Funded by the New Zealand Tertiary Education Commission and the country’s High-Value Nutrition National Science Challenge, the research stated that the improvements were explained by serotonin metabolism.
Serotonin is a natural chemical produced by the body to send messages between nerve cells and affects mood, emotions, and digestions.
“The urinary melatonin metabolite 6-sulfatoxymelatonin (aMT6s) is related to subjective and objective sleep quality measures and the urinary concentration of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) is related to mood,” the paper added.
Relative to the control group, both kiwifruit and kiwifruit powder groups saw an increase in the urinary concentration of the serotonin metabolite 5-HiAA (fresh +1.56 ± 0.4 ng/g, p = 0.001; dried: +1.30 ± 0.4 ng/g, p = 0.004; control 4.32 ± 0.4 ng/g).
Both kiwifruit and kiwifruit powder groups experienced improved esteem and less mood disturbance (p <0.1), which was not observed in the control group.
Notably, the kiwifruit powder group saw a highly-statistically significant result (p <0.05) for improved morning sleepiness, alertness upon awakening, and vigour.
Between poor and good sleepers, consumption of kiwifruits had a better impacted on the former. Among poor sleepers, the ease of awakening improved by 24% (p = 0.005) after kiwifruit powder consumption and 13% (p = 0.052) for kiwifruit consumption.
Whereas for good sleepers, they only saw a 9% (p = 0.053) improved rating of getting to sleep with kiwifruit consumption compared to the control.
On the science of kiwifruit, the researchers stated: “There are potentially at least three explanations that could explain the observed outcomes. Firstly, it is known that KF is rich in phenylalanine, tyrosine, tryptophan, glutamic acid and serotonin that are further metabolized to produce dopaminergic, serotoninergic, and GABAergic neurochemicals. These neurotransmitters are vital in the regulation of sleep/wake cycles.
“Secondly, green KF contains the enzyme actinidin, which has been shown to cause an earlier peak increase in plasma circulating amino acids in adults when compared to kiwifruit without actinidin. Therefore, potentially increasing the concentration of bioavailable amino acids to be further metabolized into neurochemicals.
“Lastly, KF may act on improving sleep via potentiating the bioaminergic responses. This has been demonstrated in in vitro studies. Given this, KF may impact sleep through the supply and modulation of peripheral neurochemical responses, which may impact systemic concentration modifications throughout the rest of the body.”
This was a randomized, single-blind crossover study involving 24 male participants, aged 18 to 35 years, conducted from January 2021 to May 2021. Participants were randomly assigned to one of the three treatments alongside a standardized evening meal - namely kiwifruit, kiwifruit powder and water control groups.
In the study, participants were directed to consume the evening meal four hours before their usual sleep time, followed by the intervention.
In the kiwifruit group, they would consume the flesh of Zespri’s fresh green kiwifruits (approximate 200g). In the kiwifruit powder group, they would consume 200ml of water with 32g of freeze-dried flesh and skin of green kiwifruits – equivalent to the dry matter of two fresh green kiwifruits. In the control group, they would only consume 200ml of water.
The study had taken subjective measurements for sleep quality and mood via some questionnaires, objective measurements for sleep via data collected from a wearable device, as well as biochemical measures for the urinary concentration of the kiwifruit remains via urine samples.
While the existing study has shown improved sleep quality and mood in male participants regardless of their baseline quality of sleep, further studies is needed to investigate the significant effect of kiwifruit powder on poor sleepers and its underlying mechanisms.
The researchers concluded that their findings help to back up the role of kiwifruit in facilitating healthy sleep regulation.
Source: Frontiers in Nutrition
“Acute effects of fresh versus dried Hayward green kiwifruit on sleep quality, mood, and sleep-related urinary metabolites in healthy young men with good and poor sleep quality”
Authors: Kanon, A., et al.