Plants contain a wide range of biologically active compounds, some of which are known as phytochemicals. There may be as many as 100000 different compounds, which determine particular properties in plants, and in the fruits and vegetables they produce, such as flavor and color. Phytochemicals are classified according to their chemical structure and functional characteristics, and include salicylates, phytosterols, saponins, glucosinolates, polyphenols, protease inhibitors, monoterpenes, phytoestrogens, sulphides, terpenes, and lectins.

It is widely believed that the health benefits of diets high in fruits and vegetables are likely to be due partly to the presence of phytochemicals. For instance, several act as antioxidants, preventing oxidative damage to cells, proteins, and DNA. It is likely that other bioactive phytochemicals have yet to be identified, and those that are known may have additional properties in the body that are not yet understood. But it is thought that nutrients, phytochemicals, and other, as yet unknown, bioactive components act together to influence physiological responses.

Although many phytochemicals are bioactive, they are not essential in the diet and there is no daily requirement, so they are not classed as nutrients. Humans have developed tastes for some phytochemicals, such as the hot flavors of mustard oil, bitter alkaloids, and irritating capsaicins. There is genetically inherited variation in sensitivity to some tastes, for example, the bitter taste of isothiocynates in cruciferous vegetables such as cabbage.

Examples of phytochemicals which can prevent cancer:

• From veggie burgers to milk to protein bars, soy beans are known as a nutrient rich ingredient- there are studies showing soy bean can prevent prostate cancers.
• Pomegranate phytochemical urolithin B suppresses estrogen production and that prevents the proliferation of breast cancer cells and the growth of estrogen-responsive tumors- Cancer Prevention Research, Jan 2010.
• Phytochemicals (vitamins A, C, K, folate, carotenoids and lutein) from leafy veggies and green tea showed lower levels of genetic changes known to give rise to lung cancer in smokers-Cancer Research, Jan 2010.
• Apples contain natural phytochemicals that have a protective effect against liver, breast and colon cancer-Journal of Agricultural and Food Chemistry, Apr 2008.
• Mangos are known to be rich in vitamins C and A, as well as fiber. Polyphenolic compounds in mangos known as gallotannins, a class of natural bioactive compounds believed to help prevent or block the growth of cancer cells-Jan 2010.  http://agnews.tamu.edu/showstory.php?id=1686

Reference: Reports from American Institute for Cancer Research

Freezing and cooling by use of natural ice and snow is a method of food preservation traditionally available only in cold climates or in winter in temperate climates. Natural ice refrigeration on an industrial scale first developed in the late 19^th century, when refrigerated containers used in trains, ships, and then later trucks, greatly increased the production and consumption of red meat. Domestic freezing, chilling, and refrigeration on a mass scale is a phenomenon mostly of the second half of the 20^th century.

Today, much perishable food is solid frozen or chilled. Together with the growth of industrial refrigeration, domestic refrigerators began to be used in the USA, Australia, and New Zealand on the scale in the 1920s, and the Europe and Japan mostly since the 1950s. In Japan, for example, household processing refrigerators increased from 9 per cent in 1960 to 91 per cent in 1970, and 99 per cent in 2004. Supermarkets with freezers, chill cabinets, and domestic refrigerators are now commonable in the cities and towns of tropical countries; poorer rural communities still rely on drying, fermenting, salting, bottling, tinning, and other methods of food preservation, as well as their own gardens and farms. It is unlikely that refrigeration itself has any direct effect on the risk of cancer. Their effects are indirect.

• Refrigeration enables consumption of fresh perishable foods including seasonal vegetables and fruits all year round, as well as of fresh meat.
• Refrigeration reduces microbial and fungal contamination of perishable foods, notably cereals (grains) and pulses (legumes).
• Refrigeration reduces the need for and use of salting, smoking, curing, and pickling as methods of preserving vegetables, fruits and meat.

It can therefore be said that refrigeration (including freezing and chilling) indirectly influences risk of those cancers, the risk of which is affected by the above factors.

Evidence mounting to a judgement of ‘convincing’ or ‘probable’ for such factors relates to cancers of the mouth, pharynx, larynx, masopharynx, oesophagus, lung, stomach, pancreas, liver, and colon.

Reference: Reports from World Cancer Research Fund and American Institute for Cancer Research

  Mycotoxins are toxins produced by certain moulds or fungi. Although moulds that contaminate foods are usually destroyed by cooking temperatures, the toxins they produce may remain. Aflatoxins are one type of mycotoxin. All naturally occurring aflatoxins are classified as human carcinogens (group 1) by International Agency for Research on Cancer; other mycotoxins, such as fumonisins, are suspected carcinogens. It is common to find co-contamination by more than one species of mycotoxin-producing fungus. In Europe, the joint FAO/WHO expert committee on Food Additives and Contaminants recommends that aflatoxin concentrations in foods be kept as low as possible.

The main foods that may be contaminated by aflatoxins are all types of cereal (grain), including wheat, rice, maize (corn), barley, and oats; and pulses (legumes)-notably peanuts. Nuts and seeds may also be contaminated. Feedstuffs for farm animals may also be contaminated with aflatoxins, which can then be secreted in milk or accumulated in tissues.

Aflatoxins, which are produced by Aspergillus flavus and A. parasiticus, are most problematic in countries with hot, damp climates and poor storage facilities. Under these conditions, food may become contaminated with fungi and then accumulate such toxins. Such foods are marketed and consumed in the countries in which they are produced; they are also exported to neighboring countries and intercontinentally. Aflatoxin contamination is therefore an international issue.

Levels of aflatoxin contamination tend to be highest in countries where rates of liver cancer are high, such as some African countries and South-East Asia, including China. In general, rates are low in Europe, but relatively high rates of contamination have on occasion been found in the USA. Aflatoxin exposure levels are low in Europe and Australia, higher in the USA, and high in many low-income countries. This is particularly the case in tropical and subtropical regions where grains and nuts are stored for long periods under non-ideal conditions.

Rates are reduced by inspection, use of fungicides, and screening of imported foods. However, monitoring of levels of aflatoxin contamination in low-income countries is generally lacking.

Excess energy from food is stored as fat in the body in adipose tissue. The amount of this body tissue varies more from person to person than any other type (such as muscle, bone or blood). The size and location of these fat stores also vary considerably between populations, people, and over the course of a person’s life.

Fat is not distributed equally around the body. It accumulates subcutaneously (beneath the skin) around the muscles of the upper arm, buttocks, belly, hips, and thighs. It also accumulates intra-abdominally or viscerally (around the organs). Increases in body weight during adulthood depend mostly on accumulation of fat rather than lean tissue, and therefore any change may better reflect fatness than adult attained weight itself, which is more dependent on lean mass.

The evidence that greater body fatness is a cause of adenocarcinoma of the oesophagus, and cancers of the pancreas, colon, breast, endometrium, and kidney is convincing. Some of the mechanisms by which body fatness increases the risk of cancer are well understood.

Obesity influences the levels of a number of hormones and growth factors. Insulin-like growth factor 1 (IGF-1), insulin, and leptin are all elevated in obese people, and can promote the growth of cancer cells. In addition, insulin resistance is increased, in particular by abdominal fatness, and the pancreas compensates by increasing insulin production. This hyperinsulinaemia increases the risk of cancers of the colon and endometrium, and possibly of the pancreas and kidney. Increased circulating leptin levels in obese individuals are associated with colorectal and prostate cancers.

Sex steroid hormones, including oestrogens, androgens, and progesterone are likely to play a role in obesity and cancer. Adipose tissue is the main site of oestrogen synthesis in men and postmenopausal women. The increased insulin and IGF-1 levels that accompany body fatness result in increased oestradiol in men and women, and may also result in higher testosterone levels in women. Increased levels of sex steroids are strongly associated with risk of endometrial and postmenopausal breast cancers, and may impact on colon and other cancers.

Obesity is characterized by a low-grade chronic inflammatory state, with up to 40 per cent of fat tissue comprising macrophages. The adipocyte (fat cell) produces pro-inflammatory factors, and obese individuals have elevated concentrations of circulating tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and C-reactive protein, compared with lean people, as well as of leptin, which also functions as an inflammatory cytokine. Such chronic inflammation can promote cancer development.  

Figure. The influences of obesity, insulin, leptin, oestrogen and insulin-like growth factor (RED color) on the cancer process.

Reference: World Cancer Research Fund International and the American Institute for Cancer Research reports.

Any chemicals that have a useful function in the production, processing, or preservation of foods or drinks may nevertheless be toxic, and possibly mutagenic or carcinogenic. For this reason, food additives and contaminants, such as traces of chemicals used in industrial agricultural production, are subject to international and national surveillance and regulations.

They are a cause for concern and vigilance because some, and in particular agricultural chemicals, are known to be toxic in experimental settings, though at levels well above those found in foods and drinks.

There is little epidemiological evidence on the possible effects of contaminants and additives as present in foods and drinks.

Because contaminants and additives are subject to international and national regulation, there is a vast amount of toxicological information from experiments on laboratory animals and other settings. Failing any other method, it seems reasonable to observe the effects of food additives and contaminants on laboratory animals at levels greatly in excess of any likely to be present in foods and drinks; and based on several assumptions and judgements, to set limits for safety in use. When such limits are used as regulatory limits, they are also subject to surveillance and special investigation when any chemical present in foods and drinks seems to be a cause for special concern.

This are remains controversial. Theoretically, it would be ideal if food supplies contained no trace of any toxic substance, including those that are or may be mutagenic or carcinogenic. However, some foods in nature contain carcinogens and the issue is not confined to methods of industrial food processing.