In recent years, attention has been developed to the protective biochemical function of natural antioxidants contained in dietary plants that are candidates for prevention or protection of oxidative damage caused by free radical species. Population studies indicated that in individuals regularly consuming nuts have reduced risk for cardiovascular disease and diabetes. In clinical and experimental trials, nuts appear to have beneficial effects on glucose profile as well
20-22. In our study, pistachio consumption insignificantly reduced blood glucose levels and supported the antioxidant system.
In the present study, we aimed to adapt the dosage of pistachio (10 oz =310 g) to rats, in order to examine the effect of pistachio consumption on glucose, SA levels and oxidant-antioxidant status in a rat model with moderate diabetes. For this purpose, we have calculated the equivalent dose of pistachio for rats as 0.88 g pistachio/week, consuming human weight as 70kg and rat weight as 200g. Accordingly, pistachio added diets that contain 1.26 % pistachio were prepared.
Experimentally, STZ produced a significant increase in serum glucose levels which was reduced insignificantly due to pistachio consumption. Nuts contain 70% to 80% fat, and most fatty acids in nuts are unsaturated (polyunsaturated and monounsaturated), which may be beneficial for glucose and insulin homeostasis. Several studies have shown that a higher intake of monounsaturated and polyunsaturated fatty acids improves insulin sensitivity21.
In literature, The Nurses' Health Study has shown that the consumption of peanuts and peanut butter 5 times a week was associated with a 27% and 21% reduction in risk of type 2 diabetes, respectively1. In the same study of 86,016 female registered nurses those consuming at least 5 oz (155 g) of nuts/week had a % 35 lowering in nonfatal myocardial infarction compared with those eating less than 1 oz of nuts23.
The polyphenol resveratrol (RSV) was detected in the aqueous extracts from the edible nut of cultivars of pistachio24 and resveratrol treatment in STZ induced diabetic rats improved blood glucose levels25. The hypoglycemic effects of RSV administration seen in STZ induced rats was explained by a reduced food intake and/or a reduced digestion and absorption of the foods by the gastrointestinal system. Furthermore, RSV is a strong antioxidant and it may improve the functional states of the metabolic machinery of the cells26. In our study, pistachio consumption insignificantly reduced the glucose levels of the control group. They are energy-dense and good sources of fiber and protein, and contain dietary factors that increase satiety ratings. Moreover people who consume nuts may tend to engage in higher levels of physical activity than non-nut eaters6. Hu et al.23 reported that nut consumption was associated with greater frequency of vigorous exercise among the Nurses Health Study participants. The Physicians' Health Study also noted that men who ate nuts frequently were more physically active27.
It was reported that human plasma has a chain breaking antioxidant capacity and ability to trap free radicals5,28. Induction of STZ significantly depleted the antioxidant activity in our experimental model. This reduction might be due to the higher production of oxidative agents. Reduced antioxidant levels as a result of increased free radical production in experimental diabetes has previously been reported29. Biological effects of ROS are controlled in vivo by a wide spectrum of enzymatic and non-enzymatic defense mechanisms, in particular SOD, which catalyzes dismutation of superoxide anions to H2O2; and catalase, which then converts H2O2 into molecular oxygen and water5,30. According to our study, decreased SOD activity in the diabetic group could be due to its depletion or inhibition as a result of the increased production of free radicals. However pistachio consumption insignificantly increased SOD levels in diabetic rats. GSH also protects tissue from damage caused by diabetes. Depletion of tissue GSH is one of the primary factors that permit lipid peroxidation. GSH participates in many cellular functions particularly elimination of H2O2, interaction with free radicals and regeneration of vitamin E. Therefore, decreased GSH content in plasma may impair antioxidant defenses and accelerate the oxidative damage31. Also, GSH significantly decreased in the diabetic group. In literature, serum GSH levels has been reported to be decreased indicating that the reduced activity of the antioxidant defense system was caused by long-term diabetes10. Administration of pistachio had no effect on GSH levels in diabetic rats.
Free radicals may cause the oxidation of protein thiol groups. TT groups in proteins serve as antioxidants by scavenging free radicals, thus sparing antioxidants and/or cellular constituents from attack. Therefore, the measurement of sulphydryl groups in proteins may be useful32. Consistent with the literature, although statistically insignificant, TT levels decreased in diabetic group compared with the control group which might result from an increased consumption to detoxify an augmented load of free radicals generated in diabetes. However, treatment with pistachio insignificantly increased total thiol levels in our diabetic group. The decrease in TT levels in diabetic rats were consistent with the findings of other investigators11,33.
Hyperglycemia is the primary symptom of diabetes and is blamed for the complications of diabetes because elevated glucose concentration directly injures cells and induces lipid peroxidation34. In our study, we observed a significant increase in TBARS levels in diabetic rats. However, pistachio consumption decreased TBARS levels. Thus, we propose that pistachio treatment may have a role in scavenging free radicals generated by STZ induction.
Sialic acid is a terminal component of the non-reducing end of carbohydrate chains of glycoproteins and glycolipids. These are essential constituents of hormones, enzymes and tissues. SA is reported to increase in humans and animals during a number of pathological situations where the contributory event is either of tissue damage, tissue proliferation or inflammation35-37. It has been demonstrated that the activity of sialidase (the enzyme that liberates sialic acid) is increased in human leukocytes from people with type 2 diabetes, in endothelial cells stimulated with advanced glycation endproducts, and in an animal model of diabetes37,38. In diabetic rats, we observed a significant increase in SA levels when compared to the control group and consumption of pistachio nuts insignificantly decreased serum SA in the diabetic group.
The results of our study support a positive effect of pistachio consumption on oxidant-antioxidant status. Other than the positive effects of nuts on blood glucose levels; pistachio nuts may be included in diets to lower the oxidative stress induced by diabetes. However, studies with longer period of observation times are needed to prove the reliability of pistachio consumption in the long term.
Acknowledgement
We are grateful to Dr. Ahmet Oral and İmam Çağdaş for providing pistachios.