Polyols (sugar alcohols, alditols)
The general formula of sugar alcohols alias polyols alias alditols is H(HCHO)n+1H – the difference is in chain length. They are polyalcohols derived by the reduction of an aldose-polyol dehydrogenase (NADP+ ), which reversibly converts aldoses to alditols. Alditols are naturally occurring sweet substances found in low amounts in plants, micro-organisms and animal bodies. Industrially they are often obtained on a large scale through the hydrogenation of sugars (except the erythritol) (Monedero et al. 2010). All polyols can be divided into hypo- or non-acidogenic groups, based on the fermentation by oral microbiota (in vitro) evident in the pH measured in dental plaque. The plaque pH, measured in vivo, is an indicator of the cariogenicity of polyols or food products. If the plaque pH decreases below 5.5 (which is associated with initial caries) after exposure to fermentable sugar alcohols because of the formation of organic acid in dental plaque, this product is classified as cariogenic. In comparison to this, the alditols are low- or noncariogenic substances. The anti-caries and/or caries preventive characteristics of polyols are of interest in the field of caries prevention. A systematic review provided by Moynihan and Kelly (2014), based on clinical studies, supports the evidence of the positive effects of the combination of dosage and frequency of fermentable dietary carbohydrates, chiefly sucrose
consumption and caries level. This has brought about an increase in the popularity of polyols in food products. The cause for this, however, is the food industry’s interest in finding a suitable, less cariogenic and low-caloric replacement for sugars. The most common alternative is sorbitol, but the trials
in oral biology research continue testing and comparing it with other alditolsbased
sugar-free food substances.
Another relevant factor in polyols is the level of sweetness, which varies in the case of different polyols. The relative sweetness of common sugar alcohols
is reported in Table 1. In addition, the molecular parameters, which determine properties of sugar alcohols, are significantly different. The metabolism of
common sugar alcohols is well studied, and therefore, it is known that alditols are mostly not metabolised by oral microbiota and polyols are only partially
absorbed in the small intestine (up to 80% for sorbitol). The absorption of dietary polyols in the gastrointestinal tract can be active or passive depending
on the molecular size and the chemical specificities of the sugar alcohols (Table 1). Payne et al. (2012) have reported that this absorption depends on the degree
When polyols dissolve, it has a cooling effect in the mouth, which is a result of an endothermic process, negative heat from the solution. Xylitol and erythritol have the strongest cooling effect, which can cause discomfort for some people. One way to relieve this is to combine different agents. Polyols are non-reactive, and therefore, they can be mixed with other sweeteners in products.
The toxicity of polyols has been the topic of several studies. However, earlier studies have been based on animal subjects but the metabolism in the
human body does not have exactly the same pathways as in animals (Lina et al. 1996). Those experiments often exposed extremely high doses of the test
components. Hence, animal tests do not allow us to infer that the toxicity for humans is similar.
The well-known side effect of almost all polyols is gastrointestinal distress via a laxative effect. Tolerance of polyols varies depending on the type of sugar alcohol, the quantity consumed and individual resistance. Side-effects are absent up to a specific amount consumed. Common complaints after taking
large doses of polyols include the following intestinal symptoms: bloating, diarrhoea, abdominal pain because of bowel movements (irritable bowel
syndrome) and nausea. Mannitol lingers in the intestinum longer than other sugar alcohols, and therefore, causes side-effects more often. One well-tolerated
polyol is erythritol. When avoiding the consumption of excessive amounts of products with sugar alcohols, they are perfectly safe. Allergic reactions against polyols are rare. Only a couple of cases, such as urticarial, have been reported by Hino et al. (2000) and Yunginger et al. (2001). Some other symptoms, such as hives, skin rashes, difficulty breathing, swelling of the mouth and hands, dizziness, vomiting, weakness, and even anaphylactic shock after consuming erythritol or others sugar alcohols have been reported by Shirao et al. in 2013.
The aim of caries prevention using polyols is to replace the carbohydrates after a meal with some less-cariogenic sugar alcohols. The most effective method is exposure to a polyol immediately after eating, indicatively 4–5 times per day. Prolonged contact between the product and the teeth is required – not less than 2–3 minutes.
The most widely used and tested polyols are xylitol (a pentitol type sugar alcohol), sorbitol, mannitol (hexitol), maltitol, lactitol (12-carbon polyols) and
mixed products (van Loveren 2004). The newest promising subject is erythritol (tetritol), tested by Kawanabe (1992) and Mäkinen (2001).
Xylitol, pentitol type 5-carbon polyol (C5H12O5), is naturally found in fruits and vegetables. Commercial xylitol is extracted from corn or birch wood or other
xylan-containing plants. The sweetness of xylitol is like white, table sugar but the caloric value is 2.4 kcal/g compared to 4 kcal/g in sugar. The human body
synthesises 5–15 g xylitol per day, where the xylitol is an intermediate product in carbohydrate metabolism in the clucoronate-xylulose cycle. The body does
not require insulin to metabolise xylitol and therefore xylitol has been approved in special dietary foods, e.g. for diabetics.
Xylitol has a long history – it has been used since the early 1970s. Xylitol, as a non-cariogenic and anti-cariogenic replacement for sugar, is popular in dietary
products and in oral hygiene articles, especially in the US and in Finland. In the Turku Sugar Study (Scheinin and Mäkinen 1975), the potential of xylitol was
thoroughly tested. In addition, numerous clinical trials have been carried out to investigate the effectiveness of xylitol.
Repeated studies over 40 years (Sheinin et al. 1975, 1985, Kandelman and Gagnon 1990, Mäkinen et al. 1995) have confirmed the significant reduction of dental caries on several occasions, both in low- or high-risk groups. Xylitol has been used as a non-cariogenic sweetener, with the sweetness similar to sugar. The oral bacteria are generally not able to metabolize xylitol (metabolism is possible by some mutans streptococci species), and therefore, no acid production follows from the use of it. The study by Twetman et al. (2003) reported that lactic acid concentration reduced significantly in dental plaque after xylitol experience. The increased pH in dental plaque does not take place and demineralisation is slower or non-existent. This could be one of the most important mechanisms of xylitol for caries prevention.
Xylitol has also been used to reduce the quantity of dental plaque – it prevents the sticking of bacteria to teeth (Mouton et al. 1975, Söderling and Hietala-Lenkkeri 2010) although recent in vitro studies did not confirm this (Giertsen et al. 2011, Decker et al. 2014). Mäkinen et al. (1989) and Isokangas et al. (1991) have shown that xylitol is able to reduce the growth of mutans streptococci in the dental plaque, both after short- and long-term use. Thabuis et al. (2013) emonstrated a similar significant reduction of the growth of three other cariogenic species (S. sobrinus, Actinomyces viscosus and Lactobacillus) in dental plaque. This effect occurs through inhibiting Streptococcus mutans metabolism (Trahan 1995, Lingström et al. 1997). This was first discovered in in vitro studies (Knuuttila and Mäkinen in 1975) and since then, the mechanism has been repeated in several clinical trials (Miyasawa-Hori et al. 2006). In the majority f studies chewing gum containing xylitol has been used (Sheinin and Mäkinen 1976, Kandelman and Gagnon 1887, 1990, Wennerholm et al. 1994, Mäkinen et al. 1995, Thabuis et al. 2013, Keukenmeester et al. 2014). Since chewing gum stimulates the secretion of saliva, which is a substantial factor in the process of pH neutralization, this could be an additional impact factor. However, xylitol can reduce dental plaque even in the form of candies (Alanen et al. 2000, Shyama et al. 2006). In addition to the preventive effect, xylitol has been found to promote tooth remineralisation. The inference has been based on the re-hardened caries lesions and negative caries lesions increment (Kandelman and Gagnon 1990). The recommended daily dosage for a caries preventive effect is 6–10 grams of xylitol (in different forms – chewing gum, tooth paste, mouthwash or candy), preferable regularly 5 times every day (Kandelman and Gagnon 1987), although studies using xylitol-sweetened chewing gum three times per day have shown Streptococcus mutans inhibitory effects (Autio 2002, Kiet et al. 2006, Holgerson et al. 2007).
The long-term use of xylitol increases the portion of xylitol-resistant mutans streptococci, which lacks the fructose phosphotransferase system. This adaptation mechanism minimises the anti-cariogenic effect of xylitol but does not imply acidity (van Loveren 2004). Trahan (1995) has shown that the percentage of MS does not increase in dental plaque but increases in saliva. One feasible explanation given by Trahan et al. (1992) is more easily shed xylitoladapted strains. The same could explain the reduction of dental plaque during xylitol intake.
Erythritol is a tetritol, 4-carbon polyol (C4H10O4). It is the first industrially produced polyol from glucose using a fermentation process with yeast, Moniella pollinis. Erythritol occurs naturally in some fruits, algae, fungi, lichens and fermented foods. The relative sweetness of erythritol is 60–70% of that of table sugar (sucrose). The cooling effect is close to xylitol –stronger than with the others alditols. The erythritol differs from others sugar alcohols, mostly because of its small molecular size and its unique digestion pathway.
The first interest from the food industry was to use erythritol as a noncaloric bulk sweetener. The new trend among consumers towards natural components accepts erythritol, which is made naturally by fermentation and satisfies the food safety requirements. Erythritol is well tolerated in the gastrointestinal tract because of its property to be well absorbed (via passive diffusion) in the small intestine but not metabolized (fermented) in the body, and therefore, it is qualified as a non-caloric polyol. Erythritol has an estimated energy value near 0, maximum 0.24 kcal/g (de Cock 1999, 2012, de Cock and Bechert 2002), and it has no effect on blood glucose and insulin levels. Excretion from the blood takes place through the kidneys (about 90%) and non-absorbed erythritol (~10%) that remains unchanged passes to the large intestine and is excreted with the faeces (Storey et al. 2007). This part of erythritol may be fermented by the microbiota of the colon. The laxative effect of erythritol is considerably smaller than after consuming others alditols because 90% is absorbed in the small intestine before moving to the large intestine.
Erythritol has been shown (by Kawanabe et al 1992 and Mäkinen et al. 2005) to be similar to xylitol in its dental plaque reducing effect. The mechanism of this effect has been examined for a shorter period and there are fewer studies about erythritol compared to alditols that have been in use for a longer period like sorbitol and xylitol. The in vitro experiment conducted by Kawanabe (1992) has shown that the species of streptococci do not produce either lactic nor other acids from erythritol. Similar test results have been shown by xylitol, but at the same time other widespread sugar alcohols indicated the same acid production. As reported by Söderling and Hietala-Lenkkari in 2010, streptococci were not able to grow and mostly not produce insoluble dextran in plaque from erythritol.
In conclusion, both erythritol and xylitol have a clinical effect on caries prevention despite their biochemical mechanisms being different (Mäkinen et al. 2005).
Riina Runnel, (doktorikraad) 2015, Eino Honkala; Kauko K. Mäkinen; Mare Saag, Oral health among elementary school children and the effects of polyol candies on the prevention of dental caries (Suutervis algklasside õpilastel ning polüoolide toime hambakaariese ennetusele), Tartu Ülikool, Arstiteaduskond.