What is a kiwi fruit?

Kiwi fruit

Kiwifruit (often abbreviated as kiwi) or Chinese gooseberry is the edible berries of several species of woody vines in the genus Actinidia ¹⁾. The most common cultivar group of kiwifruit (‘Hayward’) is oval, about the size of a large hen’s egg (5–8 cm in length and 4.5–5.5 cm in diameter) ²⁾. It has a fibrous, dull greenish-brown skin and bright green or golden flesh with rows of tiny, black, edible seeds. The fruit has a soft texture and a sweet but unique flavor. It is a commercial crop in several countries, such as New Zealand, Italy, Chile, Greece, and France ³⁾.

Kiwifruit may be eaten raw, made into juices, used in baked goods, prepared with meat or used as a garnish ⁴⁾. The whole fruit, including the skin, is suitable for human consumption; however, the skin is often discarded due to its texture. Sliced kiwifruit has long been used as a garnish atop whipped cream on pavlova, a meringue-based dessert. Traditionally in China, kiwifruit was not eaten for pleasure, but was given as medicine to children to help them grow and to women who have given birth to help them recover ⁵⁾.

Raw kiwifruit contains actinidain (also spelled actinidin) which is commercially useful as a meat tenderizer ⁶⁾ and possibly as a digestive aid ⁷⁾. Actinidain also makes raw kiwifruit unsuitable for use in desserts containing milk or any other dairy products because the enzyme digests milk proteins. This applies to gelatin-based desserts, as well, as the actinidain will dissolve the proteins in gelatin, either liquefying the dessert or preventing it from solidifying.

Among the many species of kiwi fruit, Actinidia arguta, known as the kiwi berry, has an edible soft skin and can be consumed without peeling ⁸⁾. Previous studies have shown that the skin of the kiwi fruit has considerably higher total phenolic and total flavonoid levels ⁹⁾, ¹⁰⁾. The skin of kiwi fruit also exhibits higher antioxidant capacity compared with the fleshy part ¹¹⁾, indicating that the consumption of whole kiwi berry is not only convenient but also beneficial for health-promoting effects. For this reason, kiwi berry has attracted a great deal of attention from researchers attempting to investigate and develop cultivars to improve the functional compound content and taste, as well as disease and insect resistance ¹²⁾, ¹³⁾. However, further studies are needed to identify and to investigate the underlying mechanisms of the intercellular antioxidant and the anti-inflammatory effects of kiwi berries.

The total phenolic content, total flavonoid content and vitamin C content of Actinidia extracts is shown in Table 3. The results showed a high phenolic content of the ethanol extract from Actinidia kolomikta. The total phenolic content of kiwi fruits ethanol extract displayed the following order: Actinidia kolomikta > Actinidia arguta > Actinidia chinensis. Thus, Actinidia kolomikta has the highest total phenolic content and it is a potential source of antioxidants.

Flavonoids are the most common compounds and are a widely distributed group of phenolic compounds in plants. Flavonoids are usually considered as effective antioxidants. The total flavonoid content of three Actinidia ethanol extracts showed decreasing order: Actinidia arguta > Actinidia kolomikta > Actinidia chinensis. Studies have shown that the total flavonoid content in eight different Actinidia genotypes ranged from 3.01 to 91.79 mg CE/100 g ¹⁴⁾. Thus, the flavonoid content in Actinidia is strongly influenced by species and cultivars. The Actinidia kolomikta extract had the highest vitamin C content, followed by Actinidia chinensis and Actinidia arguta. The vitamin C content in Californian-grown cantaloupe and honeydew melons are 36.7 ± 1.38 mg/100 g  and 18 ± 1.64 mg/100 g, respectively ¹⁵⁾. Thus, the vitamin C contents of kiwi fruits are generally comparable to Californian-grown melons. Therefore, based on the Californian-grown melon measurements, the kiwi fruits extracts showed high vitamin C contents.

Figure 1. Actinidia arguta (kiwi berry) – note the smooth skin

Figure 2. Actinidia deliciosa (green kiwi fruit)

Figure 3. Kiwi Fruits. (a) Actinidia kolomikta; (b) Actinidia Arguta; (c) Actinidia Chinensis (gold kiwi fruit)

Kiwi nutrition facts

A medium size kiwifruit (76 grams) provides 46 calories, 0.3 g fat, 1 g protein, 11 g carbohydrates, and 2.6 g dietary fiber found partly in the edible skin ¹⁶⁾. Kiwifruit is a rich source of vitamin C (112% of the Daily Value per 100 grams) and vitamin K, and a good source of dietary fiber and vitamin E (Table 1).

Kiwifruit seed oil contains on average 62% alpha-linolenic acid, an omega-3 fatty acid. Kiwifruit pulp contains carotenoids, such as provitamin A beta-carotene, lutein, and zeaxanthin ¹⁷⁾.

Table 1. Green kiwi fruit (raw) nutrition facts (Actinidia Deliciosa)

[Source: United States Department of Agriculture Agricultural Research Service ¹⁸⁾]

Table 2. Gold kiwi fruit (raw) nutrition facts (Actinidia Chinensis)

[Source: United States Department of Agriculture Agricultural Research Service ¹⁹⁾]

Table 3. Total phenolic, total flavonoid and vitamin C contents of Actinidia extracts

Variety of Actinidia	Total Phenolic Content (mg GAE/100 g FW)	Total Flavonoid Content (mg CE/100 g FW)	Vitamin C Content (mg ACE/100 g FW)
Actinidia kolomikta	430.03 ± 21.85	69.05 ± 0.75	211.12 ± 7.91
Actinidia arguta	362.18 ± 19.87	188.43 ± 3.65	26.97 ± 5.64
Actinidia chinensis	115.76 ± 8.97	67.63 ± 0.68	42.28 ± 0.77

[Source ²⁰⁾]

Kiwi fruits health benefits

Diets rich in fruits and vegetables offer protection against the development of cardiovascular diseases (coronary heart disease), diabetes and cancer ²¹⁾, ²²⁾. A common factor in the aetiology of these diseases seems to be damage to biomolecules caused by reactive oxygen species. Powerful endogenous antioxidant defences are thought to be augmented by dietary antioxidants, and so much of the protective effect of fruits and vegetables has been attributed to their high content of antioxidants ²³⁾. However, attempts to boost human resistance to cardiovascular disease and cancer through supplementation trials with isolated antioxidants and vitamins have proved disappointing ²⁴⁾. Hence, the emphasis of chronic disease prevention policy has shifted to whole foods and whole food products. In addition to recognised antioxidants such as vitamins C and E, carotenoids and flavonoids, fruits and vegetables contain innumerable other phytochemicals, with known or (mostly) unknown effects on human metabolism. Kiwifruit is particularly rich in vitamin C (ascorbic acid), but also contains a wide range of other phytochemicals. The common green kiwifruit, Actinidia deliciosa, has been used as a ‘model’ fruit in several trials to examine effects on biomarkers relevant to both cancer and cardiovascular disease. Typically green kiwifruit contains approximately 85 mg vitamin C per 100 mg fresh weight ²⁵⁾. A kiwifruit extract has powerful antioxidant activity in test tube studies ²⁶⁾, and in humans, regular consumption of this fruit inhibits platelet aggregation ²⁷⁾, decreases endogenous oxidation of lymphocyte DNA, protects lymphocyte DNA from oxidation in vitro, and enhances the capacity of lymphocytes to repair DNA oxidation damage ²⁸⁾. The more recently available ‘golden’ kiwifruit Actinidia chinensis, differs significantly in phytochemical make-up (with 20% higher vitamin C content), demonstrating higher antioxidant activity values ²⁹⁾ than the green kiwifruit. Based on these properties the golden kiwifruit would be expected to show stronger protection against effects of oxidative damage in-vivo. Consumption of these kiwi fruits could therefore benefit public health by countering oxidative stress factors, and by reducing the risk of thrombotic events mediated by platelet activation ³⁰⁾.

Kiwi fruit is widely reported as a functional food and a nutraceutical source. Kiwifruits (Actinidiaceae) are native to eastern Asia and their use for treatment of different types of cancers, especially cancers of the digestive system and of the mammary gland, dates as far back as 700 BC ³¹⁾. Green kiwi fruit (Actinidia Deliciosa) is the most well-known species in the genus Actinidia; the spread of other species, such as Actinidia Chinensis (gold kiwi fruit) and Actinidia arguta, is now gradually increasing. Among the effects associated with the consumption of green kiwi fruit, laxation activity is probably the most popular. Human trial data support the public belief that this fruit is a safe and effective dietary intervention for the treatment of constipation ³²⁾, ³³⁾.

Irritable bowel syndrome (IBS) is a common functional disorder of the gastrointestinal system, and is characterized by abdominal pain, diarrhea (IBS/D), constipation (IBS/C), and alternating diarrhea and constipation (IBSC/A). This study ³⁴⁾ examined the impact of a four week kiwifruit intervention on bowel function in patients diagnosed with irritable bowel syndrome with constipation (IBS/C). Fifty-four patients with IBS/C and 16 healthy adults participated in this study. All subjects participated in the 6 week, three phase study, which included a baseline phase (1 week), a dietary intervention period (4 weeks), and a post-intervention phase (1 week). Forty-one irritable bowel syndrome with constipation (IBS/C) patients and all healthy adults consumed two Hayward green (Actinida deliciosa) kiwifruits per day for 4 weeks. Thirteen irritable bowel syndrome with constipation (IBS/C) patients in the control group took two placebo capsules per day for 4 weeks. Colon transit time was measured immediately prior to and following the intervention period. All subjects completed daily defecation records. After the 4-week intervention, weekly defecation frequency significantly increased in the irritable bowel syndrome with constipation (IBS/C) group of participants who consumed kiwifruit. Colon transit time significantly decreased in the irritable bowel syndrome with constipation (IBS/C) group that consumed kiwi fruit. These findings suggest that kiwifruit consumption for 4 weeks shortens colon transit time, increases defecation frequency, and improves bowel function in adults diagnosed with irritable bowel syndrome with constipation (IBS/C) ³⁵⁾.

Kiwifruit is also rich in antioxidants, vitamins C and E, flavonoids, anthocyanins, and carotenoids, and it contains approximately twice the concentration of serotonin as tomatoes ³⁶⁾. Serotonin is an end product of L-tryptophan metabolism, which is related to rapid eye movement (REM) sleep and its low levels may cause insomnia ³⁷⁾, ³⁸⁾. Additionally, kiwifruit is rich in folate and insomnia is one of the neuropsychiatric diseases that are secondary to folate deficiency ³⁹⁾, ⁴⁰⁾. Therefore, it is possible that consuming kiwifruit may be beneficial in improving sleep quality in those who have sleep disorders.

More recently, many studies have been performed on different species of kiwi fruit and have suggested that this fruit is endowed with some additional health-promoting properties that may influence human wellness. For instance, anti-allergic activity in ovalbumin-sensitized murine models has been reported for extracts obtained from A. arguta ⁴¹⁾. Results obtained by Abe and co-workers ⁴²⁾ suggest that green kiwi fruit may exert beneficial effects against diabetes via its ability to regulate adipocyte differentiation and function. High levels of in-vitro anti-oxidant activity and protection against oxidative DNA damage or oxidative stress have also been described for green and gold kiwi fruit ⁴³⁾, ⁴⁴⁾. In addition, the anti-inflammatory properties of extracts from gold and green kiwi fruit in in-vitro models comprising lipopolysaccharide-stimulated macrophages or intestinal epithelial cells have been reported recently ⁴⁵⁾. Raw kiwi fruit extracts were used for these experiments, therefore the molecule(s) responsible for the anti-inflammatory effect remain unknown.

Generally, the health-promoting effects of kiwi fruit have been investigated using the whole fruit, and very little attention has been paid so far to the small peptides naturally occurring in kiwi fruit and to their possible biological effects on humans. Kissper is a small peptide found in variable amounts in green kiwi fruit ⁴⁶⁾. It derives from the proteolytic cleavage of the precursor kiwellin, one of the most abundant protein components of this fruit ⁴⁷⁾. The strong resistance to gastric and intestinal proteolysis suggests that kissper could travel safely through the digestive system. This observation adds further support to the hypothesis that this molecule could affect human gastrointestinal physiology. The data reported from this study ⁴⁸⁾ suggested a beneficial effect of this kiwi fruit peptide, kissper, on the physiology of human intestine and its possible anti-inflammatory effect on the physiological and pathological conditions of the gastrointestinal tract, as well as to evaluate a possible pharmaceutical use of this molecule in the therapy of intestinal inflammatory diseases.