Regular exercise has many health-promoting benefits, including the maintenance of physical functions [1] and prevention of metabolic syndromes [2] and various types of disease [3] - [5] . The Japanese Ministry of Health, Labour, and Welfare recommends regular exercise (>30 min at least twice a week). However, the percentage of people that habitually exercise is reportedly low: 33.8% in men and 27.2% in women according to a 2013 survey [6] . One reason for these low rates is the negative perception of exercise as an exhausting activity. It follows that more people would adopt the habit of regular exercise if exercise-induced fatigue could be reduced.

Fatigue is caused by specific factors in muscle cells (peripheral fatigue) and diminished activation of the central nervous system (central fatigue). Peripheral fatigue associated with the depletion of energy, accumulation of metabolites such as lactate, and oxidative stress [7] [8] . Although the mechanism of central fatigue is complex, it is thought to result from an increase in central serotonin (5-HT) level and inflammatory cytokine release from muscles [7] . Consumption of certain foods is an easy way to prevent both peripheral and central fatigue; for example, carbohydrates have been suggested to delay fatigue during prolonged exercise [9] [10] , and a mixture of specific branched-change amino acids (BCAAs; e.g., valine, leucine, and isoleucine) reportedly provides energy for muscles and inhibits central 5-HT release [11] [12] . Antioxidants such as vitamins C or E have also been shown to prevent oxidative stress and muscular fatigue [13] [14] .

Tomato (Solanum lycopersicum) is one of the most widely consumed vegetables worldwide and contains many beneficial components, including sugars, amino acids, minerals, vitamins, and the antioxidant lycopene [14] . As such, tomato can be expected to reduce fatigue via multiple mechanisms. We investigated this possibility in the present study by evaluating the anti-fatigue effects of tomato juice in humans and mice following exercise.

In the human experiment, ingestion of tomato juice attenuated the subjective feeling of fatigue following exercise, which was corroborated in a mouse models in which fatigue was assessed by locomotor activity and blood marker levels. This suggests that the result in humans was not a placebo effect, but reflected physiological changes induced by components of tomato juice. The same effects were observed in mice using tomato juice supernatant containing few water-insoluble components as well as a water solution composed of the same aqueous components (sugars, OAs, and AAs) as the supernatant.

Lycopene is a red lipophilic pigment found in tomatoes that is a potent, natural singlet-oxygen quencher. However, lycopene and most other lipophilic components were present in low amounts in the tomato juice supernatant (e.g., the lycopene content of the supernatant was only one-fifth that of tomato juice). Hence, the effect of tomato juice was largely attributable to the water-soluble substances in tomatoes. Indeed, supernatant suppressed the exercise-induced decline in locomotor activity to a greater extent than whole tomato juice. This may be explained by the inhibition of nutrient absorption by dietary fiber [17] [18] , which could weaken the anti-fatigue effect.

In mice, only the mixed solution containing all major aqueous components of tomato (sugars, OAs, and AAs) abrogated the post-exercise decrease in locomotor activity and increase in blood markers of fatigue, indicating that anti-fatigue effects of tomato were due not to a single component but to a combination of two or more. A mixture of amino acids―especially EAAs―is known to reduce fatigue, with BCAAs contributing significantly to this phenomenon. During endurance exercise, BCAAs act as a major energy source after the depletion of carbohydrates, and increased oxidation of BCAAs triggers a gradual decrease in tricarboxylic acid cycle intermediates, resulting in fatigue [19] [20] . BCAAs are also thought to influence central fatigue, which may be me-

diated by 5-HT. Brain 5-HT synthesis is stimulated in response to increased delivery to the brain of blood-borne tryptophan (Trp), an amino acid precursor of 5-HT. Trp shares the same blood-brain barrier transport mechanism as BCAAs; thus, brain 5-HT synthesis is induced by increases in the free-Trp/BCAA ratio [21] . Some studies have shown that BCAA supplementation reduces central fatigue via inhibition of brain 5-HT synthesis resulting from a decrease in blood free-Trp/BCAA ratio [22] [23] . Sugars also tend to reduce fatigue to a greater extent than water. Carbohydrates in the form of muscle and liver glycogen and blood glucose are the primary source of energy during exercise; one reason for declines in exercise performance is the depletion of muscle glycogen levels. Furthermore, hypoglycemia can occur during prolonged exercise, which can contribute to fatigue by limiting the supply of energy to working muscles. This can be countered by carbohydrate supplementation, which effectively improves endurance [9] [10] . In contrast, OAs had no effect of indicators of fatigue. Based on these findings, we speculate that EAAs or sugars are the components of tomato juice that are responsible for the observed anti-fatigue effects. However, the amounts were much lower than those reported to reduce fatigue [9] - [12] , giving rise to the possibility of synergistic effects or the contribution of other unidentified substances.

In addition to locomotor activity, blood TGF-β1 and corticosterone levels were measured as fatigue markers. TGF-β1is an anti-inflammatory cytokine that is up regulated during physical exercise [24] [25] , possibly to restrict inflammatory responses to skeletal muscle damage caused by exercise [26] . Moreover, peripheral TGF-β1 levels are higher in patients with chronic fatigue syndrome than in healthy subjects [27] [28] . On the other hand, corticosterone in rodents and cortisol in humans are glucocorticoid hormones that are secreted from the adrenal cortex via hypothalamic-pituitary-adrenal axis activation in response to psychological and physical stress [29] [30] , including energy exhaustion. Glucocorticoids maintain blood glucose levels by stimulating the release of AAs from muscle and promoting gluconeogenesis is and mobilization of fat stores [31] - [33] . Corticosterone is also a peripheral marker of central 5-HT activity [34] [35] . In our experiments, changes in TGF-β1 and corticosterone levels were correlated with decreased locomotor activity, providing evidence that these molecules are markers of fatigue. Taken together, our results indicate that TGF-β1 and corticosterone cause post-exercise fatigue via mechanisms involving peripheral muscle inflammation, peripheral energy exertion, and the central 5-HT system.

We find that ingestion of tomato juice prior to exercise attenuate exercise-induced fatigue. Tomato contains many components that could prevent fatigue via synergistic and/or additive effects. This is a typical example of the benefits of a crude food containing a cocktail of nutrients. These findings are especially important given that excessive intake of refined foods have been shown to have negative effects on human health [36] [37] . Future studies will aim to identify the effective components of tomato juice and clarify their mechanisms of action.

Kei Mukuta,Kazuyoshi Koizumi,Eisuke F. Sato,Keiichi Hiramoto,Masao Hiwatari,Satoshi Sunabori,Hiroyuki Suganuma, (2016) Administration of Tomato Juice or Aqueous Components of Tomato Reduces Fatigue Induced by Acute Treadmill Exercise. Food and Nutrition Sciences,07,524-532. doi: 10.4236/fns.2016.77054