By pairing a specific prebiotic with a specific probiotic, a unique synbiotic compound is formed, creating an opportunity to provide targeted health benefits and a huge boost in immunity. Ian L Brown, PhD, explores

The important contribution to our health made by our intestinal microflora continues to be a growing focus of both international research and commercial activity. The consumption of probiotics has been linked to a broad range of physiological benefits through the changes affected on the indigenous microflora.¹

Researchers have also focused on prebiotics — nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon.2

Although initial studies in this area focused on fermentable substrates, such as inulin and fructo-oligosaccharides, a number of other dietary components, such as resistant starch, have been identified that can act as prebiotics.³

Considerable attention has been given recently to the concept of combining probiotics and prebiotics into what is called synbiotics, in order to produce health-enhancing functional food ingredients.2 The European Union-sponsored SYNCAN project is investigating the most suitable synbiotic preparation against carcinogenesis; it issued its first published findings in 2005.4

However, rather than combining just any probiotic and prebiotic, the concept of using a synbiotic offers the opportunity to provide a targeted benefit through matching the probiotic to a specific prebiotic. The type of prebiotic and probiotic strain or combination of strains used all can alter the potential to reinforce the immune system of the body and increase resistance to disease.

Indeed, only about 10 per cent of the probiotic lactic acid bacteria studied have proven strong immunosupportive effects.5 A synbiotic combination could improve the survival of the bacteria crossing the upper part of the gastrointestinal tract, thereby enhancing their effects in the large bowel.6

Increasing knowledge of the metabolism of prebiotics by probiotics is allowing researchers to consider specifically targeting such dietary intervention tools at specific population groups and specific disease states.7 Prebiotics can thus have a range of properties, including acting as a 'culture protagonist'⁸ to protect the viability of the probiotic in the food or during its transit through the upper GI tract,⁹ as a selective substrate for the probiotic,¹⁰ or as a substrate for a complementary beneficial microorganism.

Most of the studies published until now have involved two synbiotic preparations. The first is a single strain/single-fibre synbiotic comprising fermented oatmeal fibre with Lactobacillus plantarum strain 299.11 Since 2000, a number of studies have used the trademarked Synbiotic 2000.

This consists of a mixture of four probiotics from the lactobacillus genera: Pediacoccus pentosaceus 5-33:3, Leuconostoc mesenteroides 32-77:1, Lactobacillus paracasei subsp paracasei 19 and Lactobacillus plantarum 2362. The prebiotics to match in this formulation are beta-glucan, inulin, pectin and resistant starch. This synbiotic mixture has shown benefit in arteriosclerosis, Crohn's disease and chronic liver disease.^12,13

The benefit of these probiotics is in their ability to survive in the low pH of the stomach and in the high-bile-acid content of the small intestine. Also, they can attach to colonic mucosa and temporarily colonise the large intestine. They can ferment various types of plant fibres, including inulin.5

Resistant starches
The recognition that a fraction of the starch consumed in our diets is resistant to digestion in the upper GI tract has led to scientific and nutritional interest in the physiological effects of resistant starch (RS).14

Due to the vast differences in form, structure and chemistry provided by RS, there is a corresponding range of opportunities available to tailor the type and form of RS for specific physiological effects,¹⁵ to provide appropriate functionalities for the preparation of foods,^16,17 or act as a targeted prebiotic.

RS is present, usually in small amounts, in many of the starch-containing foods that people consume.¹⁸ However, it has been suggested that in Western diets, in order to maintain good colonic health, the amount of RS needs to be increased by two to three times.¹⁹

Initially, clinical studies mainly focused on sources of RS obtained from raw potatoes and green bananas. However, the first commercial source of RS in foods was obtained from high amylose maize.

This maize has a high gelatinisation temperature, which means that the granule can maintain its resistance to digestive amylases during many of the conditions that are used in the preparation of foods.

The impact of RS in the colon is generally associated with the beneficial stimulation of the colonic microflora and its action as a prebiotic.²⁰ (Table 1)

It was observed that the inclusion of RS in the diet increased the production of short-chain fatty acids, with a special emphasis on physiologically important butyrate. The consumption of RS produces positive effects on a range of biomarkers of colonic health, including decreases in transit time, pH and various toxic metabolites, such as secondary bile acids, ammonia and phenols.

The beneficial stimulation of the colonic microflora can also lead to advantageous consequences such as increasing the bioavailability of micronutrients such as calcium,21 decreasing the symptoms of bacterially induced diarrhoea,²² improving insulin sensitivity,23 being involved in biotransformations such as the conversion of phytoestrogenic materials into more bioactive forms,²⁴ increasing the immune response25 and potentially increasing lipid metabolism in the body.26^,27

Synbiotics must match
The concept of synbiotics provides a means of promoting physiological improvements and providing the fermentative substrate to achieve this effect. The synbiotic can achieve benefits through outcomes such as the manipulation of the colonic microflora, the production of beneficial metabolites such as short-chain fatty acids, and the reduction in the numbers and activity of pathogenic bacteria.

However, for the best effect, the probiotic and prebiotic needs to be carefully matched and not be just a random combination. These targeted synbiotics may offer a new means to protect against the development of or to treat a wide variety of lower GI complaints and diseases.

In addition, it is now recognised that the functioning of the GI tract can influence physiological activity elsewhere in the body, such as lipid metabolism, immune status and insulin sensitivity. Targeted synbiotics offer a potential means of preventing or treating many socially important health concerns, including obesity.

Although it is technically challenging to effectively maintain the viability of probiotics in processed foods, it is relatively easy to include prebiotics and in combination the prebiotic may assist in maintaining the viability of the probiotic micro-organism(s).

Ian L Brown, PhD, is a professorial fellow in the Department of Health and Behavioural Sciences, Wollongong University, Australia. Respond: [email protected]. All correspondence will be forwarded to the author.

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