Green tea (Camellia sinensis
) is one of the oldest beverages known to man in that it has been consumed for thousands of years. In recent times, an increased consumption of green tea has been linked with a wide range of different health benefits such as an increase in antioxidative potential [1
], a reduction in the mortality rate from cardiovascular disease [5
], a reduced development of coronary artery disease [10
] and a lowering of plasma cholesterol [11
]. Furthermore, there has been a rise in research on the individual green tea components, particularly its major component epigallocatechin gallate (EGCG), as attractive targets for the development of nutraceuticals [12
] and functional foods [14
As the most abundant green tea (GT) constituent, EGCG has been the focus of research in relation to the reduction of morbidity and mortality as a consequence of cardiovascular disease. Several mechanisms of action have been proposed as to why EGCG may benefit cardiovascular health in humans, such as the lowering of plasma cholesterol through a reduction of cholesterol absorption [17
], a decrease in cholesterol synthesis [18
] and/or an increase in the cholesterol clearance rate through an upregulation of the LDL receptor [19
]. However, many of the studies of this type in humans have used hot/cold GT preparations or whole GT extracts in powder form that considerably vary in their content of EGCG, other catechins and other components naturally found in GT such as caffeine [18
]. Therefore, despite the fact that EGCG is the predominant compound found in these preparations it is still uncertain whether pure EGCG would exhibit the same health properties in humans as EGCG in a polyphenolic matrix such as GT or GT extracts.
The relatively recent availability of pure, crystalline, stable and affordable EGCG has enabled the design of studies with controlled amounts of EGCG [26
]. Consequently, capsules containing precisely measured amounts of EGCG can be utilised in controlled clinical trials and measured amounts of the crystalline form of EGCG can be incorporated into food products for use as nutraceuticals or functional foods. However, incorporating EGCG into food products requires dissolving the crystalline form in various liquids which could affect the stability of the EGCG and potentially lead to its degradation [29
The processing temperature and pH of EGCG solutions can influence the catechin’s structural stability and cause degradation and epimerization [31
]. Although these processes have been observed to mainly occur at high temperatures and high pH, significant degradation of resolubilised EGCG has also been reported at temperatures below 25 °C [33
]. Therefore, to minimise the loss of EGCG, solutions need to be made at pH below 4 [34
] and kept at low temperatures prior to being used. The use of reducing agents such as ascorbic acid [35
] can also be useful.
The modification and use of foods in order to deliver beneficial health outcomes, commonly referred to as functional foods, is a fast growing area of research [36
]. Apart from the health benefits that these functional foods can bring, the delivery of a functional component within a food can increase the convenience of the component’s consumption and may also reduce the fasting time. A recent study by Hirun and Roach [37
] found that EGCG imbedded in a strawberry sorbet had very good stability during frozen storage for at least 4 months, a benefit ascribed to the low temperature and pH. In addition, this study [37
] and a study by Green et al.
] showed that the stability of EGCG under simulated digestion conditions increased when GT was in the strawberry sorbet or mixed with fruit juices, respectively. However, there are no reports on the effectiveness of EGCG once it is incorporated into a food product relative to any health outcomes in humans.
Irrespective of the format in which it is ingested, EGCG needs to be bioavailable in order to have health outcomes [39
]. Studies on the bioavailability of pure EGCG in humans are limited [27
] as such studies have primarily focused on the pharmacokinetic properties of preparations where EGCG was imbedded in a polyphenolic complex, as part of GT extracts [24
]. In one study, the oral administration of pure EGCG at a dose of 1.6 g in healthy human volunteers [27
] produced physiologically-relevant plasma EGCG concentrations (greater than 1 μmol/L) capable of having beneficial health effects. Although there were variations between individuals, the peak EGCG concentrations were reached between 1.3 and 2.2 h after ingestion and the mean elimination half-life ranged from 1.9 to 4.6 h. However, in this study, only encapsulated EGCG was studied as the method of delivery and only after a 10 h fast [27
To date, only one study in humans has examined the oral bioavailability of EGCG, provided with and without food (a light breakfast) and the EGCG was imbedded in a GT extract [42
]. The findings of this study indicated that EGCG (400 and 800 mg), when taken in the form of a polyphenolic complex, showed a greater systemic absorption when the extract was taken on an empty stomach after an overnight fast and it was very well tolerated with only mild symptoms of discomfort reported [42
Based on the findings of the previous studies [24
] the present study aimed to determine whether the systemic absorption of pure EGCG would be similarly decreased by the presence of food in the form of a light breakfast, as it was for EGCG in the complex polyphenolic GT extract [42
]. Another aim was to test whether EGCG imbedded in a low pH food product such as a strawberry sorbet, could enhance the absorption of EGCG. Therefore, the systemic absorption of EGCG was tested in healthy human volunteers after an overnight fast with the EGCG administered by itself within capsules without breakfast, in capsules taken with a light breakfast or incorporated within a strawberry sorbet, to determine which of the three methods of delivery was the most effective.
The results for the concentration of EGCG in plasma over the 8 h period after ingestion (Figure 4
and Figure 5
, Table 5
) clearly revealed that EGCG taken in capsule form by itself without food gave higher plasma values; the AUC was 2.7 times higher than when EGCG was taken in capsules with a light breakfast (p
= 0.044) and 3.9 times higher than when EGCG was imbedded in the strawberry sorbet (p
= 0.019). Furthermore, there was no significant difference in the AUC values (Table 5
) between the EGCG taken as capsules with a light breakfast or taken in the strawberry sorbet (p
= 1.000). This pattern was also observed when the values for Cmax
were compared between the three ingestion conditions (Table 5
). Therefore, ingesting food at the same time as EGCG whether it was imbedded or not in food, substantially inhibited the absorption of the catechin. As with some types of medications that are affected by food, it appears that EGCG should be taken without food in order to maximise its intestinal absorption.
The concentrations of EGCG measured in the plasma after its ingestion in the present study are consistent with those of the only previous study that used similar doses of administered pure EGCG [27
]. When 500 mg of EGCG was given in capsule form without food in the present study, the plasma EGCG concentration curve (Figure 4
and Figure 5
) was very similar to the plasma EGCG values obtained when a similar dose of 400 mg EGCG was given without food to humans by Ullmann et al.
]. Furthermore, the Cmax
values in the present study, 824 ng/mL and 383 ng/mL, respectively (Table 5
), were very comparable to the Cmax
values, 862 ng/mL, and 568 ng/mL, respectively, observed in the Ullmann et al.
The results in the present study are also very consistent with the previous findings of Chow et al.
], who showed that food intake also clearly interfered with the systemic absorption of EGCG when it was given as part of a GT extract. Similar to the present study (Table 5
), Chow et al.
] also reported markedly higher plasma EGCG AUC (3.5 times higher) and Cmax
(5.7 times higher) values when the catechin extract was taken without food compared to when it was taken with a standardised breakfast consisting of muffins.
With varied food types, such as muffins [49
], a breakfast cereal plus full cream milk and a strawberry sorbet (Figure 4
and Figure 5
, Table 5
), all decreasing the plasma EGCG concentration measured after ingestion, it is evident that food is an important factor, which can affect the intestinal absorption and systemic levels of orally administered EGCG, whether it is in a GT extract or in pure form. However, it is not known how food gives rise to this effect or which particular food component plays a role in decreasing the absorption of EGCG.
This is also not entirely surprising because it is well known that the absorption of some pharmaceutical medications is decreased when they are taken with various food products. From what is known about the interactions of food with medications and plant bioactives [50
], several factors may influence the bioavailability of EGCG. These factors can be divided into three broad categories: (1) the effect of the vehicle in which the EGCG was administered; (2) the effect of the biological fluids on EGCG prior to it reaching its absorption site and; (3) effects on EGCG due to physiological responses to the ingested food products.
Relative to the first category—the effect of the vehicle in which the EGCG was administered—incorporating EGCG in a food product like a strawberry sorbet could have been expected to improve the absorption of EGCG compared to taking the catechin in capsule form with a breakfast. It is well known that EGCG is relatively unstable and susceptible to degradation [29
] at high temperatures [31
] and at pH values above four [34
]. Therefore, imbedding the EGCG in a food product like strawberry sorbet, which is stored at −20 °C and has a pH below 4, may have increased the stability of the EGCG. This was supported by the finding that the EGCG was very stable in this food product, as evidenced by the very high percentage (over 97%) observed for its recovery from the strawberry sorbet after storage at −20 °C (Table 1
). This finding indicated that the method used to prepare the strawberry sorbet and its acidic environment preserved the EGCG extremely well [37
] and therefore, the amount of chemically intact EGCG (500 mg) ingested in the 200 g of sorbet given to the human volunteers was the intended amount.
Evidently however, the fact that the EGCG was chemically intact and stable in the strawberry sorbet did not improve its bioavailability (Figure 4
and Figure 5
, Table 5
). Obviously, similar to the breakfast cereal and the full cream milk in the present study and the muffins in the study by Chow et al.
], the presence of food components in the strawberry sorbet must have played a role in reducing the bioavailability of EGCG and resulted in a significantly lower AUC for EGCG than when it was taken on its own without food.
Relative to the second category—the effect of the biological fluids on EGCG prior to it reaching its absorption site—the principal biological fluids EGCG would come into contact with, are the gastric juice and the pancreatic/biliary juices. Saliva was unlikely to be a factor, as the EGCG capsules taken with the light breakfast were very quickly swallowed with water and therefore, the EGCG was unlikely to have been exposed to much saliva.
In the stomach, little degradation of EGCG is expected to occur because of the acidic nature of the gastric secretions. In effect, as shown by Record and Lane [52
], EGCG was stable in acidic solutions (pH < 3) made up to mimic those found in the fasting stomach environment (pH 1.5–2) [53
]. However, any increase in pH caused by protein or any other component of food could have led to an accelerated degradation of EGCG and reduced its bioavailability [27
In the small intestine, the acidic chyme that is pushed down from the stomach is quickly neutralised by the bicarbonate solution secreted by the pancreas into the duodenum [50
]. This is where most of the EGCG is expected to be lost. As shown by Record and Lane [52
], EGCG is particularly unstable under conditions which mimic digestion fluids in the small intestines, with only 1% of the EGCG still measurable after an hour incubation.
Hirun and Roach [37
] and Green et al.
] showed that the stability of EGCG under these simulated digestion conditions increased when EGCG was in a strawberry sorbet or mixed with fruit juices, respectively. However, the current studies suggest that this is unlikely to happen under the true small intestine conditions in humans, as imbedding the EGCG in a strawberry sorbet in the present study did not lead to any improvement in absorption compared to taking the EGCG in capsules with the light breakfast. Most likely, the amount of bicarbonate solution secreted by the pancreas into the duodenum was enough to fully neutralise any acidity brought down to the small bowel by the strawberry sorbet.
Evidently, the possibility that the acidic nature of the strawberry sorbet could keep the pH of the small intestine less basic either did not occur or was not a factor in the bioavailability of the EGCG it contained. Clearly, other ways of protecting the EGCG from the basic pH in the small intestines are needed. Several studies have reported different methods of preserving the EGCG such as encapsulation using oil-in-water sub-micrometer emulsions [56
], lyposomes [57
] and protein/polyphenols microparticles [58
], but whether they can preserve the EGCG from degradation in the small intestine and increase its bioavailability remains to be determined.
It is possible that taking the capsules without food may have allowed the EGCG to survive longer in the small intestines and therefore, enhance its chances of being absorbed, because it did not elicit strong responses from the stomach and the pancreas. The EGCG capsules by themselves could be expected to have caused the stomach to produce much less acidic chyme than when the EGCG was taken along with food, either in the form of the strawberry sorbet or the breakfast cereal with full cream milk. Consequently, this could have elicited a less strong response from the pancreas to secrete bicarbonate solution to neutralise the chyme coming down from the stomach.
Relative to the third category—effects on EGCG due to physiological responses to the ingested food products—the small intestine is the primary absorption site for EGCG and the rate at which EGCG is presented into the upper portion of the small intestine and travels down to its absorption site can determine the bioavailability of the catechin. It is known that the ingestion of food can delay the rate of gastric emptying [51
] and that the rate of gastric emptying is one of the most important factors known to influence the absorption rate of orally administered pharmaceuticals from the gastrointestinal tract [51
In concurrence with this, EGCG taken with the breakfast and in the strawberry sorbet showed a delay in time it took to reach its maximum concentration in plasma (Tmax
in Table 5
) compared to EGCG taken without food. Therefore, a slower gastric emptying in the presence of food most likely prolonged the time needed for EGCG to travel into the upper portion of small intestine. However, given that the bioavailability of the EGCG was much lower when it was taken with food, some of the extra time is likely to have been spent transiting through the small intestine where exposure to a high pH for longer could have contributed to a greater degradation.
Another possibility, which could have explained the higher plasma EGCG concentrations observed when the catechin was taken without food, was an increased clearance rate of EGCG from the plasma when it was taken with food. However, the mean elimination half-life (T1/2
in Table 5
) was not significantly different between the three ingestion methods (p
> 0.05). Therefore, the ingestion of EGCG with food or incorporated in a food did not appear to significantly influence the clearance rate of free EGCG from the systemic circulation once it was absorbed.
The main limitation of this study is the low number of participants (n = 4) and it would be useful to repeat the study with a higher number of subjects. Nevertheless, despite the low numbers, the inhibitory effect of food on the systemic absorption of the pure EGCG was strong (3–4 times lower with food than without food) and unequivocal (statistically significant). The strength of the cross-over study design may have helped; with each participant acting as their own control in this design, the impact of inter-individual variation is minimised. Therefore, the results were unequivocal in revealing that the EGCG was best absorbed when consumed in capsule form without any food after the overnight fast, a finding which supports the results of Chow et al., the only previous study to study the effect of food on the systemic absorption of EGCG, although it was given as part of green tea extract and not as a pure catechin.