Cosmetic or Dietary Vegetable Oils Sampled in the Cameroonian Market May Not Expose Consumers to Lipid Oxidation Products Generating Oxidative Stress and Inflammation

Vegetable oils are a source of energy, essential fatty acids, antioxidants and fat-soluble vitamins useful for human health care and development. These oils also contribute to organoleptic quality of their products’ derivatives. However, their chemical and physical properties can be modified by the mode of their extraction, storage and distribution. These modifications might negatively affect the nutritional quality of the oils. The goals of this study were to: sample different vegetable oils for cosmetic or dietary use marketed in Cameroon, and verify purity and oxidation states of each kind of oil through determination of its acidity, iodine, peroxide, saponification, refractive indexes and the conformity of the labeling. The carotene content, the level of polar components and specific absorbance were also determined. As the result, six oils namely palm, palm kernel, coconut, black cumin, peanut and shea butter were collected. Apart from labeling, chemicals and physicals parameters analyzed were generally in accordance with the Cameroonian and Codex Alimentarius standard. This study suggests that vegetable oils sampled in the Cameroonian market may not expose consumers to lipid oxidation products generating pathological oxidative stress and inflammation. However, efforts in application of existing standard need to be done as far as labeling are concerned.

Lipid oxidation in food science and nutritional health: A comprehensive review

Food provides abundant nutrients for human beings, but also has sensory functions and physiological regulation.Lipids are the main components of food as well as the important structural and functional components of cells.Nevertheless, lipids are easily oxidized by different ways, such as thermal oxidation and air oxidation. Lipidoxidation has adverse effects on food quality and human health. Therefore, efforts should be made to reduce lipidoxidation and improve its stability. This review focuses on important knowledge about lipid oxidation, includingthe concept of lipids and lipid oxidation, the main pathways and mechanisms of lipid oxidation, factors affectinglipid oxidation, strategies to improve the stability of lipid oxidation, and the recent research progress of lipidoxidation in food science and nutritional health.

Mediterranean Berries as Inhibitors of Lipid Oxidation in Porcine Burger Patties Subjected to Cooking and Chilled Storage

The efifciency of extracts from Arbutus unedo L. （AU）, Crataegus monogyna L. （CM）, Rosa canina L. （RC）, and Rubus ulmifolius Schott. （RU） to inhibit lipid oxidation in raw, cooked and cooked and chilled （2°C/12 d） porcine burger patties, was investigated. The modiifcation of the fatty acid proifle during processing treatments （cooking and chilling）, the quantitative measurements of thiobarbituric acid reactive substances （TBA-RS）, and lipid-derived volatiles, were used as indicators of lipid oxidation. Polyunsaturated fatty acids （PUFA） gradually decreased during cooking and the subsequent storage of cooked burger patties with this decrease being signiifcantly greater （P〈0.05） in control patties than in those with added berry extracts. In accordance, the control patties showed signiifcantly higher TBA-RS numbers and counts of lipid-derived volatiles in all treatments when compared to the berry-added counterparts （P〈0.05）. Results from the present work show, for the ifrst time, that extracts from A. unedo, C. monogyna, R. canina, and R. ulmifolius are promising antioxidants which could enhance the nutritional, safety and sensory properties of porcine burger patties.

Changes of protein oxidation,lipid oxidation and lipolysis in Chinese dry sausage with different sodium chloride curing salt content

The effect of sodium chloride(NaCl)curing salt content on protein oxidation,lipid oxidation and lipolysis of Chinese dry sausage was investigated.Two groups Chinese dry sausages with 2%and 4%(m/m)salt content were studied.The degree of protein oxidation increased during the processes in two groups sausages,while the content of phospholipids decreased,neutral lipids and free fatty acids increased.The degree of protein oxidation,lipid oxidation and lipolysis in 4%NaCl content group was higher than those in 2%NaCl content group,while 4%NaCl content group has higher lipase activity.In conclusion,4%NaCl may facilitate the protein oxidation,lipid hydrolysis and oxidation in Chinese dry sausage,and the protein oxidation had strong correlation with lipid oxidation and lipolysis.The results could provide a basis for improving the technology of industrial production.

Changes of Intramuscular Fat Composition,Lipid Oxidation and Lipase Activity in Biceps femoris and Semimembranosus of Xuanwei Ham During Controlled Salting Stages

Fatty acid composition of neutral lipids （NLs）, phospholipids （PLs） and free fatty acids （FFAs） from intramuscular fat （IMF）, lipid oxidation and lipase activity in muscle Semimembranosus （SM） and msucle Biceps femoris （BF） of dry-cured Xuanwei ham during the 90-d salting stages were analysed. The salt content increased from 0.34 to 3.52%in BF and from 0.10 to 5.42%in SM during the 90 d salting stage, respectively. PLs of IMF in both BF and SM decreased 54.70%（P〈0.001） and 34.64%（P〈0.05）, furthermore, the saturated fatty acids （SFA）, monounsaturated fatty acids （MUFA） and polyunsaturated fatty acids （PUFA） of PLs in both muscles were hydrolysed almost isochronously. FFAs were increased from 0.46 g 100 g-1 lipids to 2.92 g 100 g-1 lipids in BF at the end of salting, which was lower than SM （from 1.29 g 100 g-1 lipids to 9.70 g 100 g-1 lipids）. The activities of acid lipase, neutral lipase and acid phospholipase all remained active in the 90 d. The thiobarbituric acid reactive substances （TBARS） was slowly increased to 1.34 mg kg-1 muscle in BF and to 2.44 mg kg-1 muscle in SM during the salting stage. In conclusion, the controlled salting process prompted the hydrolysis of PLs of IMF notably and increased the lipid oxidation of muscles within some limits.

The Inhibiting Effect of Bone Protein Hydrolysates on Lipid Oxidation in Pork Patties

Bone protein hydrolysates were prepared by limited alcalase hydrolysis （5 h）. The hydrolysates were formulated （0-3%, w/w） into pork patties to determine the antioxidant efficacy. 0.02% BHA （butylated hydroxyanisole） was used as a positive control. Lipid oxidation in patties during storage was analyzed by measuring the TBARS and protein carbonyl content. The results showed that bone protein hydrolysates possessed significant antioxidant activity, and antioxidant activity increased with the increasing hydrolysates concentration. Sensory evaluation indicated that bone protein hydrolysates improved the color and decreased lipid oxidation flavor of pork patties. The 2% bone hydrolysates possessed the highest antioxidant activity and better sensory quality, and its effect was closed to 0.02% BHA.

Estrogen inhibits lipid peroxidation after hypoxic-ischemic brain damage in neonatal rats

Sprague-Dawley neonatal rats within 7 days after birth were used in this study. The left common carotid artery was occluded and rats were housed in an 8% O2 environment for 2 hours to establish a hypoxic-ischemic brain damage model. 17β-estradiol （1 × 10-5 M） was injected into the rat abdominal cavity after the model was successfully established. The left hemisphere was obtained at 12, 24, 48, 72 hours after operation. Results showed that malondialdehyde content in the left brain of neonatal rats gradually increased as modeling time prolonged, while malondialdehyde content of 17β-estrodial-treated rats significantly declined by 24 hours, reached lowest levels at 48 hours, and then peaked at 72 hours after injury. Nicotinamide-adenine dinucleotide phosphate histochemical staining showed the nitric oxide synthase-positive cells and fibers dyed blue/violet and were mainly distributed in the cortex, hippocampus and medial septal nuclei. The number of nitric oxide synthase-positive cells peaked at 48 hours and significantly decreased after 17β-estrodial treatment. Our experimental findings indicate that estrogen plays a protective role following hypoxic-ischemic brain damage by alleviating lipid peroxidation through reducing the expression of nitric oxide synthase and the content of malondialdehyde.

Lipid Oxidation in Mechanically Deboned Chicken Meat:Effect of the Addition of Different Agents

The study evaluated the effect of sodium chloride (1.5%), sodium erythorbate (0.5% and 1.0%) and ascorbic acid (0.1% and 0.2%) on inhibiting lipid oxidation in mechanically deboned chicken meat (MDCM). The peroxide, acidity, pH, color and odor values of the samples were determined on the 1st, 3rd and 5th days. Treatments with sodium erythorbate and ascorbic acid had significant influence (p ≤ 0.05) on the peroxide, acidity and pH values. Ascorbic acid and erythorbate sodium were especially effective in reducing lipid oxidation in mechanically deboned chicken meat.

Effect of Tea Polyphenols on Lipid Oxidation of Nemipterus VirgatusSurimi Cake During Ice Storage

Surimi cake is a high value aquatic product. In this work, lipid oxidation and volatile compounds in Nemipterus virgatus surimi cake were investigated to evaluate the effectiveness of tea polyphenols in preventing or reducing lipid oxidation. Surimi cake was prepared in the presence or absence of 0.2% tea polyphenols, and was stored at 4 ℃ for 16 days. It was found that the peroxide value(POV) and thiobarbituric acid reactive substances(TBARS) values of the experimental groups(with 0.2% tea polyphenols) were much lower than those of the untreated surimi cake(p<0.05). In total, 52 compounds were presented in the volatiles of the surimi cake, such as ethanol, hexanal,(E,E)-2,4-decadienal, 2,3-octanedione, 2-hydroxy-propanamide, and 2,5-dimethyl-pyrazine. It indicated that tea polyphenols could reduce the amount of hexanal, ethanol, and(E,E)-2,4-decadienal, and delay the lipid oxidation of the surimi cake. Effects of lipid oxidation of the tea polyphenols on surimi cake were reduced during storage in ice.

The Effect of Mercury on Lipid Peroxidation and Its Relation with Vitamin （A, E） and Essential Elements in Dentals Serum

In the present study, the authors attempted to shed a light on the possible relationships between lipid per oxidation markers, serum malondialdehyde （MDA） with the effect of mercury exposure using serum samples obtained from 100 mercury exposed dentals due to their occupation. These dentals were divided into two groups according to their occupational period. The third group consists of healthy volunteers that were working outside the mercury factories as a control group. The present results showed that mercury levels increased with the increasing of their occupation period. The highest level was 5.7 μg/dl, compared to the control group which was 0.1 μg/dl., The level of MDA was elevated in the workers and the highest level was found to be 41.00 nmol/dL for workers with longest period compared to the control group 8.6 nmol/dL. Some serum antioxidants such as vitamin A, E and albumin were evaluated and were found to be lower in all workers compared to the control group. The present results showed that Mg levels increased with the increasing of their occupation period and were found to be lower in Zn, Se of all workers compared to the control group.

Terminalia arjuna:A novel natural preservative for improved lipid oxidative stability and storage quality of muscle foods

The study was conducted to explore the possibility of utilization of Terminalia arjuna as a novel natural preservative in meat products by using chevon sausages as a model system.Chevon sausages were prepared by incorporating different levels of T.arjuna viz.T1(0.25%),T2(0.50%)and T3(0.75%)and were assessed for various lipid oxidative stability and storage quality parameters under refrigerated(4±1◦C)conditions.T.arjuna showed a significant(p<0.05)effect on the lipid oxidative stability as the treated products exhibited significantly(p<0.05)lower TBARS(mg malonaldehyde/kg)values in comparison to control.A significant(p<0.05)effect was also observed on the microbial stability as T.arjuna incorporated products showed significantly(p<0.05)lower values for total plate count(log cfu/g),psychrophilic count(log cfu/g),yeast and mould count(log cfu/g)and FFA(%oleic acid)values.Significantly(p<0.05)higher scores were observed for various sensory parameters of the products incorporated with T.arjuna during refrigerated storage.T.arjuna successfully improved the lipid oxidative stability and storage quality of the model meat product and may be commercially exploited as a novel preservative in muscle foods.

Membrane Lipid Replacement: Clinical Studies Using a Natural Medicine Approach to Restoring Membrane Function and Improving Health

Functional oral supplements containing cell membrane glycerolphospholipids and antioxidants have been used to safely replace damaged membrane lipids that accumulate during aging and in various clinical conditions. This approach differs from other dietary and intravenous interventions in the composition of phospholipids and the presence of fructooligosaccharides that protect the phospholipids against oxidation and bile and enzymatic damage. Various chronic clinical conditions are characterized by membrane phospholipid oxidative damage, resulting in loss of cellular function. Recent clinical trials have shown the benefits of Membrane Lipid Replacement in replenishing damaged membrane lipids and restoring mitochondrial function, resulting in reductions in fatigue in aged subjects and patients with a variety of clinical diagnoses. Recent in vitro experiments with nonphysiological concentrations of phospholipids did not result in enhancement of mitochondrial electron transport enzyme activities. This can be explained by the use of the wrong phospholipid fatty acids, over-dilution of membrane constituents and mitochondrial swelling. A similar phenomenon was seen when human sperm were incubated in vitro with high concentrations of glycerolphospholipids and their motility was assessed. Only lower, more physiological concentrations of glycerolphospholipids stimulated sperm motility. Additional studies are needed to determine the functional effects of Membrane Lipid Replacement on other cellular membranes, such as the plasma membrane and other intracellular membranes of various cells and tissues.

Anti-hypertensive effects of rosiglitazone on renovascular hypertensive rats:role of oxidative stress and lipid metabolism

This study investigated the anti-hypertensive mechanismof rosiglitazone in renovascular hypertensive rats,and examined its relationship to oxidative stress and lipid metabolism. The renovascular hypertension was induced by stenosis of the left renal artery. Four groups of rats were selected： control,induced untreated,rosiglitazone（ 20 mg / kg） and captopril（ 10 mg / kg）. After 14 d of administration,compared with induced untreated group,rosiglitazone group reduced the renovascular hypertensive rats ＇ systolic blood pressure and diastolic blood pressure,and decreased total cholesterol（TCH）,triglyceride（TG）,angiotensin II（ Ang II） and angiotensin receptor（ AT1） levels（ P ＆lt; 0. 05）. Meanwhile,rosiglitazone remarkably decreased the levels of malondialdehyde（ MDA） and hydrogen peroxide（ H2O2） while improved the levels of supperoxide dismutase（ SOD） and reduced glutathione（ GSH）. These results suggested that rosiglitazone could effectively decreased the blood pressure in renovascular hypertensive rats,and this might be performed by regulating the activity of angiotensin and the lipid metabolismand improving the oxidative stress.

Effect of dietary magnolia bark extract supplementation in finishing pigs on the oxidative stability of meat

Background:Magnolia bark extract(MBE)is a natural supplement with antioxidant,anti-inflammatory,and antimicrobial activities.Its properties suggest that the dietary supplementation in livestock could improve the quality of products.Therefore,the aim of this study was to investigate,for the first time,the effect of dietary MBE supplementation(0.33 mg/kg)in finishing pigs on the oxidative stability of meat.Oxidative stability is of paramount importance for pork,as it affects storage,retail,and consumer acceptance.For the purpose,the fatty acid profile,cholesterol,fatsoluble vitamins,antioxidant enzymes(catalase,glutathione peroxidase,and superoxide dismutase),non-enzymatic antioxidant capacity(TEAC,FRAP,and Folin-Ciocalteu assays),color stability,and lipid stability of pork were assessed.Results:Concerning carcass characteristics,dietary MBE did not affect cold carcass yield,but reduced(P=0.040)the chilling weight loss.The meat from pigs fed MBE had a lower(P=0.031)lightness index than the control meat.No effect on intramuscular fat,cholesterol,and fatty acid profile was observed.Dietary MBE did not affect the content of vitamin E(α-tocopherol andγ-tocopherol)in pork,whereas it reduced(P=0.021)the retinol content.The catalase activity was 18%higher(P=0.008)in the meat from pigs fed MBE compared with the control group.The MBE supplementation reduced(P=0.039)by 30%the thiobarbituric acid reactive substances(TBARS)in raw pork over 6 d of aerobic refrigerated storage.Instead,no effect on lipid oxidation was observed in cooked pork.Last,the meat from pigs fed MBE reduced Fe3+-ascorbate catalyzed lipid oxidation in muscle homogenates,with a lower(P=0.034)TBARS value than the control group after 60 min of incubation.Conclusions:Dietary MBE supplementation in finishing pigs delayed the lipid oxidation in raw meat.This effect was combined with an increased catalase concentration.These results suggest that dietary MBE could have implications for improving the shelf-life of pork.

The application of Melissa officinalis L.essential oil nanoemulsions protects sea bass(Lateolabrax japonicus)against myofibrillar protein and lipid oxidation during refrigeration

Objectives:The nutrient rich sea bass is prone to oxidation of lipid and protein during refrigeration.Materials and Methods:The research was to investigate the effect of different concentrations of Melissa officinalis L.essential oil(MOEO)nanoemulsions on myofibrillar protein(MP)and lipid oxidation in sea bass(Lateolabrax japonicus)during refrigeration at 4°C.Results:The results of thiobarbituric acid reactive substances and mitochondrial membrane potential showed that carboxymethyl chitosan/locust bean gum active coating solutions incorporating 2%MOEO nanoemulsions(C/L-2M)was the most effective in inhibiting lipid oxidation that occurred in sea bass under attack by reactive oxygen species.Low-field nuclear magnetic resonance results showed that C/L-2M maximally slowed the conversion of bound water to free water during storage.The oxidation of lipids and MP disrupted the secondary and tertiary conformations of MP and accelerated protein aggregation and degradation.Conclusions:C/L-2M slowed the oxidation of lipids and proteins by inhibiting the oxidation of reactive oxygen species.C/L-2M is a very promising preservative emulsion for the preservation of sea bass.

Changes and Correlations of Duroc Muscle pH,Glycogen,Lactic Acid and TBA under Different Storage Condition

The changes and correlations of muscle pH, glycogen, lactic acid and in- tramuscular fat oxidation in Duroc pigs 10 d after their slaughter, and the effects of different storage temperature and time on Duroc muscle pH value, water loss rate, glycogen, lactic acid and 2-thiobarbituric acid （TBA） were studied. The results showed that during the 10 h after the slaughter, the pH value was decreased rapid- ly, the lactic acid content was increased significantly, while the glycogen and TBA contents were remained stable. At the storage temperature of 4 ℃, storage time showed no significant effects on Duroc muscle pH value and glycogen, lactic acid and TBA contents. At the storage temperature of -20 ℃, storage temperature had significant effects on pH value, while no significant effects on other indicators. The correlation analysis demonstrated that during the 10 h after the slaughter, the TBA content was negatively related to glycogen content （P〈0.05）, but positively related to lactic content （P〈0.05）; the pH value was negatively related to lactic acid content （P〈0.05）. At the storage temperature of 4 ℃, the TBA content was negatively relat- ed to water loss rate （P〈0.01） and lactic acid content （P〈0.05）; the water loss rate was positively related to pH value （P〈0.01） and lactic acid content （P〈0.05）. At the storage temperature of -20 ℃, the TBA content was negatively related to pH value （P〈0.01） and positively related to water loss rate （P〈0.05）; the water loss rate was negatively related to pH value （P〈0.01） and lactic acid content （P〈0.05）.

Inhibition of lipid oxidation in foods and feeds and hydroxyl radical-treated fish erythrocytes:A comparative study of Ginkgo biloba leaves extracts and synthetic antioxidants

This study explored the effects of butylated hydroxytoluene(BHT) and ethoxyquin(EQ) and ethyl ether extracts, ethyl acetate extracts(EAE), acetone extracts, ethanol extracts and aqueous extracts of Ginkgo biloba leaves(EGbs) on lipid oxidation in a linoleic acid emulsion, fish flesh and fish feed and in hydroxyl radical(·OH)-treated carp erythrocytes. The linoleic acid, fish flesh and fish feed were incubated with BHT,EQ and EGbs at 45℃ for 8 d, respectively, except for the control group. The lipid oxidation in the linoleic acid emulsion, fish flesh and fish feed was then measured by the ferric thiocyanate method or thiobarbituric acid method. The carp erythrocytes were treated with BHT, EQ. or EGbs in the presence of40 μmol/L FeSO_4 and 20 μmol/L H_2 O_2 at 37℃ for 6 h,except for the control group. Oxidative stress and apoptosis parameters in carp erythrocytes were then evaluated by the commercial kit. The results showed that BHT, EQ and EGbs inhibited lipid oxidation in the linoleic acid emulsion, fish flesh and fish feed and·OH-induced phosphatidylserine exposure and DNA fragmentation(the biomarkers of apoptosis) in carp erythrocytes. Furthermore, BHT, EQ and EGbs decreased the generation of reactive oxygen species(ROS),inhibited the oxidation of cellular components and restored the activities of enzymatic antioxidants in·OH-treated carp erythrocytes. Of all examined EGbs, EAE showed the strongest effects. The effects of EAE on lipid oxidation in the linoleic acid emulsion and on superoxide anion and malonaldehyde levels,catalase activity and apoptosis in. OH-treated carp erythrocytes were equivalent to or stronger than those of BHT. Moreover, these results indicated that the inhibition order of EGbs on the generation of ROS and oxidation of cellular components in fish erythrocytes approximately agreed with that for the food and feed materials tested above. And, the antioxidative and anti-apoptotic effects of EGbs were positively correlated with their flavonoid content. Taken together, these results revealed that the fish erythrocyte system can be used as an experimental model to evaluate lipid oxidation in food and feed ingredients. The EAE can be used as a potential natural antioxidant or apoptosis inhibitor. The inhibition effects of EGbs on lipid oxidation and apoptosis may be due to the presence of flavonoid compounds.

Risk assessment of chemical substances of safety concern generated in processed meats

This manuscript is reviewing the presence of substances of safety concern that may be generated in different types of meat products.Such substances include N-nitrosamines that can be generated under certain conditions when using nitrite as preservative,the polycyclic aromatic hydrocarbons generated as a consequence of particular smoking processes,heterocyclic aromatic amines generated under particular cooking conditions,compounds released from the oxidation of lipids and proteins,Maillard reaction products like acrylamide and carboxymethyl lysine,and amines that can be generated and accumulated in fermented meats.These hazardous compounds,their mechanisms of generation,risks for health and ways of preventing its presence in meat products are briefly described in this review.

Application of Natural Antioxidants from Strawberry Tree(Arbutus unedo L.) and Dog Rose(Rosa canina L.) to Frankfurters Subjected to Refrigerated Storage

The effect of the addition of natural antioxidants from strawberry tree （Arbutus unedo L.; AU） and dog rose （Rosa canina L.; RC）, in frankfurters elaborated with or without the addition of antioxidant additives （sodium ascorbate and nitrite） was studied. Six different types of experimental frankfurters were prepared depending on the addition of phenolic-rich extracts from RC and AU and the presence （P） or absence （C） of antioxidant additives. Thiobarbituric acid reactive substances （TBARS）-numbers signiifcantly increased during chilled storage of C-frankfurters while additives and fruit phenolics inhibited lipid oxidation in P-frankfurters. The amount of protein carbonyls signiifcantly increased in all treatments except in P-AU frankfurters. The discoloration process that occurred during the chilled storage was reduced by the addition of substances with proven antioxidant activity （P-frankfurters）. Texture characteristics as hardness, springiness, cohesiveness and gumminess also suffered a signiifcant deterioration in C-frankfurters. The use of phenolic fruit extracts in combination with traditional antioxidant additives is a successful strategy to enhance the oxidative stability of frankfurters without modifying their color and texture properties.

Overweight and Obesity-Induced Oxidative Stress in Children

Objective To investigate whether overweight and obesity might cause oxidative stress and potential oxidative damage in overweight and obese children, and to explore its possible mechanism. Methods Eighty-five overweight and obese children （OOC）, and eighty-five age-matched healthy children （HC） were recruited in this case-control study. The present study analyzed spectrophotometrically vitamin C （VC）, vitamin E （VE）, and β-carotene （β-CAR） in plasma, as well as the activities of superoxide dismutase （SOD）, catalase （CAT）, and the level of malondialdehyde （MDA） in erythrocytes. Results Compared with those of VC, VE, β-CAR, SOD, CAT and MDA in the HC group, the average values of VC, VE, β-CAR, SOD, and CAT in the OOC group were significantly decreased （P〈0.001）, while the average value of MDA in the OOC group was significantly increased （P〈0.001）. The regression analysis demonstrated that VC, VE, β-CAR, SOD, and CAT were negatively correlated （P〈0.05-0.01）, and MDA was positively correlated with BMI （P〈0.05）. Fitting to the model of multiple stepwise regression of BMI on VC, VE, β-CAR, SOD, CAT, and MDA in 85 OOC was Y = 27.0041 ＋ 0,2541MDA - 2.1448β-CAR -- 0.0090CAr, where F = 43.8088, P〈0.001, r = 0.7866, r^2= 0.6187, adjusted r^2= 0.6046. The findings from the reliability analysis for VC, VE, β-CAR, SOD, CAT, and MDA used to reflect increased oxidative stress and potential oxidative damage in the OOC showed that the reliability coefficients （alpha, 6 items） = 0.7231, P〈0.0001, and that the standardized item alpha = 0.9207, P〈0.0001, Conclusion The present study suggests that there exists an increased oxidative stress in overweight and obese children.