each meat type. The main cooking methods for each type of meat were fairly similar among the four gender-dialect groups studied. Boiling or steaming was the most common cooking method in this population for fish (45.6%), pork (48.2%) and poultry (37.9%) by weight. Stir-frying was the most common method of cooking crustaceans, commonly prawns (45.9%), and was used to cook 18.5% of all pork and 13.5% of all poultry by weight. Pork and poultry were also cooked by open roasting/barbecuing or grilling; these methods accounted for a mean intake of 6.6 g of meat per day for each subject in our study population. Figure 1 shows the distribution by cooking method of the four most common meat types in this population. Among all study subjects combined, intake by weight of fish was highest, and 44.3% of fish by weight was also most likely to be cooked using high-temperature methods (specifically, pan-frying) (Figure 1). Documented high-temperature methods (open roasting/barbecuing, grilling, pan-frying), combined with stir-frying, accounted for 35.1% of pork, 25.6% of poultry and 47.2% of crustaceans consumed.

Figure 1.

Intake of meat by cooking method (weight in grams per person per day) among 986 Singapore Chinese men and women.

Full figure and legend (22K)

Table 3 - Meat intake by type, cooking method, gender and dialect group among 986 Singapore Chinese men and women.

Full table

Discussion

There is increasing evidence from basic and epidemiologic research of the potential role of HCAs as a cause of cancer in populations that consume a Western diet (Layton et al, 1995; Zimmerli et al, 2001). To our knowledge this is the first study to describe intake of meat and the cooking methods used in a Chinese population. The strengths of this study are that data covered both foods consumed at home as well as outside the home, and information on cooking method was complete. The use of in-person interviews for all 986 subjects also enabled standardized measurement of portion size, and interviewers were able to clarify any uncertainties regarding the dishes described by participants. We show that the highest proportion of meat eaten, by weight, comprises fish and pork, and identify the specific preparations which employ high-temperature methods and thus represent potentially significant sources of dietary HCAs.

Several characteristics of the study population will aid interpretation of the data. The study population, which represented a randomly generated subpopulation of a much larger population-based cohort of men and women from the Singapore Chinese Health Study, was very similar to the parent cohort in mean age, educational level, marital status, birthplace and length of residence in Singapore, all of which are factors that can affect choice of lifestyle and dietary habits.

Singapore is a tropical country situated one degree north of the equator. There is therefore no seasonal variation in the food consumed throughout the year other than a few food items associated with festivities, which are usually not consumed in any large amount. Singapore imports almost all meats from the neighboring countries, and common meats such as chicken, fish, seafood, pork, beef and mutton are freely available in the markets located in all residential estates. Most middle- and older-aged Chinese are either first- or second-generation Singaporeans. While many younger Chinese may consume a diet that is more 'Westernized', the older Chinese still retain closely the eating habits associated with their Chinese roots, in this case, from Fujian and Guangzhou, both in the southern part of the Chinese mainland. This is also inferred from the present study, in which very few of the 986 subjects reported having consumed traditionally 'Western' foods such as hamburger (one person), bacon (one person), Western sausages (three persons) and beef steak (zero persons) in their 24-h dietary recalls. Boiling and steaming, and stir-frying are characteristic of the regional cuisine in this area, and may explain the propensity towards these cooking methods. In addition, a large proportion of Chinese in Singapore are Buddhists or Taoists, and strict adherents may choose to abstain from beef as part of their religious practice.

The results of the present study indicate that, while mean meat intake among this study population lies within the range described for European populations in the EPIC cohorts, which ranged from 47 to 124 g/day for women, and from 79 to 234 g/day for men, the pattern of consumption differs markedly from populations in Europe (Linseisen et al, 2002), the United States (Byrne et al, 1998) and New Zealand (Ferguson, 2002). Compared with the United States (Byrne et al, 1998), Chinese in this population have higher fish consumption (US: 6–15 g/day, current study: 36–44 g/day), lower beef consumption (US: 20–30 g/day, current study 1 g/day), and fairly similar consumption of chicken (US NHS and HPFS data: 21–27 g/day, current study: 13–28 g/day).

With regard to cooking method, Chinese in Singapore appear to consume a larger proportion of their meat boiled or steamed, and a smaller proportion fried, roasted or grilled compared with Western populations. In the EPIC cohort, 28.1% of meat and fish consumption events assessed by 24-h recalls involved cooking by frying (including deep-frying and meat breaded or battered before frying), 14.2% involved boiling and 5.1% were classified as stir-frying (Rohrmann et al, 2002). In a Swedish population survey, 61% of the meat consumed was fried, 20% baked or roasted, 15% boiled and 4% grilled (Augustsson et al, 1999). Similarly, an estimated 47% of meat in the Swiss diet was fried, 19% grilled and 9% oven-roasted (Zimmerli et al, 2001). In the US, pan-frying was the most common method for cooking chicken (56%) and fish (54%); whereas between 34% (for steak) and 63% (for hamburgers) of beef was cooked by broiling/grilling (Bogen & Keating, 2001). Pan-fried meats have been identified as the largest source of HCA in the US diet, and chicken the largest source among the various meat types (Keating & Bogen, 2001). In contrast, studies in Japan show that grilled fish is a major dietary source of HCAs, and more than 50% of HCA intake is derived from this source (Kobayashi et al, 2002). Not surprisingly, most of the meat items for which HCA concentrations have been published in the literature are fried or grilled (Skog et al, 1998).

The relevance of cooking method to HCA formation lies in the maximum internal temperature attained as this parameter is thought to combine the processes of heat transfer, water loss and cooking-surface reactions, which in turn affect HCA formation (Skog et al, 1995). Significant amounts of HCAs begin to form in meats and model systems at temperatures of 150^oC (302^oF) or higher (Bjeldanes et al, 1982; Knize et al, 1985). Pan-frying, barbecuing and grilling/broiling are high-temperature cooking methods which heat food by radiative and conductive processes, and have the greatest potential to generate significant amounts of these compounds. Oven-roasting and baking are methods that cook meat by indirect convection and at lower temperatures, thus producing low or intermediate levels of HCAs. Boiling, steaming, and stewing without previous browning or braising are cooking methods at temperatures at or below 100^oC (212^oF) and usually produce insignificant amount of HCAs (Bjeldanes et al, 1983; Skog et al, 1998; Bogen and Keating, 2001).

While approximately half of the meat consumed by this population, being boiled or steamed, is unlikely to represent a significant source of HCAs, we document a propensity for pan-frying fish, which has been shown to produce high amounts of HCAs (Wakabayashi et al, 1993). Secondly, barbecued or roast pork (known locally as 'char siew' and 'siew yoke') is a significant pork item. This is traditionally prepared by grilling on a rotating spit over an open fire in a large drum till parts of the surface are crispy or burnt.

Our data also show that stir-frying is a common method used to cook pork and chicken, and therefore needs to be explored further. In stir-frying, all ingredients including meat, are finely chopped or sliced before cooking, and a large surface area is consequently exposed to heat. The oil is first heated in a wok or skillet till smoking, the essence of the technique being that the temperature should be high enough for the food to be cooked very quickly, often in a matter of minutes or even seconds, to retain the moisture and flavor. As the food is fried, it is stirred almost continuously so that the hot oil covers every part of the food, hence its name. Recent work suggests that flipping of meat patties lowers the surface temperature intermittently and may reduce the amount of HCAs formed (Salmon et al, 2000), although simulation studies could not fully explain these reductions based on a thermal (cooking time or temperature-related) effect (Tran et al, 2002). While more studies on the mechanism of this effect are needed, it remains possible that the effect of very high temperatures may in fact be mitigated by the short cooking time and constant stirring. Stir-frying remains an important and unique aspect of the Chinese diet and its significance in relation to HCA exposure deserves further study.

Finally, in epidemiologic studies where HCA exposure needs to be estimated from dietary recalls or questionnaire data, surface browning or 'done-ness' of meat is widely accepted as a surrogate for cooking time, weight loss, and indirectly for HCA content, primarily for beef and bacon (Sinha and Rothman, 1997; Skog et al, 1998). This indicator has a more limited utility where fish, pork and to a lesser extent, chicken, are concerned, since these meats do not share the same variation in 'done-ness' as beef and bacon.

In conclusion, our findings highlight aspects of the Chinese diet which are relevant to HCA formation, and call for novel approaches to estimating individual exposure to dietary HCAs. We believe these results provide a framework for the development of a database representative of the Chinese diet that will facilitate further investigation into their association with cancer risk in this, and similar populations.

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Acknowledgements

This study was supported by the National Medical Research Council in Singapore (NMRC 0521/2001). The Singapore Chinese Health Study is supported by National Cancer Institute Grants R01 CA55069, R35 CA53890 and R01 CA80205. (P.I. Mimi C. Yu).