Figure 2. Mechanisms for the visceral fat-lowering effects of egg white protein [9,21,28,29,30]. Mechanisms for the visceral fat-lowering effects of egg white protein [9][21][28][29][30].
An allergy study found that the ovalbumin in EWP is not easily degraded by pepsin in unheated egg whites but is easily degraded in heated egg whites
[31]. Therefore, if ovalbumin in LE is less likely to be degraded by pepsin, the digestibility of pepsin may be involved in the visceral fat-reducing mechanism. Therefore, when
wresearche
rs evaluated the pepsin degradability of ovalbumin in LE, it was found that the ovalbumin in LE was not easily degraded by pepsin
[21].
Pepsin is a gastric digestive enzyme, indicating that the main factor in the visceral fat-reducing effect of EWP may be occurring in the gastrointestinal tract. The triglyceride absorption into the lymph of rats undergoing thoracic lymph duct cannulation surgery was also reduced
[28]. It was shown that the suppression of lipid absorption by ovalbumin in unheated egg whites may have reduced the visceral fat level. In an in vitro study by Handa et al. (1999), ovalbumin was reported to bind to fatty acids
[32]. This indicated that ovalbumin, which is not easily affected by pepsin, can bind to fatty acids degraded by lipase in the digestive tract, thereby suppressing fatty acid absorption.
However, while the suppression of lipid absorption by EWP is approximately 20% higher than that by casein
[28], the visceral fat reduction effect is 30% less than that of casein
[9]. Despite the differences in experimental conditions, these tests were conducted by feeding diets containing 20% casein or EWP, and the visceral fat reductions by EWP could not be explained solely by the suppression of lipid absorption. Other mechanisms for reducing visceral fat using EWP include increased β-oxidation in the muscle and liver, as well as a reduction in the visceral adipocyte area
[9].
The enhancement of β-oxidation in the liver was reported in a study in which rats were fed diets containing 20% casein or EWP. The results showed that EWP consumption resulted in significantly higher activity levels for carnitine palmitoyltransferase and acyl-CoA oxidase, which are involved in β-oxidation in the liver, when compared to casein
[9].
The enhancement of β-oxidation in the muscle was identified in a similar study in which rats were a fed diet containing 20% casein or EWP, and the EWP diet was found to increase gastrocnemius muscle mass and showed higher levels of enzyme activities related to β-oxidation in the muscle when compared to casein
[33].
Studies investigating the decrease in visceral fat area in rats have also shown an increase in the expression of a series of genes, including peroxisome proliferator-responsive receptor (PPAR) γ2 and adiponectin in visceral adipocytes, as well as improved insulin sensitivity
[29,30][29][30]. The reductions in visceral fat by EWPs were also thought to be caused by these effects.
The following two possibilities were considered as explanations for the increased β-oxidation in the muscle and liver and the increased gene expression in the visceral adipocytes. The first was the possibility of a secondary effect for lipid absorption suppression. The second was the possibility of a direct effect of the EWP-derived components on the liver, muscle, and adipose tissue. For the latter, the absorption rate of the unheated EWP was reported to be over 90%
[21]. In addition, EWP was digested into peptides in the gastrointestinal tract
[34]. Therefore, the absorption of peptides derived from EWP may generate these effects. In the liver, soy-derived peptides reportedly enhance the gene expression of enzymes related to β-oxidation in a mouse model for diabetes
[35]. For white fat cells, it has been reported that soy hydrolysate enhances the gene expression of PPARγ2, albeit this has only been found for in vitro results
[36]. Based on these findings,
wresearche
rs expected to find a peptide with similar effects in the EWP. The identification of the peptide sequence that exerts this effect in EWP will be a subject for future studies.
Reductions in visceral fat using EWP in humans have not yet been investigated. Like EWP, lactoferrin and β-conglycinin from soybeans have also been reported to reduce visceral fat
[37,38][37][38]. Lactoferrin is reported to reduce visceral fat at 300 mg daily and β-conglycinin at 5 g per day
[37,38][37][38]. The inhibition of lipid absorption has been reported as a mechanism for visceral fat reduction using β-conglycinin
[39], and the inhibition of lipid absorption by ovalbumin, ovotransferrin, and lysozyme has been reported as a visceral fat reduction mechanism for EWP
[28,40][28][40]. To consume 5 g of these three components in equal measure, 8 g of LE as EWP needs to be consumed.
At the same time, the function of ovotransferrin in EWP was found to be similar to that of lactoferrin. If ovotransferrin has the same physiological activity as lactoferrin, it was hypothesized that consuming 3 g of lactic-fermented egg white per day as protein would reduce the level of visceral fat. Based on this, the minimum effective amount of lactic-fermented egg white was determined to be 3–8 g of protein per day.
Therefore, the effects of LE containing 6 or 8 g of EWP per day or 8 g of whey protein per day was evaluated on the visceral fat levels of 22 Japanese adult males (overall) with a BMI ≥ 24 and waist ≥ 85 cm for eight weeks. The results showed that the consumption of lactic-fermented egg white, which was equivalent to 8 g of EWP per day, reduced the visceral fat area when compared to that before consumption. A further analysis in subjects with a BMI > 25 showed that 8 g per day of lactic-fermented egg white as EWP significantly reduced the visceral fat area and visceral fat/subcutaneous fat ratio when compared to pre-consumption levels or when whey protein was used instead
[21]. The results indicate that 8 g of EWP per day may reduce the total visceral fat area.
4. Antifatigue Effects of Egg White Protein Hydrolysate
The physiological functions of EWP have been described. However, various health effects of hydrolyzed proteins, which are artificially prepared using enzymes, especially peptides, derived from milk and soybeans have also been reported
[57,58,59][41][42][43]. The physiological functions of egg white protein hydrolysate (EWH: mainly Peptifine
®, Kewpie Corporation, Tokyo, Japan) were, thus, assessed.
The absorption and nutritional value of whey protein hydrolysate with a molecular weight of 500 and EWH with a molecular weight of 2500 were assessed (Egg White Peptide EP-3, Henningsen Foods Inc., Omaha, NE, USA). The results showed that for the EWH, the absorption rate of amino acids into the portal vein was faster than with the whey protein hydrolysate, despite its larger molecular weight. The PDCAAS and DIAAS of egg white hydrolysate as well as whey protein and its hydrolysate. In addition, the net protein utilization was significantly higher for EWH than whey protein hydrolysate
[22].
One of the ongoing challenges for EWP is its bitter flavor. To address this, a new hydrolysate that was less bitter was developed by enzymatically degrading egg whites and collecting the soluble fraction (Peptifine
®)
[60][44].
Egg whites have high amounts of sulfur-containing amino acids and branched-chain amino acids (BCAA)
[61][45]. Since sulfur-containing amino acids are the source of glutathione
[62][46], they are expected to have antioxidant properties. Antioxidant effects are known to prevent various diseases, including atherosclerotic diseases
[63][47], and have also been reported to lead to antifatigue effects. In addition, BCAAs have been reported to improve muscle fatigue
[64][48], and since the blood BCAA concentration increases when EWP is ingested and decreases after exercise
[65][49], an antifatigue effect of the BCAAs can also be expected (
Figure 3).
No antifatigue effects have been reported for proteins, but they have been reported for peptides. Imidazole peptides derived from chickens reportedly have antioxidant properties, and consequently antifatigue effects
[66][50]. Peptides from whey have been reported to increase swimming times in forced swimming mice by chelating radical scavengers and iron
[67][51].
EWH has also been reported to have antioxidant properties
[68][52]. In addition, the pepsin degradation products of egg whites have also been reported to have in vivo antioxidant and antifatigue effects
[69][53].
The effects of EWH (Peptifine
®) on the swimming times of weight-loaded forced swimming mice were consequently investigated. Seven-week-old male ddY mice were divided into a casein group, an EWH group, and an EWP group and fed diets containing each for 14 days. The swimming times were evaluated daily starting on the 11th day of the study. On the last day of the test, blood samples were taken for a blood test. The results showed that the swimming time of the EWH group was significantly longer than for the casein and EWP groups on the 14th day of the testing. The reason for this was a decrease in the concentration of hexanoyl lysine in the blood of the EWH group. In summary, the egg white hydrolysate was found to prolong the swimming time in forced swimming, and the mechanism of the prolonged swimming time was thought to be due to the antifatigue effect of its antioxidant action (
Figure 3)
[70][54].
In healthy marathon runners, 7.5 g of EWH (Peptifine
®) per day for eight weeks was found to improve the subjective symptoms of physical and overall fatigue when compared to the controls. Additionally, at this time, serum aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase were found to be significantly lower than in the controls at week 8
[71][55].
Similarly, when marathon runners were given 2.5 g and 5 g of EWH per day for eight weeks, physical fatigue was significantly lower in the group given 5 g of EWH per day when compared to the control group
[71][55].
These results indicate that the minimum effective dose of EWH to exert its antifatigue effect is 5 g. In addition, when athletes were continuously given 5 g of EWH (Peptifine
®) per day for two weeks, the psychological fatigue was reduced when compared to the control
[72][56]. It was, thus, determined that EWH could be used effectively to improve the exercising capacity of humans, as it is a good source of protein and raw materials for muscles due to its amino acid composition, and it is quickly absorbed and improves physical fatigue through its antioxidant effects.
Figure 3. Mechanisms for the antifatigue effects of egg white protein hydrolysate [61,62,63,64,65,70]. Mechanisms for the antifatigue effects of egg white protein hydrolysate [45][46][47][48][49][54].