1 Introduction

For general aspects of food, see → Foods, 1. Survey; for food additives, see → Foods, 3. Food Additives. In general, the human diet provides all nutrients essential for human metabolism to maintain optimal health and physical performance. However, the awareness of the importance of the nutrients for human health is increasing in the population, and scientific evidence is growing that some food components other than the recognized classical nutrients may have additional beneficial effects. In particular, nutrition science considers the human diet not only as a way to avoid nutrient deficiencies, but also as the concept of providing the human body with components that are biologically active and potentially beneficial for health. As many traditional food products contain such biologically active components, new foods are developed typically by increasing the amounts of potentially beneficial components in these foods, either by using sources different from those for traditional foods or by modifying the composition of the foods through various techniques. This category of foods with higher amounts of potentially beneficial components is called functional foods or health value added foods (1).

Many consider the term functional food as a mere marketing term. This is because all food can be considered functional or as having an inherent health value by providing energy and nutrients required to maintain biological function (2). The term nutraceutical is frequently used interchangeably with functional food, but nutraceutical refers to any biologically active component in supplemental form, whereas functional food refers to components solely given in food form. However, there are still considerable overlaps in the use and practical application of both terms (3). In contrast, the terms medical food and dietary supplement are not considered functional foods, with medical foods being applied by a medical doctor for the management of a disease and dietary supplements being a nonfood product intended to supplement the diet.

2 Definitions and Categories of Functional Foods

As functional foods as a food category are not defined by legislation yet, there is no regulation on the exact wording. According to the European Food Information Council, functional foods are considered as foods intended to be consumed as part of the normal diet, which contain certain biologically active components offering the potential of enhanced health or reduced risk of disease (4). It is, however, not clearly defined what type of biological activity a component has to provide to be classified as biologically active and what exactly is understood by the term normal diet.

Several definitions of functional foods from different bodies and organizations exist, and the Academy of Nutrition and Dietetics collated all these definitions as listed in Table 1 (5). The consensus definition largely covers this wide field (1): “A food can be considered as being functional, the moment it can be satisfactorily demonstrated that it affects beneficially one or more target functions in the host body, beyond an adequate nutritional effect, to improve the state of health or well-being, or reduces the risk of a disease.” Based on this definition, functional foods have to go beyond the basic functions of enriched or fortified foods (e.g., vitamins, minerals). Furthermore, it must be possible to consume this foodstuff in quantities comparable and compatible with a normal, well-balanced, and varied diet.

A more detailed classification of functional foods is found in Table 2 (8).

Examples of each of these categories of functional foods can be found in the common marketplaces (6).

The simplest type of functional food are foods that have been fortified or enriched with additional nutrients, either commonly present in the original food or originating from other foods. Examples for this category are breakfast cereals fortified with folic acid, various fruit juices with addition of vitamin C, or dairy products with higher concentration of calcium, etc. Enriched foods, containing additional components normally not found in large amounts in this food, have also been introduced on the market for some decades now. Fruit juices containing additional calcium or margarine spreads containing phytosterols, which have been shown to decrease blood cholesterol levels, are some examples.

Foods can also be altered with respect to particular food ingredients that are replaced by potentially more beneficial components, ideally without having any impact on food quality, sensory characteristics, and consumer acceptance. Fat replacers, such as sucrose esterified with fatty acids, providing similar properties as fat, but largely being indigestible and thus not providing the same amount of energy, have been introduced onto the market, although economically rather unsuccessfully.

A further category of functional foods are foods that have been developed by modern biotechnology (→ Biotechnology, 6. Special Applications). In this way, high-lysine food plants and grains with high amounts of carotenoids such as golden rice have been produced, with the aim of improving the intake of critical nutrients via foods that are consumed traditionally by many individuals who do not have access to those nutrients from other sources due to low availability or low affordability.

New technologies such as microencapsulation also lead to functional food. They aim at providing new ways to increase the bioavailability of naturally occurring bioactive compounds. Isoflavons and polyphenols extracted from rich sources such as black currant or even byproducts such as pomegranate peel have been packed into small maltodextrin microcapsules through spray drying, which may offer the potential of bioactive components to be better bioavailable than in their naturally occurring form.

Besides the examples indicated above, functional foods have gained public awareness by the introduction of specific microorganisms with potential beneficial effects classified as probiotics. Up to now, much scientific and public interest has focused on these probiotics (5-10). Although these theories of modulation of the intestinal ecosystem were introduced almost a hundred years ago (11), only recently has the scientific basis of probiotics been established and sound clinical studies on some bacterial strains have been published, providing a good understanding of the physiological and nutritional properties of some strains. Thus, it was verified that some strains are probiotic, with documented and well-published examples of their ability to maintain and promote the health of their host (12, 13). In contrast, the authorities in Europe responsible for the approval of health claims have not accepted yet any health claim related to the benefits of probiotics and numerous applications have either been redrawn or have received negative opinions.

3 Prebiotics, Probiotics, and Synbiotics

Probiotics are probably the category of functional food with the largest market share. Probiotics have been defined by the FAO/WHO as microorganisms capable of surviving the passage through the digestive tract, capable of proliferating in the gut, and able to confer defined health benefits to the host. Consequently, a food containing a probiotic (i.e., a living bacterium) in line with this definition is a functional food. The most commonly used probiotics are lactic acid bacteria, such as lactobacilli or bifidobacteria, which are added to fermented milk products or are responsible for the fermentation of the milk matrix itself, or they are given as a supplement in a lyophilized form (→ Milk and Dairy Products)

In addition to the probiotics, the term prebiotic has been introduced. Prebiotics are fermentable, nonviable food components (dietary fibers), which demonstrate a beneficial effect on the intestinal microflora of the host. This benefit to the host is mediated through a selective stimulation of the growth and/or activity of one or a limited number of species of the colonic microflora. As relevant substances, dietary fibers, lactulose, fructo- and galacto-oligosaccharides, and inulin (e.g., chicory extract) have been identified (→ Cereals and Cereal Products, → Inulin). They must not be hydrolyzed or absorbed in the upper gastrointestinal tract. In general, they must alter the microbiota in the colon to a healthier composition and should induce luminal or systematic effects that are beneficial to host health.

Finally, the term synbiotics has been coined, which refers to a combination of a probiotic microorganism and a prebiotic substance. This mixture should benefit the host by improving the survival and implantation of the selected living bacteria in the gastrointestinal tract, by selectively stimulating the growth, and/or by activating the metabolism of one or a limited number of health-promoting bacteria (9). A combination of a bifidobacterial strain with fructose- or galactose-containing oligosaccharides for example, is a common and feasible approach. Only prebiotic bifidogenic factors have been concisely reported so far.

4 Functional Foods Containing Phytochemicals

Phytochemicals are a group of secondary plant components in contrast to primary plant components such as carbohydrates, proteins, fats, vitamins, and minerals. While primary plant components are required for the normal metabolism of the plant itself, secondary plant components are produced to provide color (to attract organisms required for plant reproduction) or protection from vermins. They are hypothesized to be responsible for much of the disease protection associated to diets high in fruit, vegetables, pulses, and cereals (→ Cereals and Cereal Products). Chemically, phytochemicals are a heterogenous group of different compounds including polyphenols, flavonoids, phytosterols, and phytoestrogens. While there is substantial scientific evidence from epidemiological studies that a diet high in fruits and vegetables is associated with a reduction of chronic diseases, the evidence of phytochemicals being responsible for these effects is limited. Consequently, for most of the phytochemicals, no claim indicating a beneficial effect for human health has been authorized.

Phytosterols, however, have been shown to reduce the blood cholesterol level, and that effect is allowed to be claimed for functional foods enriched with certain phytosterols. About 44 different phytosterols have been identified so far, including β-sitosterol, campesterol, and stigmasterol. Due to their structural similarities with cholesterol, phytosterols can reduce cholesterol absorption in the small intestine and thus lower LDL cholesterol levels in the blood stream. Products enriched with phytosterols include yogurts, yogurt drinks, and margarines. Labeling these products with a claim relating to their contribution to the maintenance of normal blood cholesterol levels is authorized only in combination with the information that the beneficial effect is obtained with a daily intake of at least 0.8 g of plant sterols, a statement that the product is not intended for people who do not need to control their blood cholesterol level, and that these products are consumed as part of a healthy diet and life style with sufficient fruit and vegetables consumption.

5 Functional Foods with Modified Fatty Acid Profiles

The typical western diet is considered to be low in long-chain polyunsaturated fatty acids, mainly ω-3 fatty acids (→ Fatty Acids). The most important source of this type of fatty acids is fatty marine fish, which is normally consumed in relatively low amounts by the average person. To improve the intake of these fatty acids, foods not naturally containing large amounts of ω-3 fatty acids have been enriched with fats high in ω-3 fatty acids, thus classifying them as functional foods. Typically, these fats are derived from marine fish, resulting in the challenge of incorporating these fats without sensory impairment and with sufficient stability regarding lipid oxidation. To improve the oxidation stability, various antioxidants can be used, including tocopherols, ascorbyl palmitate, phenolic acids, or spice extracts. Microencapsulation has been shown to be a feasible way of introducing ω-3 fatty acids into foods with sufficient stability thus satisfying rheological and sensory requirements (10). A wide range of products has been introduced on the market so far, including milk and dairy products, breads, sausages, and others.

6 Regulation of Functional Foods

As there is no legally binding definition of functional food, many products have entered the market using any of these terms or variations of these to promote their benefit for human health. Both in Europe and in the USA, health effects related to foods have to be approved by the authorities before they are authorized for any form of communication (e.g. labeling, marketing material, product information). In Europe, the Health Claim Regulation (Regulation (EC) No 1924/2006 of the European Parliament and of the Council on Nutrition and Health Claims Made on Foods (11)), and the Nutrition Labeling and Education Act (NLEA) in the USA (12) provide the legal framework for such claims. Both regulations consider a relationship made between a food, a food component, or a dietary ingredient and the risk of a disease to fall under this legislation and require scientific evidence for these relationships to be demonstrated before authorization is made. In practice, these requirements will almost always apply to functional food as any effect attributed to functional food would be considered a health claim. In other words, if one intends to introduce a food onto the market with additional effects on human health, producers are obliged to prove this effect with sound science in the target population of the intended health effect through the corresponding authorities. Not authorized food–health relationships have not been allowed since the legal requirements came into force.

At present, in the EU only 14 health claims have been authorized that relate to the reduction of a disease, while 229 claims are allowed to be made for the role of a nutrient or other substance in growth, development, and the functions of the body, psychological and behavioral functions, slimming and weight-control, reduction in the sense of hunger or increase in the sense of satiety, and to the reduction of the available energy from the diet (13). Table 3 lists the presently authorized claims relating to reduction of a disease.

7 Outlook

In addition to the categories of functional foods discussed above, new products and ingredients potentially providing additional benefits to the human diet are continuously developed. Bioactive proteins and peptides derived from milk and colostrum (14), meat (15), soy (16), seafood (17), or dietary fiber products (18) with functional properties, or chemically modified ingredients of plants (19) are extensively researched. Whether this research leads to new products with approved health claims, however, remains to be seen.

In view of the growing awareness of consumers that their diet is important for their health and that present dietary behavior may be insufficient to provide the optimum amount of biologically active components, the importance of functional food will increase both from a scientific point of view and from an economic point of view. Scientifically, health effects have to be substantiated by appropriate studies in humans, but achieving high levels of evidence remains a challenge. Economically, the effort required to run the required studies and the return on investment from better competitiveness on the food market remain a challenge. Nonetheless, it is expected that the market of functional food will grow. The global market of functional food was estimated to amount to at least 33 × 10⁹ $ in 2003 (20), while others even estimated this market to be 47.6 × 10⁹ $ with the United States being the largest market segment, followed by Europe and Japan. Japan, frequently regarded as the birthplace of functional food, has a large market of these products with more than 1700 functional food products launched between 1988 and 1998. The European market for functional foods was estimated to be between 4 and 8 × 10⁹ $ in 2003 depending on the definition of a functional food (21). This value has increased to around 15 × 10⁹ $ by 2006 (22). However, all of these estimates were made before the regulations on health claims came into force, and it remains to be seen whether the forecasted growth can be maintained. New technologies may lead to innovative products in the sector of functional food providing clear benefits for many consumers, however, consumer's acceptance of these products and different consumer expectations in respect to foods (e.g., trends towards organic foods, foods free of genetically modified organisms, etc.) may counteract this development.