The obtained data show that a single dose of GTE ingested with the test meal may limit the digestion and absorption of dietary starch in humans. The dose of GTE (4 g, EGCG content −257.6 mg), established according to pharmacokinetic studies, is equivalent to at least several cups of green tea16,17. As the usual daily intake of green tea is smaller than the dose used in our study, its effect may be not as pronounced. Some commercially available green teas may differ in total amount of polyphenols, even to as little as 15% polyphenols. To the best of our knowledge, this is the first study evaluating the effect of pure GTE on the digestion and absorption of starch with the use of a plausible method.

To study the effect of physicochemical characteristics of starch digestion, we used a CO 2 starch breath test. Lacroix et al. used this method for the first time in measuring the 13CO 2 excretion in breath after ingestion of naturally 13C-labelled glucose18. The principle of the method is based on the fact that the glucose, previously hydrolyzed from starch, is absorbed and further metabolized to CO 2 . Breath tests are considered to be a reliable method to study the carbohydrate absorption and are widely used in gastrointestinal studies19.

The key enzyme in the digestive system –α-amylase (of pancreatic as well as salivary origin) breaks down the starch contained in cornflakes. In the initial step, α-amylase catalyzes the hydrolysis of starch to smaller oligosaccharides consisting of maltose, maltotriose and a number of α-(1–6)- and α-(1–4)-oligoglucans, which are further degraded by α-glucosidases to glucose. This process may lead to the elevated post-prandial hyperglycemia occurring in diabetes. Hence, effective inhibition of enzymes such as α-amylase is crucial in the control of this disease. Most inhibitors, such as acarbose, cause serious side effects (diarrhea, vomiting). For this reason, much attention has been paid to herbs and plant extracts that offer similar benefits without the side effects20,21. Although high doses of EGCG (e.g. 800 mg) might cause mild headache and fatigue17, the usage of 257.6 mg EGCG did not cause any side effects in the subjects during and after the study beyond leaving a taste of bitterness in 5 subjects.

Honda and Hara have previously reported that GTE inhibited human salivary α-amylase22. Another study evaluated the inhibitory effects of GTE and its catechins on α-amylase and α-glucosidase activity in comparison with acarbose. GTE (especially EGCG) was a stronger inhibitor than the latter20. Similar results evaluating EGCG activity were obtained in starch-fed mice23. Despite a few studies conducted in animal models24,25,26,27, little is known about the impact of pure GTE on starch digestion and absorption in humans. The research carried out in healthy Asians provided evidence that a beverage containing 0.1 g black, 0.1 g green and 1.0 g mulberry teas caused carbohydrate malabsorption of 25% (~60 kcal) compared to placebo in healthy adults as assessed by breath hydrogen analysis28. Although suggestive of an influence of green tea on inhibiting starch digestion and absorption, this data is confounded by the complicated mixture of three teas and the use of a rice-based meal rather than pure starch. In contrast to the aforementioned study we used GTE alone. We avoided different biological interactions between compounds of three teas this way. The variety of green tea extract components as well as their effects on humans have been extensively studied. The study of Gao et al. shows that GTE and its polyphenols namely EGCG, strongly suppress the α-glucosidase in vitro29. Based on the half maximal inhibitory concentration (IC 50 ) values, GTE, green tea polyphenols and EGCG alone demonstrate 800–1000 times the efficacy of acarbose (IC 50 values of GTE, green tea polyphenols and EGCG against α-glucosidase were 4.421 ± 0.018, 10.019 ± 0.017 and 5.272 ± 0.009 respectively, whereas of acarbose against α-glucosidase values were 4,822.783 ± 26.042). In regard to α-amylase, it was not strongly inhibited by these substances. Similar results were obtained in the study of Yang et al. who proved, consistently with the previous report, that the inhibition of α-glucosidase by tea polyphenols is noncompetitive13,30. Furthermore, Gao et al. also reported that the combination of acarbose and GTE, EGCG, or green tea polyphenols show combined inhibitory effects at certain concentrations29. Therefore, it could be expected that joined therapy with GTE or green tea polyphenols or EGCG may diminish the dose of acarbose needed in therapy, hence weaken the side effects of acarbose alone. The aforesaid findings indicate that green tea or functional food based on green tea could be applicable for complementary therapy in postprandial hyperglycemia.

We used naturally 13C abundant cornflakes as a source of starch. The physical form of starch, the method of its processing or the size of the particle are a vital factors that may influence the hydrolysis and glycemic response in subjects31. Commercially available cereals used in the study, were produced via an extrusion process, which gives starch a high degree of gelatinization. Based on the study of Hiele et al. starch in this formation is more rapidly hydrolysed than native starch19. Maize, a representative of C4 photosynthesizing plants incorporates more 13C atoms into the starch than C3 plants do (e.g. European grain, potatoes, rice). The typical Polish diet (in contrast to the American diet) contains foodstuffs like potatoes, rye, wheat, beet sugar and (to lesser extent) maize, therefore maintaining the conditions of the test was easier.15

Our data suggest that the use of GTE is a viable alternative to pharmaceutical inhibitors of glucoside hydrolase enzymes. This plant extract is widely available, inexpensive and well tolerated, so it has potential utility for weight control and the treatment of diabetes. Our study supports the concept that pure GTE inhibits starch digestion and absorption. However, the clinical significance of each green tea catechin and the exact mechanism responsible for this action in humans remain to be determined.