Certain mushrooms are widely used in traditional Chinese medicine and have been investigated for their anti-cancer properties.
They are also known to offer high-quality protein, vitamins, unsaturated fatty acids and fibre but a precise study of carbohydrate breakdown has not yet previously been carried out.
Scientists at the University of Illinois at Urbana-Champaign found that the six mushroom varieties tested - in raw and cooked forms and at various harvest times and maturity levels - are rich in cholesterol-lowering chitin and the heart healthy beta-glutan.
The findings, published in an early online edition of the Journal of Agricultural and Food Chemistry, could help health food makers looking for a natural source of heart health fibres.
Heart disease costs the UK €87.85 per capita, or €5.2 billion a year, but research suggests that functional foods could help prevent the disease and reduce risk factors.
The mushrooms studied were white button, crimini and portabella, all of which represent different maturity levels of Agaricus bisporus, and maitake (Grifola frondosa), shiitake (Lentinus edodes) and enoki (Flammulina velutipes). The latter two mushrooms were analyzed only in their consumed cooked form.
"The maitakes and shiitakes tended to be very similar in their nutrient concentrations, and quite a bit different than the others," said lead author Cheryl Dikeman.
"Portabellas were off on their own in terms of their contents of oligosaccharides, beta-glucans and chitin."
Chitin concentrations were 8 per cent in raw, mature portabellas and 6 per cent in raw, immature ones. When cooked, chitin content fell to 2.7 per cent in both forms, but their levels of total dietary fibres went up significantly.
Raw enokis showed the same pattern, with a 7.7 per cent chitin content. Cooking also lowered it to 2.7 per cent but total dietary fibres jumped from 29.3 per cent in raw to 41.6 per cent in cooked.
Raw, mature white buttons and cooked, mature shiitakes boasted chitin levels of 3 per cent and 3.6 per cent, respectively.
Raw, mature portabellas also had the highest level of beta-glucan (0.2 per cent), while most of the other mushrooms had 0.1 per cent. Enokis and maitakes had none. Relatively small amounts are required to provide cardiovascular benefits, Fahey said.
Cooking tended to increase starch, total dietary fibres and fat contents and to decrease chitin concentrations in all of the mushrooms.
"Some nutrients went up after cooking, while some went down," Dikeman said. "Part of that you'd expect to happen as water is cooked out."
The researchers also measured oligosaccharide levels, sugar molecules that are only partially digestible. The undigested components are considered prebiotics as they boost growth of healthful bacteria in the colon.
Raw, immature portabellas had a total oligosaccharide concentration of 5,272 micrograms per gram (ug/g). Also found to have more than 1,000 ug/g were raw, mature portabellas and cooked, immature crimini. None were detected in enokis, maitakes or shiitakes.
Most of the total oligosaccharides were in the form of glucooligosaccharides, but fructooligosaccharides (FOS) accounted for the total concentrations in cooked, immature white buttons. FOS did not appear in other samples.
Figures released in November by Ireland's food and drink body Bord Bia showed that consumers are buying significantly more mushrooms, spending €34 million on mushrooms in retail outlets - an annual increase of 13 per cent for the year ending May 2004.
The study was funded by the Mushroom Council of Dublin, California.