Introduction

LINDA VIKSTRÖM BERGANDER AND INGEMAR PONGRATZ

Department of Biosciences and Nutrition, Karolinska Institute, SE-141 83 Huddinge, Sweden

1.1 General Introduction

Food consumption is a global issue involving a complex chain of food producers, food handling, transporting and packaging, among others. Today, there is a substantial knowledge of the various hazards ending up in foodstuffs. These hazards range from simple physical hazards to biological hazards, including pathogenic bacteria and naturally occurring toxins, as well as chemical hazards such as pesticides and heavy metals. There is a large heterogeneous group of compounds present, both naturally and man made, in the environment that is causing adverse health effects. These chemical compounds that disturb hormonal pathways are often known as endocrine disrupting chemicals (EDCs).

Exposure to chemical contaminations from the diet is the main critical route for humans, as well as wildlife, to persistent bioaccumulative (fat-soluble compounds with a tendency to build up and reach high levels in an organism) EDCs. Basically, the hormonal or endocrine disruptors are chemicals with the potential to interfere with the function of endocrine systems. Thus, this book will be focusing on diet-derived hazardous substances that disturb/influence nuclear receptor signaling and thereby target the hormonal systems.

1.1.1 Endocrine Disruptive Chemicals

Environmental pollutants and their effects on the environment, humans and animals are a significant concern in today's society. During recent years there has been substantial awareness that a variety of environmental pollutants can intervene with the hormonal system. Many man-introduced compounds influence the hormonal system of animals and may be responsible for developmental and reproductive abnormalities seen in wildlife. Natural sources of EDCs are present in various types of foods and are susceptible to metabolic degradation; however, synthetic industrial chemicals, such as inorganic contaminants, agrochemicals, industrial chemicals, plasticizers, plastics, and pharmaceutical agents, that leak into the soil, have the ability to end up in the food chain and thereby bioaccumulate in animals and humans.

The term endocrine disruptor was evolved at the Wingfield meeting in 1991, where a group of researchers with diverse backgrounds was united to discuss the effects of mammalian exposure to environmental chemicals. As a result of the meeting, a consensus statement was set by the participants: "We are certain of the following: a large number of man-made chemicals that have been released into the environment, as well as a few natural ones, have the potential to disrupt the endocrine system of animals, including humans". It was also concluded that the effects of such chemicals are diverse when comparing embryo, fetus, and perinatal organisms to adults and that detectable effects are commonly seen solely in the offspring. Later on, the so-called "endocrine disruptor hypothesis" was published in the book Our Stolen Future, which essentially claimed that certain synthetic chemicals interfere with hormone synthesis and, thus, disrupt endocrine networks in animals and humans. To clarify the concept of EDCs, the U.S. Environmental Protection Agency (EPA) defined EDCs as "exogenous agents that interfere with the production, release, transport, metabolism, binding action, or elimination of the natural hormones in the body, responsible for the maintenance of homeostasis reproduction and the regulation of developmental processes".

In 1962, prior to the Wingfield meeting, Rachel Carlson wrote the alarming book Silent Spring. This groundbreaking book recapitulates a small fictional world on the road to ruin as a result of accumulated separate disasters; how- ever, all were picked from real life. She discussed the widespread use and the danger of environmental chemicals, such as pesticides and herbicides, on wildlife development and reproduction. This warning for man-made chemicals was first of a kind and, hence, a precursor to the debates on the use of chemical pesticides that later on would result in a ban of the heavily used insecticide DDT, as well as polychlorinated biphenyls (PCBs) in the USA.

1.1.2 Biological Pathways Affected by EDCs

The scientific community has become increasingly concerned that humans experience health problems and wildlife populations are adversely affected following exposure to chemicals that interact with the endocrine system. A well functioning endocrine system, a hormonal balance, is a central function and a key issue for maintaining physiological homeostasis and a healthy body. One hormone in imbalance affects other hormones in the body.

The basics of the endocrine system are a number of glands that secrete the chemical messages that we call hormones. The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal body, and the reproductive organs (ovaries and testes). These glands release a diversity of hormones directly into the bloodstream, where they target an organ and thereby regulate various processes, like growth, metabolism, development, reproduction, and sexual characteristics.

Hormones exert their action through a range of receptors by a lock-and-key model. These receptors are either (i) membrane bound and linked to ion channels, G-proteins, or enzymes or (ii) intracellular and localized in the nucleus or the cytosol. The membrane-bound receptors mediate the cellular response to hormones either by a depolarization of the membrane or by the generation of so-called second messengers or signal transducing molecules. The intracellular receptors, on the other hand, mediate the cellular response by modulating gene expression in target cells.

Absolute receptor specificity is rarely encountered and, hence, hormone receptors may bind exogenous compounds other than their primary endogenous ligands. By this means, a fraction of exogenous pharmaceuticals, as well as agricultural/industrial chemicals or EDCs released into the environment, may bind to hormone receptors and activate the receptor in a manner similar to endogenous compounds. These compounds may also interfere with the binding and actions of endogenous ligands without activating the receptor itself.

The basis for endocrine disruption is not fully known; however, knowledge of the mechanism of action of the hormonal-disrupting chemicals is advancing. Basically, it has developed from a narrow hormone receptor point-of-view to a broader approach of targets related to nuclear receptors, non-nuclear steroid hormone receptors, non-steroid receptors, and orphan receptors, as well as enzymatic pathways concerning steroid metabolism. With the wide range of EDC actions, it is hard to distinguish if the diversity of the biological end points induced by EDCs is due to direct or indirect effects of EDC exposure. However, it has been noticed that a majority of these compounds work by mimicking or interfering with the normal actions of endocrine hormones, including estrogens, androgens, thyroid, hypothalamic, and pituitary hormones. Today, chemicals that mimic or antagonize the female estrogenic hormones, the male androgenic hormones, or the thyroid hormones are gaining the most attention.

1.2 Chemicals Contaminating Our Food

There is a broad spectrum of compounds with a wide range of physical properties compromising endocrine disrupting qualities. Owing to the heterogeneity of EDCs, the only similarity being small molecular masses, it is hard to predict possible endocrine disrupting actions of chemicals. Also, as only a fraction of all potentially physiological disrupting compounds in the environment has been investigated, it is challenging to develop techniques to improve the analysis of these compounds. However, the European Union has summarized a report categorizing chemicals on the basis of available evidence of endocrine disrupting effects. Out of 146 high production volume chemicals and/or highly persistent substances, a group of 60 compounds were considered to have high exposure risks regarding endocrine disruption.

As previously described, a large diverse group of hormone-disruptive chemicals is present naturally, as well as man introduced, in the environment. The natural source of EDCs present in various types of foods has been termed phytoestrogen and defined as any plant compound structurally and/or functionally similar to ovarian and placental estrogen and its active metabolites. Apart from the beneficial health effects of phytoestrogens, including the prevention of cancer, atherosclerosis, menopausal syndromes, and bone density loss, adverse health effects of phytoestrogens are emerging with a potential for endocrine disruption.

The phytoestrogens are divided into two major classes: the polyphenolic flavonoids and the lignans. Among all, the most well-known phytoestrogens are the soy and chickpea isoflavones genestein and daidzein and the clover-derived comuestrol, as well as the lignans, mainly found in grains, seeds, and other fiber-rich foods. Flavonoids are highly consumed by the Asian population whereas the lignans are generally more consumed by Europeans. Both flavonoids and lignans are, however, in general ingested as precursors and converted into active compounds by the microbial system.

As described above, the synthetic group of EDCs is widespread in the ecosystem and a variety of these chemicals has been designed to be long lasting in the environment and are, therefore, not easily degraded. Chemicals that were banned a long time ago are still found in the ecosystem, even at locations far away from where they were initially utilized. Typical synthetic contaminants found in the food chain are industrial chemicals, like combustion by-products including PCBs and dioxins, the polybrominated flame-retardants (PBB and PBDEs), and biocides and pesticides including tributyltin and DDT. Other contaminants in food are inorganic compounds like heavy metals and metalloids, such as mercury, cadmium, lead, and arsenic, as well as pharmaceutical or synthetic hormones, such as diethylstilbestrol (DES). A more recently emerging food-contaminating group is the chemicals originating from packing materials. Phthalates, used as plasticizers, and the plastic monomer bisphenol A (BPA), a high production chemical used all over the world, are leaking out from packing materials, subsequently resulting in animal and human exposure. Other emerging chemical contaminants are the ubiquitously used polyfluorinated chemicals (PFCs), like perfluorooctanesulfonic acid (PFOS), in non-stick coatings and food packing. Recent reports are showing bioaccumulation in wildlife and humans of PFOS, as well as endocrine disrupting properties.

1.2.1 Sources and Routes of Food Contaminating Chemicals

Over the past century, humans have introduced substantial amounts of chemical substances into the environment, all with an unpleasant ability to enter the body by absorption. The exposure route proceeds by means of inhalation, i.e. absorption through the lungs, absorption through the skin and, most importantly, oral ingestion and absorption through the digestive system. Chemical hazards can be found in the natural environment (air, water, soil) from industrial and environmental pollution. They also exist in food products as natural chemicals or chemicals produced during manufacturing and processing procedures and eventually arise in the food supply. All of these chemicals have a risk to affect health adversely. Fetuses, children, and adults are all at high risk of exposure to chemicals originating from contaminated food absorbed in the digestive system and ending up in blood and stored in tissues.

High exposure of persistent EDCs is associated with high consumption of fatty food; consequently a reduction of dietary fat should correspond to a reduction of organic contaminants. Chemicals are preferentially eliminated from the body by making them water soluble and ready for excretion. Some chemicals, the hydrophobic or fat soluble, prefer fatty surroundings and accumulate in tissues rather than being extracted as a water-soluble product. This means that EDCs have the ability to accumulate and thereby concentrate in tissues and that a low level of chemicals in water, soil, or plants can be concentrated higher up in the food chain owing to elevated consumption. One of the major routes of removing accumulated chemicals is through breast milk to a nursing baby. According to a recent study, there is a country-specific pattern of EDCs in breast milk, where the Danish population has a higher exposure to persistent bioaccumulative chemicals than, for example, Finnish mothers. These results are interestingly correlated with a higher frequency of male endocrine disrupting disorders in Denmark.

In 2006, the WWF launched a report about the food link in the chain of contamination. A wide range of food items, like dairy products, meat, fish, bread, honey, and olive oil, selected from seven EU countries, was analyzed and toxic residues were found in all products. High amounts of phthalates were detected in olive oil, PBDEs in minced beef, and DDE, PFOS, and PCBs in pickled herring.

Mercury has been well known as an environmental pollutant for several decades. It is a global pollutant of major concern with numerous environmental sources, such as the mining and charcoal industries as well as the healthcare sector. Mercury enters the food chain as a more toxic form, methylmercury (MeHg). It is converted from elemental mercury released into the environment by bacteria and bioaccumulates, especially in fish. Exposure to MeHg in vertebrates results in, among all, embryo toxicity, endocrine disruption, and altered reproductive behavior. In the early 1970s a major MeHg poisoning catastrophe occurred in Iraq, owing to the use of MeHg as a fungicide for treatment of seed grain. Interestingly, a recent publication described mercury as a compound that radically changes birds' mating behavior and a cause of homosexuality in male ibises.

The naturally occurring EDCs, or the phytoestrogens, described above are bioactive compounds structurally and/or functionally similar to the endogenous estrogen and its active metabolites and, thus, have hormone-like activity. Phytoestrogens usually show a weaker estrogenic activity than the endogenous hormones but are highly consumed, especially soybeans, nuts, and seeds. Hence, there is an emerging concern regarding the daily exposure to soy infant formulas, which may result in exposure of infants to high amounts of isoflavones.

1.2.2 Vital Topics Regarding Dose and Mixture Effects

A lot of evidence has been presented during the years regarding the large number of chemicals that pose a risk to human and environmental health. However, the full range of EDCs present in the environment remains largely unknown. The knowledge on EDCs is complicated when considering the effects of chronic low-dose exposure of chemicals through the diet. Extra care should be taken when addressing the exposure during early life, as fetus, infants, and young children, since chronic low-concentration exposure to EDCs is often seen in long-term health effects. In that way, as reproductive disorders typically affect younger individuals, the implications may only become apparent many years later. The concern for chronic low exposure of EDCs is also amplified when considering exposure to a mixture of compounds. We are all exposed, daily, to a cocktail of chemicals, although the knowledge of such combinational exposure is still under elucidation and the regulations of chemicals in food are based on tests of the individual compounds. However, there is growing concern that the substances in combination may cause a greater risk compared to exposure to individual substances. Toxic effects may occur during simultaneous exposure of chemicals owing to chemical interactions that alter the absorption, biotransformation, or excretion of one or both of the interacting chemicals. Up to now, risk assessments for toxicity are set on separate chemicals based on NOAEL, "no adverse effect level". This means the highest dose at which no adverse effects have been detected; however, combinational effects can be expected even at doses well below NOAELs, provided a sufficiently large number of chemicals is present. Different chemicals in a mixture could affect each other in either an antagonistic (weakening) way or in a synergistic way, i.e. the combined effects are stronger than the additive effect given the knowledge of each chemical's toxic quality alone.