Nasopharyngeal cancer – Influence of fruit and preserved foods

Nasopharyngeal cancer (NPC) shows a very skewed racial and geographic distribution. The incidence rate among Caucasian males in Westernised societies is <1/100,000 but it can be 20-40 in southern Chinese. Intermediate rates are found in several indigenous populations in Southeast Asia, and in natives of the Arctic region, North Africa, and the Middle East. A study on Chinese immigrants in California showed that the risk of NPC for the third generation was less than half that for the second generation but was still 10-fold greater than Caucasians. Alternatively, Caucasians born and raised in southern China have increased rates compared to those remaining in western countries. These findings indicate both genetic and environmental factors contribute to the disease, and several candidate genes have now been identified. About 90% of the world population is infected with the EpsteinBarr virus at a young age and this also has been shown to be a necessary but not sufficient factor in the onset of NPC. Incidence in males is 2-3 times that in females, and the effect of smoking on NPC is an order of magnitude less than with lung cancer or other respiratory tract malignancies.

If NPC is relatively rare in Caucasians, perhaps we in Australia should focus our health-directed efforts on other more pressing problem areas? But we are a multicultural society with an ever-increasing ethnic population, and to our benefit and enjoyment these people have very successfully introduced their national cuisines to the wider community here. Who doesn’t enjoy going to a Chinese restaurant or cooking up one of their traditional meals? However a number of these foods and cooking practices were shown more than 40 years ago to be strong risk factors for NPC, and to the extent that we consume these meals we may all be subject to them, with resulting increased risk of NPC. The following summary of a study in the southern province of Guangdong, China (BMC Cancer 2010, 10:446) describes some of the dietary factors that can exacerbate or minimise the disease.

Nasopharyngeal carcinoma (NPC) is rare in most parts of the world but is a common malignancy in southern China, especially in Guangdong. Dietary habit is regarded as an important modifier of NPC risk in several endemic areas and may partially explain the geographic distribution of NPC incidence. In China, rapid economic development during the past few decades has changed the predominant lifestyle and dietary habits of the Chinese considerably, requiring a reassessment of diet and its potential influence on NPC risk in this NPC-endemic area. To evaluate the association between dietary factors and NPC risk in Guangdong, a large-scale, hospital-based case-control study was conducted. 1387 eligible cases and 1459 frequency matched controls were recruited. Odds ratios (ORs) were estimated using a logistic regression model adjusting for age, sex, education, dialect, and habitation household type. Observations made included the following.
Consumption of:
  • Canton-style salted fish, preserved vegetables and preserved/cured meat were each significantly   associated with increased risk of NPC, with enhanced ORs of 2.45, 3.17 and 2.09 respectively   in the highest intake frequency stratum during childhood. The effect for preserved/cured meat   was more pronounced in adults (OR of 9.69) than in children;
  • fresh fruit was associated with reduced risk and a significant dose-dependent relationship with   ORs of 0.38 and 0.12 for monthly and weekly or more consumption respectively during   childhood;
  • fermented pastes showed some evidence of enhanced NPC in childhood, and
  • Canton-style herbal tea and slow-cooked herbal soup were significantly associated with   decreased risk, with ORs of 0.84 and 0.58 respectively in the highest intake frequency stratum.
 In multivariate analyses, these associations remained significant. It can be inferred that despite rapid   urbanisation and changing dietary habits, previously established dietary risk factors in the Cantonese  population are still contributing to a high incidence of NPC.

This was a case control study where people with confirmed NPC were compared with subjects of similar age, gender, education etc but who did not have NPC. The reliability of this comparison is always dependent on how well the healthy controls match the case cohort or whether it is unknowingly influenced by other factors not considered. Prospective studies where healthy people are entered into a study, then followed for a certain time during which cases develop, and then identifying factors that were different between cases and those who remained healthy, provide stronger evidence. Nevertheless, when case-control studies with similar findings in many different studies are considered together they can provide valuable information.

To facilitate statistical analysis in the above work, intake levels for particular foods were grouped into three levels – less than monthly (serving as the reference), monthly, and weekly or more. Children defined as <12 years old were generally more susceptible to both positive and negative dietary effects than adults. Interviewing the mothers and family of people who developed NPC revealed that salted fish was one of the most common solid foods given to children when weaning, and these NPC cases were more likely to have other members in the family with NPC. The most extreme OR of 9.69 was for adults consuming preserved/cured meats in the highest strata of ‘weekly or more often’, highlighting the danger of eating these foods. It represents almost a 10 times greater risk of developing NPC, and in earlier studies this has been >40-fold. The effect of preserved vegetables and fermented pastes was not as pronounced, but still cautions against too frequent consumption.

Teas, herbs and spices have some of the highest antioxidant phytochemical levels of all plant-based foods, and since they’re thought to play a central role in minimising cancers, it might be expected that they would be of benefit in NPC. Their impact is muted because the amounts typically consumed are much smaller than for other staple foods, but they can still represent the major source of antioxidants when diets are insufficiently balanced and adequate. Levels in a given plant species vary greatly depending on geography, variety, climatic and soil conditions, fertilisation and management, harvest, storage conditions etc, plus when consumed with other foods in a meal, absorption and effects may be variably compromised. This scenario is further complicated when several herbs are involved and the effectiveness and reproducibility of products becomes increasingly unpredictable. As a result it’s not surprising that the effects of Chinese teas and soups made from 30 different species as in this study may have shown beneficial effects whereas in others the reverse has been seen.

As with all cultures worldwide, there has been a pressing need in the past to be able to preserve foods in times of plenty to cover periods when less was available. Historically the principal means of achieving this were dehydration (sun-dried), smoking, fermentation and salting. We now know that all these have their drawbacks nutritionally and newer less-destructive or harmful techniques such as autoclaving and pasteurisation, spray drying, micro-wave treatment, high pressure sterilisation, gamma radiation, freeze drying etc have been introduced. But the foods produced by the traditional techniques have been so strongly integrated into national cuisines over centuries that it’s difficult to achieve meaningful changes – old habits and practices that should be abandoned or minimised prove very resistant. There has been some reduction in incidence of NPC in southern China following gradual adoption of foods subject to more modern processing techniques and the advice of public health authorities, but NPC incidence still remains much higher than in the West.

With still incomplete understanding of the disease, appropriate dietary habits remain the principal means of prevention. The damage from traditionally processed foods in NPC is development of carcinogens such as nitrosamines which may then be eaten and/or inhaled. Salt preserved foods are a dietary staple in all endemic NPC populations, with increased incidence in lower socioeconomic groups less aware or receptive to public health advice. The message from the present study and others suggests that consumption of preserved/cured meats and other salted products, including eggs, fruit and vegetables should be limited, particularly in childhood, however much loved or enjoyed. Eating fresh fruit regularly can do wonders for prevention, reducing the risk of NPC by about 90%.