History
In the twentieth century, kiwifruit came a long way from being a wild species partially exploited by man to being a commercial crop of international economic importance [
7]. Kiwifruit are native to the temperate forests of the mountains and hills of southwest China. Missionaries in the nineteenth century made many contributions to the advancement of botany and the distribution of horticultural plants [
8]. The first botanical specimens of
A. chinensis were sent to Europe by the Jesuit priest Père Pierre Noël Le Chéron d’Incarville around the 1750s and later by Robert Fortune, a plant collector. Robert Fortune was sent to China by the Horticultural Society of London (1843–1845) to “collect seeds and plants of an ornamental or useful kind”, and one of Fortune’s specimens of
A. chinensis was held at the Royal Botanic Gardens at Kew, London. The first fruits of
A. chinensis to be seen in Europe were sent, preserved in spirit, to Kew in 1886. Today New Zealand is a major producer of kiwifruit, and all early commercial varieties of kiwifruit plants in New Zealand and around the world can be traced back to a Church of Scotland mission station in Yichang, China, in 1878. Early in the twentieth century, the seeds and plants were regarded as ornamental curiosities with no mention of the edible fruit. The introduction of kiwifruit to New Zealand can be traced to a school teacher, Isabel Fraser, who in 1904 returned from a visit to China with seeds [
7]. Around 1922, Hayward Wright, a nurseryman living near Auckland, New Zealand, offered plants in his catalogue, listing the plant as “a wonderful fruiting climber” and promoting it as a highly valuable new fruit because it ripens in the winter over a long period, thus making the fruit a valuable addition to the short supply of winter fruits.
The Hayward cultivar has been sold widely from the late 1930s and the dominance of this cultivar worldwide is now complete. The first commercial orchards and large-scale plantings occurred around this time. Orcharding kiwifruit required brave and courageous decisions as the work was hard, there were no proven patterns of management by growers and agronomic problems were faced as they occurred. World War II and then agricultural and marketing incentives from the 1950s to the present day resulted in the rapid geographical expansion of orchards in New Zealand, Australia, Chile, USA and Europe, mainly Italy, France and Greece. In Italy, the high content of vitamin C gave kiwifruit the reputation of being the “frutto della salute”—the health fruit [
8].
The last 100 years have seen the domestication of the kiwifruit from being a wild plant (the so-called “Chinese gooseberry”) to a stage where it is now an important crop in several countries. The name “kiwifruit” was proposed by Turners and Growers Ltd, an exporting firm in Auckland, after the flightless bird, which is endemic to, and often taken as, the emblem of New Zealand. Servicemen were also commonly known as “Kiwis”, and by 1969 the name kiwifruit was well established and accepted.
The process of domestication of kiwifruit is a fascinating and complex story. It includes botanical identification, the collection of seeds and propagating material, cultivation techniques to grow and manage the plant, the management of a dioecious perennial climber, selection of the best cultivars, the commercial discoveries of the cultural conditions affecting yield, harvesting, storage, packing to extend the season and transporting across the globe [
8].
Of all the different species of
Actinidia, the main cultivar of economic importance is
A. deliciosa, and all the commercial plantings in New Zealand can be traced back to the seeds introduced by Isabel Fraser. The geographic range, the diversity of the wild population and subsequent development of cultivars, including gold and red-fleshed varieties, indicate that the gene pool, mostly sourced from wild types in China, offers many opportunities for breeding programmes for many desirable attributes, including very high levels of vitamin C [
5,
9]. Whilst the kiwifruit season requires winter growing, the fruit can be stored very well once harvested and also is produced in both the northern and southern hemispheres. This means that kiwifruit is available throughout the year which is important for those interested in regular consumption for its health benefits [
10].
Digestive health
Early Chinese pharmacopoeia from the Tang Dynasty onwards (AD 618–907) list a whole variety of medicinal uses for “mihoutao” fruit, the Chinese name generally used for Actinidia species, including aiding digestion, reduction of irritability and curing of dyspepsia and vomiting.
Functional gastrointestinal disorders (FGIDs) are common and distressing [
107]. FGIDs include functional dyspepsia (FD) and irritable bowel syndrome (IBS), affecting an estimated 3–28% of the global population [
108], particularly the elderly and women, and may severely affect the individual’s quality of life and wellbeing [
107,
109]. Upper gastrointestinal disorders include gastric reflux, stomach ache, delayed gastric emptying, nausea and vomiting, and lower gastrointestinal disorders include constipation, indigestion, bloating and diarrhoea. Current interventions for FGIDs include lifestyle and dietary modifications as well as pharmacological interventions targeting pain, motility, laxation and the gut microbiota [
108].
The worldwide growth in the incidence of FGIDs has created an immediate need to identify safe and effective food-based interventions. For example, constipation may be present in up to 29% of the population, depending on the definitions used [
110‐
112]. Food ingredients such as psyllium and wheat bran are the most studied for maintaining a healthy gut and to manage abdominal discomfort. Additionally, it is generally regarded that adequate intakes of fibre-rich fruits and vegetables daily with sufficient water will prevent constipation. Whole green kiwifruit have been used and promoted for many years to maintain abdominal comfort [
113] and have been studied more recently under controlled settings [
114,
115]. The components found in kiwifruit have been shown to increase faecal bulking and softness and enable better lubrication, assisting the propulsion of content along the colon [
116,
117].
It is thought that the unique combination of soluble and insoluble fibres, polyphenols and actinidin, present in kiwifruit, confers the gastrointestinal benefits, improvements in laxation and reduction of abdominal discomfort, both in individuals with either constipation-predominant irritable bowel syndrome (IBS-C) and in normal healthy people suffering from constipation without reported side effects. The putative mechanism of kiwifruit on maintenance of normal GI function has recently been reviewed [
95]. The review discusses the physiological functions of the digestive system, the pathophysiological mechanisms behind functional constipation, a summary of the work covering the effects of green kiwifruit on the gut as well as hypothetical mechanisms behind the gastrointestinal effects of green kiwifruit.
Lack of dietary fibre is a contributing factor in people with constipation [
118], and both soluble and insoluble fibres can add bulk, increase water retention in the colon [
119,
120] and change faecal consistency [
121,
122]. Dietary fibre can also decrease transit time [
122,
123]. Soluble dietary fibres are the main substrate for the microflora in the GI tract [
60]. When setting the Dietary Reference Value (DRV) of 25 g /day for dietary fibre, the EFSA NDA Panel used the role of fibre in bowel function as the most suitable criterion [
124]. Consuming 2 green kiwifruit per day would provide approximately 6 g of fibre (24% DRV), therefore, depending on habitual dietary fibre intake this may be a significant contribution to the total daily intake. Kiwifruit typically contain about two-thirds insoluble fibre, and one-third soluble fibre [
125], and as previously mentioned, kiwifruit fibre has an impressive water retention capacity [
57,
58]. In the native state, the capacity of kiwifruit fibre to swell, defined as the volume fibre has in water after passively settling [
126], is more than six times higher than that of apple fibre, and one and a half times higher than psyllium [
58], but is significantly reduced when subjected to processing conditions such as dehydration [
127]. Feeding studies in pigs [
128,
129] as well as observations in human studies [
114,
115,
130] have demonstrated that feeding kiwifruit increases water retention and faecal bulking, however animal studies suggest the pectic substances of kiwifruit are highly susceptible to fermentation in the hind-gut [
131,
132]. Such fermentation may produce short-chain fatty acids capable of stimulating colonic motility [
133] and contribute to the effects of kiwifruit, however the role of kiwifruit fibre in human digestive function is yet to be fully understood. In contrast, but consistent with earlier findings of changes associated with processed kiwifruit, the fibre of a dried kiwifruit product consumed as a part of a mixed fibre diet, did not demonstrate a significant contribution to faecal bulking in the rat [
131]. A reduction in GI transit time has been linked to actinidin [
128]. Although a considerable proportion of short chain fatty acids have recently been shown to be derived from the fermentation of non-dietary gut materials [
134], kiwifruit fibre may also contribute to favourable changes in the human colonic microbial community [
135] and their metabolites [
136] which are associated with intestinal health [
137].
The proteolytic enzyme actinidin from green kiwifruit has been shown in in vitro studies to aid protein digestion both in the stomach and small intestine [
2,
3]. For example, a range of common protein sources derived from soy, meat, milk and cereals were incubated with a kiwifruit extract containing actinidin and pepsin at pH 1.9 (a simulation of gastric digestion in humans) [
3]. Results in this gastric digestion model showed that for milk, soy and meat protein sources, the presence of kiwifruit extract enhanced digestion to a greater extent than pepsin alone [
13]. Likewise, in an in vitro, small intestine digestion model, actinidin-containing kiwifruit extract was particularly effective in improving the digestion of whey protein, zein, gluten and gliadin [
2]. These studies suggest that actinidin may assist with protein digestion in the gastric and ileal regions, that may be of benefit particularly to individuals with compromised digestive function [
138]. Under in vitro conditions, gastric lipase remained active, however actinidin effectively inactivated amylase suggesting that when cooked starch is consumed together with kiwifruit it is possible that starch digestion may be retarded [
139].
There is growing evidence that kiwifruit have beneficial effects on digestive health and general wellbeing, a potentially important characteristic in the light of the increasing proportion of the elderly population in ageing societies that experience impaired bowel function, changes in gastrointestinal function [
138], and gastrointestinal discomfort.
Table
3 summarises the findings from human clinical trials with fresh green kiwifruit. The daily consumption of two kiwifruit was found to increase stool frequency, including the number of complete spontaneous bowel motions per week, reduce gastrointestinal transit time and improve measures of intestinal comfort. These early human studies [
50,
114,
130,
140‐
142] were carried out in different countries and included different study populations (e.g., differing in age, health status), and the lack of a common protocol may have led to results that were not applicable to the larger normal healthy population. Most studies consider the effects of prolonged kiwifruit consumption, however recently Wallace et al. [
143] investigated the acute effects of green kiwifruit on gastric emptying following consumption of a steak meal, using a computerised SmartPill™, and measures of digestive comfort. Although the SmartPill™, did not provide reliable data following the meal event, there was a significant reduction in bloating and other measures of gastric discomfort [
143]. A multi-country, randomised, cross-over, controlled human intervention study is currently underway to evaluate further the effects of green kiwifruit on digestive function [
144]. Changes in bowel function in the general population such as reduced transit time, more frequent bowel movements, increase faecal bulk or softer stools are considered by EFSA to be beneficial physiological effects, provided they do not result in diarrhoea [
32]. Similarly, reducing gastrointestinal discomfort [e.g., bloating, abdominal pain/cramps, borborygmi (rumbling)] are considered appropriate outcome measures in human studies that include the use of validated questionnaires on severity and frequency of symptoms. The EFSA Panel on Dietetic Products Nutrition and Allergies (NDA) [
32] has also stated that IBS patients or subgroups of IBS are generally considered an appropriate study group to substantiate health benefits on bowel function and GI discomfort.
Table 3
Summary of findings from human studies with fresh green kiwifruit for digestive health
Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols (FODMAPs) are rapidly fermentable, poorly absorbed carbohydrates found in food that can cause digestive discomfort, especially for people with IBS [
145]. The action of FODMAPS is likely via multiple pathways [
146], and includes the release of gases from the bacterial fermentation of oligosaccharides and the proportion (if any) of malabsorbed fructose, polyols, and lactose [
147]. Symptoms associated with FODMAPs include abdominal bloating, pain, cramps, excessive flatulence and altered bowel habit [
146]. Low FODMAP diets are effective in the treatment of functional gastrointestinal symptoms [
148,
149].
Kiwifruit are certified as low FODMAP fruits by the Monash University low-FODMAP diet digital application (
https://www.monashfodmap.com/i-have-ibs/get-the-app/), based on their relatively low proportions of fructose and fructans per single fruit serve. A recent pilot study demonstrated that the consumption of two green kiwifruit is not associated with clinically significant evidence of colonic fermentation as shown by hydrogen and methane on breath testing [
150], lending support for the low FODMAP status for kiwifruit.
Metabolic abnormalities such as dyslipidaemia [increased blood total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), triglycerides (TG), lower high density lipoprotein cholesterol (HDL-C)], hypertension, vascular inflammation, abnormal glucose metabolism and haemostatic disorders all play important roles in the pathophysiology of the major causes of morbidity and mortality such as diabetes, cardiovascular disease (CVD), stroke and dementia [
151‐
153]. A number of studies have investigated the effects of green and gold kiwifruit on some of these metabolic markers, including the effects of kiwifruit on glucose and insulin balance, and on bodyweight maintenance and energy homeostasis.
Green and gold kiwifruit have clinically measured glycaemic indices (GIs) of 39 and 48, respectively [
65], which puts them in the GI “low” category (GI < 55). The glycaemic response to a fruit depends not only on GI, but also the amount of carbohydrate consumed in the fruit. As kiwifruit contains only about 12% available carbohydrate and a low GI; the impact kiwifruit produces on plasma glucose levels is low enough for the fruit to be suitable in managing diets for people with reduced tolerance to glucose. The fibre content of kiwifruit may cause a delay in carbohydrate digestion and absorption by way of swelling action that reduces the rate of glucose diffusion [
57,
127]. This reduction in glycaemic response by 200 g kiwifruit (approximately two fruits) has been demonstrated in a human intervention study conducted by Mishra et al. [
154]. The authors concluded that the low in vivo glycaemic impact could partly be attributed to the carbohydrate in kiwifruit being fruit sugars (fructose) and partly to the non-digested fibre components reducing the rate of intestinal processes such as digestion, sugar diffusion and mixing of intestinal contents. This partial substitution of starch-based staple foods, such as a high carbohydrate breakfast cereal, with kiwifruit could be an effective and healthy way to improve glucose homeostasis [
154]. Further exploration of this effect was investigated by Mishra et al. [
64] to better understand the role of non-sugar components in kiwifruit in modulating glycaemic response. Kiwifruit consistently reduced the amplitude of the glycaemic response of participants following a series of wheat-based cereal meals adjusted to match the available carbohydrates from kiwifruit leading the investigators to conclude that components other than the available carbohydrate in kiwifruit, such as cell wall remnants or phenolic compounds, may be involved in the improved glycaemic response to co-ingested foods [
64]. The energy value of foods is also an important dietary aspect in managing risk factors for metabolic syndrome. Using an in vivo–in vitro model that determines the available energy (AE) content based on ATP yield at the cellular level [
155], Henare et al. [
156] found the AE of green and gold kiwifruit was significantly less than that predicted by the traditional Atwater system, suggesting kiwifruit are useful in dietary weight management strategies. Further studies will explore the use of kiwifruit as an effective dietary strategy to reduce postprandial hyperglycaemia while at the same time increasing the amount of essential nutrients consumed.
Regular consumption of green and gold kiwifruit can also affect beneficially certain physiological biomarkers, particularly in individuals with metabolic abnormalities related to major causes of morbidity and mortality, such as diabetes, cardiovascular disease (CVD), stroke and dementia [
157]. For example, Chang, Liu [
158] investigated the effects of two kiwifruits on the lipid profile, antioxidants and markers of lipid peroxidation in hyperlipidaemic adult men and women in Taiwan. After 8 weeks of the intervention, the HDL-C concentration was significantly increased, whilst the LDL-C/HDL-C ratio and TC/HDL-C ratio were significantly decreased. Vitamin C and vitamin E, the antioxidant nutrients, together with plasma antioxidant status, also increased significantly in fasting blood samples.
Gammon et al. [
100] found that consumption of two green kiwifruit per day for 4 weeks favourably affects plasma lipids in a randomised controlled trial in 85 normotensive and pre-hypertensive hypercholesterolaemic men compared with the consumption of a healthy diet alone. Small, but significant, differences occurred, including an increase in HDL-C and a decrease in TC: HDL-C ratio and TG’s. There were no significant differences, however, between the two interventions for plasma TC, LDL-C, insulin, high-sensitivity C-reactive protein (hs-CRP), glucose and blood pressure (BP). In a further exploration of the study, no beneficial effects on markers of cardiovascular function, or on BP were noted [
159].
In 2012, Karlsen et al. [
80] demonstrated that intake of three kiwifruit per day for 3 weeks promoted pronounced anti-hypertensive effects, as well as antithrombotic effects in male, middle-aged and elderly smokers. The authors commented that this dietary approach may be helpful in postponing pharmacological treatment in individuals with high-normal blood pressure or hypertension. From a further randomised, controlled study over a period of 8 weeks, Svendsen et al. [
79] concluded that among men and women aged between 35 and 69 years with moderately elevated BP, the intake of 3 kiwifruit added to the usual diet was associated with lower systolic and diastolic 24-h BP compared with one apple a day. The authors observed these results were in contrast those of Gammon et al. [
159], noting the differences in study population criteria (normotensive [
159] versus moderately elevated BP [
79]) may have been a contributory factor. Although Svendsen et al. [
79] found no differences in measures of endothelial function in their study, they suggested that an increase in plasma antioxidant status (lutein), and in increased dietary potassium, resulting from the kiwifruit intervention, could be an explanation for the improvements in BP observed.
In vitro studies on antioxidant and fibrinolytic activities have also indicated the potential cardiovascular protective properties of kiwifruit extracts [
160]. Evidence that consumption of kiwifruit can modulate platelet reactivity towards collagen and ADP in human volunteers was provided in a study by Duttaroy, Jørgensen [
84]. The authors concluded that kiwifruit may have the potential to increase the effectiveness of thrombosis prophylaxis.
Habitual intakes of high levels of fruits and vegetables have long been associated with beneficial effects that lower the risk of chronic diseases, including CVD in humans [
161]. The presence of antioxidant components such as vitamin C, vitamin E, polyphenols [
162], a favourable Na
+/K
+ ratio [
52], and other bioactive components in kiwifruit could explain their beneficial physiological effects [
157].
This review highlights the nutritional attributes and health benefits of green and gold kiwifruit. The nutritional composition, particularly the high amount of vitamin C, supports its position as a highly nutritious, low energy fruit. With the plethora of man-made, processed health foods available to the consumer, one aspect that sets kiwifruit apart is that it is a natural, whole food. Nature compartmentalises many bioactive and nutritional components within the complex structure of cell walls, cells and the matrix in between. Human digestion interacts with fresh whole foods to break down the structures and digests the complex carbohydrates slowly. Many health care professionals now recognise whole foods are ideal for the release and delivery of nutrients and health components to various locations along our digestive tract.
There is a growing body of evidence to support the beneficial effects of kiwifruit in gastrointestinal function in healthy individuals as well as in individuals with constipation and other gastrointestinal disorders [
143,
144,
163], and recognition for the role of kiwifruit in their management [
164]. This presents an evidence-based opportunity for health care professionals to adopt dietary recommendations, and for consumers to recognise the impact of diet, in particular whole foods, on specific body function, and their health and well-being. Green and gold kiwifruit are well characterised and the mechanisms of action for the benefits on gastrointestinal function and modulation of glycaemic responses now being better defined.
Overall, the scientific evidence for the health benefits of kiwifruit needs to be expanded through the conduct of well-designed and executed human intervention studies that clearly define the study populations, the amount and duration of kiwifruit consumption and the specific beneficial physiological effects. A greater understanding of the mechanisms of action of kiwifruit and its bioactive constituents in promoting health also needs to be fully elucidated.
The increased research data identifying the nutritional and health benefits of kiwifruit and their growing consumer acceptance as a part of a balanced diet, will undoubtedly offer opportunities to tackle some of the major health and wellness concerns around the world.