## 1 Introduction  
Non-communicable diseases (NCDs) are today the leading causes of death around the world. One of the four major NCDs is diabetes, while more than 90% of diabetes cases are type 2 diabetes. Furthermore, diabetes and obesity may lead to all other diet-related chronic diseases [1, 2, 3, 4, 5, 6]. One of the risk factors for the development of type 2 diabetes is an intake of products/meals with a high glycemic index that leads to raised blood glucose levels [5, 7]. Carbohydrates, predominantly complex ones (starch), should comprise 40-70% of the total daily energy requirement [8]. At the same time, most of the starchy foods (e.g., wheat breadstuff), which underlie our regular diet, exhibit a high glycemic index [9]. It is impossible to make an accurate assessment of the calories provided by a specific glycemic carbohydrate component based on its quantity alone due to variations in both the rate and extent of its digestion and absorption [10]. Hence, the rate and extent of starch digestion are important in controlling postprandial blood sugar [11].  
One of the ways to minimize the risk of type 2 diabetes or expand the scope of foods acceptable to eat by people already suffering from this disease is to provide food rich in natural bioactive ingredients, e.g., polyphenols. Polyphenols are known to inhibit starch digestibility through two mechanisms: interactions with \(\alpha\)-amylase and interactions with amylose [12]. The action of \(\alpha\)-amylase consists of bonding the hydroxyl groups with the active site of the enzyme and making the hydrophobic interactions between the aromatic groups of polyphenols and the enzyme [13]. The second mechanism of inhibition of starch digestibility is also grounded on hydroxyl bonding and hydrophobic interactions between polyphenols and amylose, and as an effect, non-digestible complexes are built [14]. There have already been studies that tested various products enriched with whole/parted fruits or polyphenol extracts and their effect on inhibiting the digestibility of starch in vitro [15; 16; 17; 18; 19; 20; 21]. The mentioned research showed the retarding activity of polyphenol-enriched products on starch digestion. However, whole/parted fruits contain not only polyphenols but also other bioactive compounds, e.g., fiber, which can have an impact on starch digestibility or its properties. In terms of polyphenol extracts, they have an unknown composition and could be mixtures of different polyphenols in different doses. Studies were also conducted to examine the impact of single polyphenols on starch digestibility [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. Although these studies tested single polyphenols in a pure form, they differ from each other in the applied experimental conditions and features of the reagents that were used, so it is impossible to compare their results. A pilot study was also carried out [37]. However, the results only indicated differences in the amount of one starch fraction (resistant starch) and only in wheat starch.  
Thus, the results already obtained do not give a clear answer to the question of which polyphenols have the strongest inhibiting effect on starch digestibility and, at the same time, lower glycemic index. The present research was conducted to fill the information gap in the literature. The chosen phenolic compounds were commonly found in popular food raw materials and starches and are most widely used in the manufacture of high-starch products. The objectives of the proposed research were (i) to determine whether there were differences between different polyphenols in the reduction in starch digestibility and glycemic index and (ii) to find the most effective phenolic compound in this respect.