## 1 Introduction
Fiber consumption can impact metabolic parameters by the regulation of dietary carbohydrate absorption and gut microbiome [11]. This way, it could even potentially act as an anti-inflammatory agent. The observations confirm that fiber consumption is inversely associated with metabolic syndrome risk including body mass, waist circumference, and BMI, insulin resistance and diabetes type 2 risk, and is associated with lipid profile improvement (HDL level increase and TC decrease) [12; 13; 14]. These observations are also confirmed in interventional human studies in which consumption of arabinaxplan decreased fasting glucose levels and insulin resistance parameters in patients with the prediabetic condition [15; 16; 17]. The main antioxidants that can be found in apples are flavonoids and anthocyanins [18]. Among them, quercetin is the main flavonoid found in apples, mainly in apple peel [8]. It is suggested to present protective properties against metabolic syndrome and other cardiovascular disease risk factors [7; 19; 20]. These suggestions come also from observational studies in which quercetin intake was inversely correlated with BMI, waist circumference, and fat mass in obese patients and from interventional studies in which the supplementation resulted in lipid profile improvement (total cholesterol (TC) and low-density lipoprotein cholesterol (LDL) and oxidized LDL levels reduction [21; 22]). Cyanidin is present only in red apple peels and it also presents antioxidant properties [7]. It can potentially reduce inflammation related to obesity [23]. Even though its pure supplementation did not significantly impact lipid profile, the supplementation of the mixture of natural phenolic compounds including cyanidin improved glucose and LDL levels [24; 25; 26], so this suggests that phenolic compounds could potentially act as bioactive agents once they are mixed instead of supplementing pure compounds. As described above, apple peels are rich sources of multiple bioactive compounds that could be potentially protective against metabolic syndrome, but the results of human studies regarding separate compounds are not always consistent. On the other hand, the studies involving applications of complete food products, e.g., juices, presented such potential. This could suggest the possible cooperation between the single compounds and their additive metabolic effect. That is why apples as a set of compounds could potentially act as functional foods. Moreover, industrial production of apple-derived products often leads to a large amount of apple peel as industrial waste. The dwarf kiwi (_Actinidia arguta_ Miq.) is also locally called Polish kiwi fruit. It is a smaller alternative to traditional kiwi fruit, and it is possible to harvest in moderate climates and eat without peeling, which is why it has become popular among consumers during the last few years [27; 28; 29; 30]. The _Actinidia arguta_ species was shown to contain higher levels of antioxidant agents compared to other kiwi species [31; 32]. Among the flavonoid group, flavanols are the main contributors to antioxidant properties, however, other compounds such as quercetin and chlorogenic acid were also present in dwarf kiwi fruits [33]. Similarly to apples, the concentration of antioxidant agents is higher in the peels than in the flesh, which is also worth noting as the Polish kiwi can be consumed without peeling [32]. The dwarf kiwi has not been widely studied in humans, however, the results from animal model studies are very promising. It was shown that dwarf kiwi supplementation can present antioxidant properties, which can be used in conditions linked to inflammation such as cancers, dermatology [34; 35] or gastroenterology [36]. Moreover, in terms of metabolic properties, it was shown that Polish kiwi supplementation can present hepatoprotective properties and improve lipid profile in rats with induced hypercholesterolemiaemia [37]. In humans, _Actinidia arguta_ was only tested in terms of allergic diseases [38].