Relative bioavailability of the antioxidant flavonoid quercetin from various foods in man.

Quercetin is a strong antioxidant and a major dietary flavonoid. Epidemiological studies suggest that consumption of quercetin protects against cardiovascular disease, but its absorption in man is controversial. We fed nine subjects a single large dose of onions, which contain glucose conjugates of quercetin, apples, which contain both glucose and non-glucose quercetin glycosides, or pure quercetin-3-rutinoside, the major quercetin glycoside in tea. Plasma levels were then measured over 36 h. Bioavailability of quercetin from apples and of pure quercetin rutinoside was both 30% relative to onions. Peak levels were achieved less than 0.7 h after ingestion of onions, 2.5 h after apples and 9 h after the rutinoside. Half-lives of elimination were 28 h for onions and 23 h for apples. We conclude that conjugation with glucose enhances absorption from the small gut. Because of the long half-lives of elimination, repeated consumption of quercetin-containing foods will cause accumulation of quercetin in blood.

Absorption and disposition kinetics of the dietary antioxidant quercetin in man.

Quercetin is a dietary antioxidant that prevents oxidation of low-density lipoproteins in vitro by scavenging to free oxygen radicals. Its intake was inversely associated with coronary heart mortality in Dutch elderly men. However, data on absorption of quercetin in man are scarce and contradictory. We studied the time course of the plasma quercetin concentration in two subjects after ingestion of fried onions containing quercetin glucosides equivalent to 64 mg of quercetin aglycone. Peak plasma levels of 196 ng/ml were reached after 2.9 h, with a half-life of absorption of 0.87 h. The half-life of the distribution phase was 3.8 h, and of the subsequent elimination phase 16.8 h. After 48 h the plasma concentration was about 10 ng/ml. We conclude that quercetin glucosides from onions are absorbed and are eliminated slowly throughout the day. Thus, the dietary antioxidant quercetin could increase the antioxidant capacity of blood plasma.

Consumption of high doses of chlorogenic acid, present in coffee, or of black tea increases plasma total homocysteine concentrations in humans.

BACKGROUND: In population studies, high intakes of coffee are associated with raised concentrations of plasma homocysteine, a predictor of risk of cardiovascular disease. Chlorogenic acid is a major polyphenol in coffee; coffee drinkers consume up to 1 g chlorogenic acid/d. OBJECTIVE: We studied whether chlorogenic acid affects plasma total homocysteine concentrations in humans. For comparison we also studied the effects of black tea rich in polyphenols and of quercetin-3-rutinoside, a major flavonol in tea and apples. DESIGN: In this crossover study, 20 healthy men and women ingested 2 g (5.5 mmol) chlorogenic acid, 4 g black tea solids containing approximately 4.3 mmol polyphenols and comparable to approximately 2 L strong black tea, 440 mg (0.7 mmol) quercetin-3-rutinoside, or a placebo daily. Each subject received each of the 4 treatments for 7 d, in random order. RESULTS: Total homocysteine in plasma collected 4-5 h after supplement intake was 12% (1.2 micromol/L; 95% CI: 0.6, 1.7) higher after chlorogenic acid and 11% (1.1 micromol/L; 95% CI: 0.6, 1.5) higher after black tea than after placebo. Total homocysteine in fasting plasma collected 20 h after supplement intake was 4% (0.4 micromol/L; 95% CI: 0.0, 0.8) higher after chlorogenic acid and 5% (0.5 micromol/L; 95% CI: 0.0, 0.9) higher after black tea than after placebo. Quercetin-3-rutinoside did not significantly affect homocysteine concentrations. CONCLUSIONS: Chlorogenic acid, a compound in coffee, and black tea raise total homocysteine concentrations in plasma. Chlorogenic acid could be partly responsible for the higher homocysteine concentrations observed in coffee drinkers. Whether these effects on homocysteine influence cardiovascular disease risk remains to be established.

Catechin intake might explain the inverse relation between tea consumption and ischemic heart disease: the Zutphen Elderly Study.

BACKGROUND: Epidemiologic studies suggest that tea consumption may reduce the risk of cardiovascular diseases, but results are inconsistent. Catechins, which belong to the flavonoid family, are the main components of tea and may be responsible for the alleged protective effect. Taking catechin sources other than tea into account might clarify the reported associations. OBJECTIVE: The objective was to evaluate the association between catechin intake and the incidence of and mortality from ischemic heart disease and stroke. DESIGN: We evaluated the effect of a high catechin intake by using data from the Zutphen Elderly Study, a prospective cohort study of 806 men aged 65-84 y at baseline in 1985. RESULTS: The mean (+/-SD) catechin intake at baseline was 72 +/- 47.8 mg, mainly from black tea, apples, and chocolate. A total of 90 deaths from ischemic heart disease were documented. Catechin intake was inversely associated with ischemic heart disease mortality; the multivariate-adjusted risk ratio in the highest tertile of intake was 0.49 (95% CI: 0.27, 0.88; P for trend: 0.017). After multivariate adjustment, catechin intake was not associated with the incidence of myocardial infarction (risk ratio in the highest tertile of intake: 0.70; 95% CI: 0.39, 1.26; P for trend: 0.232). After adjustment for tea consumption and flavonol intake, a 7.5-mg increase in catechin intake from sources other than tea was associated with a tendency for a 20% reduction in ischemic heart disease mortality risk (P = 0.114). There was no association between catechin intake and stroke incidence or mortality. CONCLUSION: Catechins, whether from tea or other sources, may reduce the risk of ischemic heart disease mortality but not of stroke.

Plasma concentrations and urinary excretion of the antioxidant flavonols quercetin and kaempferol as biomarkers for dietary intake.

Flavonols are antioxidants that may reduce the risk of heart disease. Two major flavonols in the diet are quercetin and kaempferol, and their main sources in The Netherlands are tea and onions. We investigated whether plasma concentrations and urinary excretion of quercetin and kaempferol in humans could be used as biomarkers of intake. We provided 15 subjects with strong black tea (1600 mL/d) or fried onions (129 g/d) for 3 d each in random order separated by a 4-d washout period. The tea provided 49 mg quercetin and 27 mg kaempferol daily and the onions provided 13 mg quercetin and no kaempferol. Flavonols from both foods were clearly absorbed. However, the excretion of unmodified quercetin was 0.5% of intake after tea and 1.1% after onions. Thus, the absorption of quercetin from tea was half of that from onions. The onion treatment was repeated 7-14 d later to estimate within-subject CVs as a measure of reproducibility when the same treatment is given twice. CVs for quercetin were 30% in plasma and 42% in urine. The magnitude of these variations relative to actual variations of approximately 60% between free-living subjects indicates that concentrations of quercetin in plasma and urine are applicable as biomarkers of its intake. We conclude that flavonols in plasma and urine reflect short-term flavonol intake and that they could be used as biomarkers to distinguish between high and low flavonol consumption in epidemiologic studies.

Absorption of dietary quercetin glycosides and quercetin in healthy ileostomy volunteers.

Quercetin is a dietary antioxidant that prevents oxidation of low-density lipoproteins in vitro. Intake of quercetin was inversely associated with coronary heart disease mortality in elderly Dutch men. However, the extent of absorption of quercetin in humans is unclear. The aim of this study was to quantify absorption of various forms of quercetin. Nine healthy ileostomy subjects were studied, to avoid losses caused by colonic bacteria. They followed a quercetin-free diet for 12 d; on days 4, 8, and 12 they received a supplement of fried onions at breakfast (rich in quercetin glucosides) equivalent to 89 mg aglycone, pure quercetin rutinoside (the major quercetin compound in tea) equivalent to 100 mg aglycone, or 100 mg pure quercetin aglycone, in random order. Subsequently, participants collected ileostomy effluent and urine for 13 h. In vitro incubations of quercetin or its glycosides with gastrointestinal fluids showed minimal degradation. Absorption of quercetin, defined as oral intake minus ileostomy excretion and corrected for 14% degradation within the ileostomy bag, was 52 +/- 15% for quercetin glucosides from onions, 17 +/- 15% for quercetin rutinoside, and 24 +/- 9% for quercetin aglycone. Mean excretion of quercetin or its conjugates in urine was 0.5% of the amount absorbed; quercetin excretion in urine was negatively correlated with excretion in ileostomy effluent (r = -0.78, n = 27). We conclude that humans absorb appreciable amounts of quercetin and that absorption is enhanced by conjugation with glucose.

Urinary salicylate excretion in subjects eating a variety of diets shows that amounts of bioavailable salicylates in foods are low.

Intake of acetylsalicylic acid reduces the risk of cardiovascular disease and is associated with a decreased risk for colorectal cancer. Amounts of salicylates in foods are thus of interest, but data are scarce and controversial. We gave 58 mumol (10.5 mg) pure acetylsalicylic acid or 66 mumol (9.1 mg) salicylic acid to six volunteers and recovered 77-80% in 24-h urine samples. Thus, urinary excretion is a valid indicator for intake of free forms of (acetyl)salicylic acid. To estimate the bioavailable salicylate contents of diets, we subsequently studied salicylate excretion in 17 volunteers from 14 countries and four continents who ate a wide variety of self-selected diets. Median 24-h urinary salicylate excretion was 10 mumol (range: 6-12 mumol). Values increased with the fiber content of the diet (r = 0.73), suggesting that vegetable foods are the main sources of salicylates. However, amounts of salicylates in a variety of diets are evidently low and probably insufficient to affect disease risk.

Effects of the flavonoids quercetin and apigenin on hemostasis in healthy volunteers: results from an in vitro and a dietary supplement study.

Intake of dietary flavonols and flavones was inversely associated with risk for cardiovascular disease in several epidemiologic studies. This may have been due to effects on hemostasis because flavonoids have been reported to inhibit platelet aggregation in vitro. We indeed found that 2500 micromol/L of the flavonol quercetin and the flavone apigenin significantly inhibited collagen- and ADP-induced aggregation in platelet-rich plasma and washed platelets by approximately 80-97%. However, lower concentrations, such as might occur in vivo, had no effect. To test this in vivo we fed 18 healthy volunteers 220 g onions/d providing 114 mg quercetin/d, 5 g dried parsley/d providing 84 mg apigenin/d, or a placebo for 7 d each in a randomized crossover experiment with each treatment period lasting 2 wk. Onion consumption raised mean plasma quercetin concentrations to 1.5 micromol/L; plasma apigenin could not be measured. No significant effects of onions or parsley were found on platelet aggregation, thromboxane B2 production, factor VII, or other hemostatic variables. We conclude that the antiaggregatory effects of flavonoids seen in vitro are due to concentrations that cannot be attained in vivo. Effects of dietary flavonols and flavones on cardiovascular risk are possibly not mediated by hemostatic variables.

A RP-HPLC method for the determination of tea catechins.

An HPLC method with gradient elution for the quantification of catechins ((-)-epigallocatechin (EGC), (+)-catechin (C), (-)-epicatechin (EC), (-)-epigallocatechingallate (EGCg) and (-)-epicatechingallate (ECg)) in tea was developed. The method was used to determine catechins in black tea, green tea and oolong tea. Sample preparation was simple because only filtration and adjustment of pH was required. The levels of catechins in different teas varied between <5 and 287 (EGC), 17 and 94 (EC), 18 and 408 (EGCg), and 8 and 85 mg/l (ECg). (+)-Catechin was not present in any of these teas.

Acetylsalicylate and salicylates in foods.

Acetylsalicylic acid is effective in the prevention of cardiovascular disease. It was suggested that fruits and vegetables provide unknown amounts of acetylsalicylic acid. We could not find any acetylsalicylic acid in 30 foods using HPLC with fluorescence detection (detection limits: 0.02 mg/kg for fresh, and 0.2 mg/kg for dried products). We showed that urinary excretion of salicylates is a valid indicator for intake, and found a median salicylate excretion of 10 micromol (1.4 mg) in 24 h urine of 17 volunteers eating a variety of diets. Our data suggest that the content of (acetyl)salicylic acid of diets may be too low to affect disease risk.

Consumption of quercetin and kaempferol in free-living subjects eating a variety of diets.

Quercetin and related flavonoids are anticarcinogenic in rats, but little is known about human intakes. The intake of five major flavonols and flavones was calculated using 1-day dietary records of 17 volunteers from 14 countries, and using both 3-day records and a food frequency questionnaire of eight Dutch adults. Total consumption (+/- SD) was 27.6 +/- 19.5 mg/day in the international subjects, 34.1 +/- 31.2 mg/day in the Dutch adults according to 3-day records, and 41.9 +/- 23.7 mg/day according to questionnaires. Quercetin contributed 68-73%, and kaempferol 22-29%, the major sources being tea and onions. A brief food frequency questionnaire may be a suitable method for ranking individuals by flavonol intake.

Bioavailability of the dietary antioxidant flavonol quercetin in man.

Quercetin, a dietary antioxidant flavonoid, has anticarcinogenic properties. We quantified the absorption of quercetin in ileostomists. Absorption was 52 +/- 5% for quercetin glucosides from onions, 17 +/- 15% for quercetin rutinoside, and 24 +/- 9% for quercetin aglycone. The plasma quercetin concentration in subjects with an intact colon, after ingestion of fried onions, apples and pure quercetin rutinoside, decreased slowly with elimination half-lives of about 25 h. Thus, repeated dietary intake of quercetin will lead to accumulation in plasma. The relative bioavailability of quercetin from apples and rutinoside was one-third of that from onions. Absorption kinetics and bioavailibility might be determined by the type of glycoside. Dietary quercetin could increase the antioxidant capacity of blood plasma.

Salicylates in foods.

It was recently suggested that the intake of salicylates from food may have contributed to the decline in cardiovascular mortality in the United States. However, a normal mixed diet provides far too few salicylates to have an effect on disease risk.

Health effects and bioavailability of dietary flavonols.

Flavonoids are polyphenolic compounds that are ubiquitously present in foods of plant origin. Flavonoids are categorised into flavonols, flavones, catechins, flavanones, anthocyanidins, and isoflavonoids. They may have beneficial health effects because of their antioxidant properties and their inhibitory role in various stages of tumour development in animal studies. It is estimated that the human intake of all flavonoids is a few hundreds of milligram per day. Flavonoids present in foods used to be considered non-absorbable because they are bound to sugars as beta-glycosides. However, we found that human absorption of the quercetin glycosides from onions (52%) is far better than that of the pure aglycone (24%). The sugar moiety is an important determinant of their absorption and bioavailability. Flavonol glycosides might contribute to the antioxidant defences of blood. The average intake of the flavonols quercetin, myricetin and kaempferol and the flavones luteolin and apigenin in the Netherlands was 23 mg/day. The intake of these flavonols and flavones was inversely associated with subsequent coronary heart disease in some but not all prospective epidemiological studies. A protective effect of flavonols on cancer was found in one prospective study; two others showed no association. Thus the epidemiological evidence does not yet allow a decision on the involvement of flavonols in the etiology of either cardiovascular diseases or cancer.

Addition of milk does not affect the absorption of flavonols from tea in man.

Tea is a major source of flavonols, a subclass of antioxidant flavonoids present in plant foods which potentially are beneficial to human health. Milk added to tea, a frequent habit in the United Kingdom, could inhibit absorption of tea flavonoids, because proteins can bind flavonoids effectively. Eighteen healthy volunteers each consumed two out of four supplements during three days: black tea, black tea with milk, green tea and water. A cup of the supplement was consumed every 2 hours each day for a total of 8 cups a day. The supplements provided about 100 micromol quercetin glycosides and about 60 - 70 micromol kaempferol glycosides. Addition of milk to black tea (15 ml milk to 135 ml tea) did not change the area under the curve of the plasma concentration-time curve of quercetin or kaempferol. Plasma concentrations reached were about 50 nM quercetin and 30 - 45 nM kaempferol. We conclude that flavonols are absorbed from tea and that their bioavailability is not affected by addition of milk.

The sugar moiety is a major determinant of the absorption of dietary flavonoid glycosides in man.

Flavonoids are antioxidants present in plant foods. They occur mainly as glycosides, i.e. linked with various sugars. It is uncertain to what extent dietary flavonoid glycosides are absorbed from the gut. We investigated how the nature of the sugar group affected absorption of one major flavonoid, quercetin. Quercetin linked with glucose, i.e. quercetin glucoside and quercetin linked with rutinose, i.e. quercetin rutinoside, both occur widely in foods. When we fed these compounds to nine volunteers, the peak concentration of quercetin (Cmax) in plasma was 20 times higher and was reached (Tmax) more than ten times faster after intake of the glucoside (Cmax = 3.5+/-0.6 microM (mean +/- SE); Tmax < 0.5 h) than after the rutinoside (Cmax = 0.18+/-0.04 microM; Tmax = 6.0+/-1.2 h). The bioavailability of the rutinoside was only 20% of that of the glucoside. We suggest that quercetin glucoside is actively absorbed from the small intestine, whereas quercetin rutinoside is absorbed from the colon after deglycosylation. Absorption of other food components might also be enhanced by attachment of a glucose group.

Protection of UV-induced suppression of skin contact hypersensitivity: a common feature of flavonoids after oral administration?

In this study we investigated the effect of the dietary ingredients fruit and vegetable, green tea phenol extract (GTP) and the specific flavonoid components quercetin and chrysin on the UV-induced suppression of the contact hypersensitivity (CHS) response to picryl chloride (PCl). The SKH-1 mice were fed with test diet from 2 or 4 weeks before and during the UV irradiation (daily, 95 mJ/cm2) and tested for the CHS ear-swelling response 10 weeks after the onset of the irradiation. For the CHS, mice were immunized with PCl by epicutaneous application on nonirradiated sites. Four days after sensitization all mice were challenged on both sides of each ear by topical application of one drop PCl. In addition, from mice fed with the fruit and vegetable mixture the number of Langerhans cells (LC) were scored in the skin and from mice fed with quercetin, quercetin levels in plasma were measured at week 11 after the start of UV irradiation. It was found that fruit and vegetable (19% in the diet), GTP (0.1% and 0.01% in the drinking water), quercetin (1% in the diet) and chrysin (1% and 0.1% in the diet), prevented statistically significantly the UV-induced suppression of CHS to PCl. In the skin of mice fed with fruit and vegetables combined with UV irradiation the number of LC were comparable to the control mice, whereas the number of LC were significantly diminished in mice treated with UV only. This protective effect on the presence of LC in the epidermis after UV irradiation, which was also observed in a previous study with quercetin, may play a role in the prevention of UV-induced immunosuppression by the flavonoids tested. In conclusion, we found protection of flavonoids against UV-induced effects on CHS, which may be a common feature of most flavonoids.

Bias and error in the determination of common macronutrients in foods: interlaboratory trial.

Chemical analyses of nutrient values in foods form the basis of much of the science and practice of nutrition and dietetics, but little is known about the accuracy and precision of common macronutrient analyses. Therefore, an interlaboratory study was set up. One batch each of egg powder, full-fat milk powder, whole rye flour, whole wheat flour, biscuits, and French beans (snap beans, "haricots verts") was thoroughly homogenized. Samples were sent to 19 leading food analysis laboratories in Europe and the U.S., and each performed analyses of macronutrients by its own routine methods. Most were government or semi-government laboratories and major contributors to national nutrient data banks. The results for dry matter content and ash agreed well between laboratories. For protein, the coefficient of variation between laboratories (CV between) ranged from 2.8% to 6.4%. The reproducibility within laboratories was sometimes quite poor. The CV between for total fat ranged from 5.4% to 54%. For "available" carbohydrates, the CV between ranged from 9% to 27%. The CV between for total dietary fiber ranged from 23% to 84%. Only part of the variability could be explained by the use of methods of different principle. It is concluded that leading laboratories produce widely different values for macronutrients in common foods. Quality control programs and reference materials of certified nutrient concentration are urgently needed.

Absorption, metabolism and health effects of dietary flavonoids in man.

Flavonoids are polyphenolic compounds that occur ubiquitously in foods of plant origin. Over 4,000 different flavonoids have been described, and they are categorized into flavonols, flavones, catechins, flavanones, anthocyanidins and isoflavonoids. Flavonoids have a variety of biological effects in numerous mammalian cell systems, in vitro as well in vivo. Recently, much attention has been paid to their antioxidant properties and to their inhibitory role in various stages of tumour development in animal studies. Quercetin, the major representative of the flavonol subclass, is a strong antioxidant, and prevents oxidation of low density lipoproteins in vitro. Oxidized low density lipoproteins are atherogenic, and are considered to be a crucial intermediate in the formation of atherosclerotic plaques. This agrees with observations in epidemiological studies that the intake of flavonols and flavones was inversely associated with subsequent coronary heart disease. However, no effects of flavonols on cancer were found in these studies. The extent of absorption of flavonoids is an important unsolved problem in judging their many alleged health effects. Flavonoids present in foods were considered non-absorbable because they are bound to sugars as beta-glycosides. Only free flavonoids without a sugar molecule, the so-called aglycones, were thought to be able to pass through the gut wall. Hydrolysis only occurs in the colon by microorganisms, which at the same time degrade flavonoids. We performed a study to quantify absorption of various dietary forms of quercetin. To our surprise, the quercetin glycosides from onions were absorbed far better than the pure aglycone. Subsequent pharmacokinetic studies with dietary quercetin glycosides showed marked differences in absorption rate and bioavailability. Absorbed quercetin was eliminated only slowly from the blood. The metabolism of flavonoids has been studied frequently in various animals, but very few data in humans are available. Two major sites of flavonoid metabolism are the liver and the colonic flora. There is evidence for O-methylation, sulphation and glucuronidation of hydroxyl groups in the liver. Bacterial ring fission of flavonoids occurs in the colon. The subsequent degradation products, phenolic acids, can be absorbed and are found in urine of animals. Quantitative data on metabolism are scarce.

Catechin intake and associated dietary and lifestyle factors in a representative sample of Dutch men and women.

OBJECTIVE: To study the intake of catechins in the Dutch population and to assess the relation between catechin intake and other dietary factors. Catechins, dietary components that belong to the flavonoid family, potentially protect against chronic diseases such as cancer and cardiovascular diseases. Catechins are the major components of tea, but they are present in many other plant foods as well. DESIGN: Data were used from a nationwide dietary survey carried out in 1998 among a representative sample of 6200 Dutch men and women aged 1-97y. Dietary data were collected using a 2 day dietary record method. RESULTS: The average daily catechin intake was 50 mg (s.d. 56 mg/day). Catechin intake increased with age, and the intake was higher in women (60 mg/day) than in men (40 mg/day). Tea was the main catechin source in all age groups, whereas chocolate was second in children, and apples and pears were second in adults and elderly. Catechin intake was lower in smokers than in non-smokers, and increased with socio-economic status. A high intake was associated with a high intake of fiber (r = 0.20), vitamin C (r = 0.17) and beta-carotene (r = 0.10). CONCLUSIONS: Catechins are quantitatively important bioactive components of the daily diet, which should be taken into account when studying the relation between diet and chronic diseases. Catechin intake is only moderately associated with the intake of other nutrients, but much stronger with certain health behaviours such as smoking.