Plant Foods for Human Nutrition

, Volume 63, Issue 2, pp 71–76

Banana Prevents Plasma Oxidative Stress in Healthy Individuals

Authors

    • Hospital of Yanbian University
  • Jishu Quan
    • Department of BiochemistryMedical College of Yanbian University
  • Takemichi Kanazawa
    • Institute of Food Factors Science for Health
Original Paper

DOI: 10.1007/s11130-008-0072-1

Cite this article as:
Yin, X., Quan, J. & Kanazawa, T. Plant Foods Hum Nutr (2008) 63: 71. doi:10.1007/s11130-008-0072-1
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Abstract

There is increasing evidence that lipid peroxidation and oxidative modification of low density lipoprotein (LDL) is important in atherogenesis. The present study was designed to study the effects of a single banana meal on plasma lipids and lipoprotein profile, plasma oxidative stress and susceptibility of LDL to oxidation in 20 healthy volunteers. Lipid and lipid peroxide (LPO) levels were measured before the meal (baseline, fasting) and 2 h after it (post-dose). The susceptibility to copper-induced oxidation of baseline and post-dose LDL was measured as conjugated diene (CD) formation. Results showed that the LPO contents in plasma, very low density lipoprotein (VLDL), LDL and high density lipoprotein (HDL) decreased significantly in the 2 h post-dose phase. Prolongation of lag phase and decrease of CD formation during LDL oxidation indicated that post-dose LDL was less susceptible to oxidative modification than the homologous fasting LDL. In conclusion, the consumption of banana reduces the plasma oxidative stress and enhances the resistance to oxidative modification of LDL.

Keywords

BananaHumanLDL OxidationLipid peroxidesOxidative stress

Abbreviations

CD

conjugated diene

TC

cholesterol

EDTA

ethylenediaminetetraacetic acid

HDL

high density lipoprotein

LPO

lipid peroxide

LDL

low density lipoprotein

PL

phospholipid

TG

triglyceride

VLDL

very low density lipoprotein

Introduction

Studies demonstrate that there is a link between postprandial state and risk of cardiovascular disease. Postprandial levels of triglycerides and triglyceride-rich lipoproteins correlate better with the risk for coronary heart disease than those in the post absorptive state [1]. Postprandial hyperlipemia induces alterations in metabolism and composition of all major lipoproteins [2, 3] and LDL isolated during postprandial lipemia is more susceptible to oxidation in vitro than fasting isolated LDL [4]. Moreover, postprandial LDL induces a higher cholesterol accumulation into cultured macrophages than fasting LDL [4]. The exact process of biochemical reactions that regulate the relationship between postprandial hyperlipemia and coronary heart disease remains unclear, but the balance between antioxidant/pro-oxidant species in food may represent the key factor [5].

Considerable epidemiological evidence suggests an association between consumption of fruit and vegetables and a decreased risk of cardiovascular disease and certain forms of cancer [57]. It is not known what dietary constituents are responsible for this association, but it is often assumed that antioxidants contribute to the protection [68]. However, the results from intervention trials have not been conclusive regarding the protection of supplementation with pure antioxidants [9, 10]. It is therefore plausible that the putative beneficial effects of a high intake of fruit and vegetables on the risk of diseases may not result exclusively from the action of antioxidants, such as the well-characterized vitamins E and C or β-carotene. Rather, they may result from the action of minor compounds or from a concerted action of a combination of different antioxidants present in these foods. Cao et al. [11] have found that, in general, more than 80% of the total antioxidant capacity in fruits and vegetables comes from ingredients other than vitamin C, indicating the presence of other potentially important antioxidants in these foods. Flavonoids and other phenolic compounds appear to be antioxidants that contribute to the high antioxidant capacity observed in certain fruits and vegetables [12]. Banana is an excellent tropical fruit with the highest consumption in the world. The banana has an agreeable flavor and a high nutritional value. The content of sugars, fiber, vitamins, and minerals of bananas is high, while the content of fat is low [13]. Wang et al. [14] examined the antioxidative potency in several fruits and fruit juices and reported that banana had a medium antioxidative potency among these fruits. Kanazawa et al. [15] had also examined the antioxidative potency of several fruits and found that tropical fruits had strong activity. They found that banana extract suppressed the autoxidation of linoleic acid after incubation in an emulsion system, as determined from the peroxide value and thiobarbituric acid reactivity, and identified dopamine in bananas as a strong water-soluble antioxidant.

The present study was designed to investigate the effects of a single banana meal on plasma lipids and lipoprotein profile, plasma oxidative stress and susceptibility of LDL to oxidation.

Materials and Methods

Reagents and Chemicals

Total cholesterol (TC), phospholipid (PL) and triglyceride (TG) enzymatic kits were purchased from Wako Pure Chemical Industries Ltd, Japan. Lipid peroxide (LPO) test kit was obtained from Kyowa Medex Company Ltd, Japan.

Subjects and Test Meal

Fourteen men (body mass index < 26 kg/m2, 31 ± 5 years) and six women (body mass index < 28 kg/m2, 30 ± 6 years) were selected from the laboratory staff. All study participants were considered in good health based upon a medical history questionnaire, physical examination and normal results of clinical laboratory tests. The study protocol was approved by the Human Investigation Review Committee of Yanbian University. The subjects were asked to keep their diet as constant as possible during the study period and none of them was taking any drug or vitamin supplement. In the evening before the day of sampling, subjects were fasted overnight. In the morning of the sampling day, a 10-ml blood sample (baseline sample) was obtained from fasting subjects, following which they were given a glass of beverage containing 400 g of banana blended in 300 ml water. Following the zero time blood sample, additional blood samples (post-dose) were collected at 2 h after consumption of the treatment meal. The consumption of water was not limited during the collection period, while other foods or beverages were not allowed.

The nutrient and vitamin intake in banana meal is presented in Table 1. Nitrogen content was obtained by applying the Kjeldahl method, and the protein content was calculated by using a nitrogen factor of 6.25 [16, 17]. Carbohydrates and lipids were also determined according to the method described by the Association of Official Analytical Chemists (AOAC) [16]. Dietary fiber was determined according to the methods proposed by Prosky et al. [18]. Ascorbic acid was determined according to Sgherri and Navari-Izzo’s procedure [19]. Carotenoids were determined with high-performance liquid chromatographic method proposed by Miller et al. [20].
Table 1

The nutrient composition in banana

Component

Content (%)

Protein

1.6

Carbohydrate

18.8

Lipid

0.27

Fiber

2.4

Ascorbic acid

1.0 × 10−2

Carotenoids

9.1 × 10−5

Plasma Samples Preparation

Baseline and post-dose blood was collected in tubes containing ethylenediaminetetraacetic acid (EDTA) (1 mg/ml) and plasma was immediately separated by centrifugation at 1,000×g for 15 min at 4 °C. Plasma samples for antioxidant assay were stored at −80 °C until the analysis.

Lipoprotein Preparation and Determination of Biochemical Indices in Plasma and Lipoproteins

VLDL, LDL, and HDL were isolated from plasma by sequential ultracentrifugation, according to a modified method of Havel et al. [21] in a HITACHI himac cpα 100 ultracentrifuge. Protein was measured by the modified Lowry procedure of Markwell et al. [22], using bovine serum albumin as standard. TC, TG, PL and LPO levels were determined by enzymatic methods using commercial test kits.

Resistance to Copper Ion-mediated Oxidation of LDL

For oxidation experiments, the isolated LDL was dialyzed in the dark against pH 7.4, 10 mM phosphate buffered saline (PBS) buffer for a total of 20 h at 4 °C. Following dialysis, the protein concentration in lipoprotein fractions was adjusted to 0.1 g/l. LDL oxidation was stimulated by addition of copper sulfate in PBS to produce a final copper ion concentration of 5 μM. The resistance of baseline and post-dose LDL to oxidative modification was measured by recording the formation of CDs in a Shimadzu UV-1202 spectrophotometer at 234 nm over a period of 3 h. The indices of lipid peroxidation measured were lag time and oxidation rate. The length of the lag phase was defined as the lag time to the intercept of the tangent of the absorbance curve in the propagation phase with baseline. Propagation rate was expressed as the slope of tangent (change in absorbance per minute) [23, 24].

Statistical Analysis

All the results were presented as mean±SD. Non-parametric Wilcoxon test was performed using Statistics Package for Social Science (SPSS) 11.5 software (SPSS Inc., USA). Differences were considered as being statistically significant at p < 0.05.

Results

Assessment of Blood Lipids, Total Protein, Lipoproteins and Lipid Peroxides

No significant treatment effect was found for plasma TC, PL and total protein assessed at baseline and 2 h post-dose. However, the plasma TG rose significantly by 8.1% at 2 h post-dose than baseline (Table 2).
Table 2

Changes of blood lipid, protein and LPO

Sample

Time (h)

TC (mg/dl)

TG (mg/dl)

PL (mg/dl)

Protein (mg/l)

LPO (μmol/l)

Baseline

0

184 ± 29

99 ± 50

217 ± 35

2,552 ± 304

6.4 ± 1.8

Post-dose

2

185 ± 31

107 ± 49*

214 ± 38

2,532 ± 316

4.4 ± 1.1*

The results are expressed as means±SD of 20 subjects

TC: cholesterol, TG: triglyceride, PL: phospholipid, LPO: lipid peroxide

*p < 0.05, compared with baseline

In this experiment, the lipid peroxidation was studied as an index of oxidative damage to the whole plasma. The concentration of plasma LPO decreased significantly by 26.6% at 2 h over baseline (Table 2).

Beyond a decrease in plasma LPO level, banana intake induced substantial modifications in the plasma lipoproteins. In the post-dose phase, an expected increase of TC, TG and PL content in VLDL was observed, while the protein content of VLDL was not changed (Table 3). A small but significant decrease was apparent for LDL protein level while there was no significant change of TG, TC and PL levels in LDL (Table 3). Interestingly, LDL-TC, the main lipid in LDL, increased significantly relative to protein level. In HDL, the protein did not change significantly during the 2-h period, but HDL-TG decreased significantly at 2 h post-dose than baseline. HDL-TC and PL also tended to decrease, though the difference was not statistically significant (p = 0.083 and 0.118, respectively, Table 3). However, HDL-TC and PL, the main lipids in HDL, decreased significantly relative to protein level.
Table 3

Changes of lipid, protein and LPO in lipoproteins

Components

Baseline

Post-dose

VLDL

LDL

HDL

VLDL

LDL

HDL

TC(mg/dl)

18 ± 10

119 ± 28

44 ± 12

21 ± 11*

120 ± 26

43 ± 13

TG(mg/dl)

65 ± 47

24 ± 4

9.5 ± 2.8

74 ± 48*

24 ± 4

8.8 ± 2.3*

PL(mg/dl)

22 ± 15

86 ± 19

111 ± 29

26 ± 16*

84 ± 18

107 ± 32

Protein (mg/l)

208 ± 92

966 ± 195

1,388 ± 237

213 ± 88

923 ± 192*

1,380 ± 251

LPO (μmol/l)

2.5 ± 1.3

1.8 ± 0.9

1.9 ± 0.6

1.8 ± 0.9*

1.4 ± 0.5*

1.5 ± 0.6*

The results are expressed as means±SD of 20 subjects

VLDL: very low density lipoprotein, LDL: low density lipoprotein, HDL: high density lipoprotein, TC: cholesterol, TG: triglyceride, PL: phospholipid, LPO: lipid peroxide

*p < 0.05, compared with baseline

LPO levels in VLDL, LDL and HDL fractions, the index of oxidative damage to the plasma lipoproteins, decreased significantly by 25–29% during the 2-h post-dose period (Table 3).

Assessment of LDL Susceptibility to Oxidative Modification

The oxidative stability of LDL, an accepted index for the occurrence of oxidative modifications to particle structure and composition, was measured as a marker of plasma oxidative stress. When the LPO content in LDL was expressed on a per mg cholesterol basis, then the post-dose LDL was found to contain less LPO molecules at the beginning of copper ion-induced oxidation than the baseline LDL (p < 0.05, Table 4).
Table 4

Changes of susceptibility of LDL to oxidative modification

Sample

Time (h)

Initial LPO (nmol/mg cholesterol)a

Lag time (min)

CD (nmol/mg protein)b

Propagation rate (ΔA234/min × 10−2)

Baseline

0

1.6 ± 0.7

65 ± 29

60 ± 27

3.5 ± 1.3

Post-dose

2

1.2 ± 0.5*

76 ± 32*

49 ± 21

2.8 ± 1.2

The results are expressed as means±SD of six subjects

CD: conjugated diene, LPO: lipid peroxide

aInitial LPO contents in LDL at the beginning of oxidation were expressed on a per mg cholesterol basis

bAbsorbance units were converted to molar units of CD using the molar extinction coefficient 29,500 M−1cm−1

*p < 0.05, compared with baseline

The results also indicated an increased lag phase of LDL oxidation curve and a mean prolongation of test LDL lag phase by more than 11 min in comparison with fasting LDL. The total formation of CD and propagation rate of LDL oxidation curve also non-significantly decreased than baseline fasting LDL. Although the observed differences did not reach statistical significance, the kinetics of CD formation indicated that, post-dose LDL was less susceptible to oxidative modification than baseline LDL.

Discussion

According to the most widely accepted theory of atherogenesis, oxidatively modified LDL activates a series of cellular events in the arterial wall ultimately leading to plaque formation [25]. Yet, the mechanism of formation in vivo of modified LDL is still uncertain, even though the evidence that human plasma contains oxidatively modified lipoproteins has been achieved [26, 27]. The novelty of our approach consists in studying the effect of a single meal of banana, looking at a direct acute effect rather than to a stabilized increase of antioxidant resistance following a long-lasting banana intake.

Our result indicates that, the post-dose LDL was more resistant to oxidative modification than baseline LDL. In vitro susceptibility of LDL to oxidative modification depends on the content of LPO from which metal ions generate initiating free radicals, on the content of antioxidants, on the fatty acid composition, and, reasonably on some intrinsic structural factors not yet clearly identified. With the exception of the fatty acid composition, banana could affect almost the totality of these factors.

Our results showed that during the 2-h period after the banana meal consumption, the LPO decreased significantly in the whole plasma, VLDL, LDL and HDL. Banana consumption also significantly decreased the post-dose LDL protein level which might reflect a reduction in the circulating LDL particle number. LDL-TC, the main lipid in LDL, increased significantly relative to protein level, probably indicating increased LDL particle size. In HDL, in contrast to unchanged protein level, HDL-TC and PL, both the main lipids in HDL, decreased significantly relative to protein level, probably indicating decreased HDL particle size. Though the reasons for the results need to be explained, the LPO levels in plasma, as well as in VLDL, LDL and HDL, still negatively correlates with the risk for coronary heart disease [28, 29]. Banana feeding could have protected the lipoproteins in vivo by forming particles with diminished concentrations of preformed lipid hydroperoxides. Preformed hydroperoxides enhance the initiation rate of oxidation, which, in turn, influences the length of the lag time. Accordingly, the prolonged lag times during banana intake could reflect lower initiation rates because of reduced hydroperoxide formation in vivo.

Plant foods contain various antioxidants, tocopherols, ascorbic acid, carotenoids, and flavonoids. Antioxidants in banana could minimize the postprandial increase of lipid hydroperoxides in plasma [30], and then in LDL by protecting against the buildup of new peroxides in the digestive tract and/or by contributing (if bioavailable) to the plasma antioxidant capacity. It has been reported that banana contains leucocynadin, a flavonoid [31]. The possibility of the incorporation of a small amount of phenolics into LDL, as reported for soybean isoflavones [32], cannot be excluded. It was also revealed that banana contained a strong antioxidant, dopamine, in large amounts and ascorbic acid at a considerable level [14]. The antioxidative potency of dopamine was greater than that of flavonoids, glutathione, and catechin, and similar to that of the strong antioxidants gallocatechin, gallate and ascorbic acid [14]. Banana is therefore one of the best sources of antioxidants. Our results also agree well with the findings of Prior et al. [33], who have demonstrated that consumption of certain fruits was associated with increased plasma antioxidant capacity in the postprandial state and consumption of an energy source of macronutrients containing no antioxidants was associated with a decline in plasma antioxidant capacity.

In conclusion, we found that the lipid peroxidation of plasma and lipoproteins, and the susceptibility to oxidative modification of LDL of healthy individuals were significantly decreased following the consumption of banana meal. However, without further long term clinical studies, one cannot necessarily translate decreases in lipid peroxidation of lipoproteins into a potential decreased risk of chronic degenerative disease.

Acknowledgement

This study was supported by the grant from the National Natural Science Foundation of China (No. 30360113).

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© Springer Science+Business Media, LLC 2008