Dietary olive oil supplementation [20
], and more recently olive oil phenols [21
] have been recommended as important therapeutic interventions in preventive medicine. However, a question remains to be addressed: what are the effects of olive oil and its minor phenolic compounds on obesity-induced cardiac metabolic changes?
To the best of our knowledge this is the first study that evaluated the relative potency of olive oil and its phenolic constituents, oleuropein and cafeic acid, on some markers of metabolic pathways in cardiac tissue of obese rats, as well their relationships with calorimetric parameters and oxidative stress. The present study brought new insights into the long term and low-dose intake of olive oil and its compounds on cardiac energy metabolism.
To demonstrate the efficiency of obesity induction [19
] despite the same food intake, Ob rats showed higher final body weight, surface area and BMI than C (Table ). Prospective studies [35
] demonstrated that a relatively low RMR and a high RQ [36
] are predictors of body weight gain. In fact, as previously observed [34
] obesity was characterized in Ob rats from reduced RMR, oxygen consumption, VO2
/final body weight and VO2
/surface area, corroborating with lower fat oxidation (Table ). There was also higher energy balance indicating that energy intake exceeded energy expenditure for vital functions (RMR), thus providing greater amount of energy to be stored as fat.
The detected fasting carbohydrate oxidation in Ob group was associated with liver glycogen storage due hypercaloric diet intake in these animals. Since the influence of the meal had essentially ended in the post absorptive state, the fasting RQ value gives an indication of the glycogen storage [27
As previously found [20
], in both dietary conditions there were no significant alterations in morphometric parameters by olive oil and its phenolic compounds. There were no significant changes in heart weight, heart weight/body weight, as well as in cardiac protein, indicating adequate protein supply in both dietary conditions.
The lower RMR in C-Oleuropein rats, as well the higher RMR in C-Olive and C-Cafeic rats were not enough to significantly change the energy available for fat storage (Table ). Note that in C-Olive and C-Cafeic rats there was a better fat utilization and this was evidenced by the lower RQ [34
] in these animals. The detected carbohydrate oxidation in C-Oleuropein rats was associated with the highest RQ, RQ/body weight and RQ/surface area found in these animals (Table ). Therefore, the lower fat oxidation in C-Oleuropein animals was not surprising, since the change in fuel selection is controlled by carbohydrate intake, and when carbohydrate oxidation rises in response to intake, there is a profound counterregulatory suppression of fat oxidation, because triacylglycerol lipase enzyme from the adipose tissue is inhibited by the insulin secretion [11
]. Oleuropein enhanced glucose uptake in tissues, increasing insulin response [6
] and the glycogen storage [36
It was widely accepted that oxygen consumed for carbon dioxide liberation, during substrate oxidation depends upon the oxygen amount in dietary nutrient. Because fat oxidation requires more oxygen, there was a significant reduction in the RQ, RQ/body weight and RQ/surface area in Ob-Olive, Ob-Oleuropein and Ob-Cafeic rats (Table ).
Several mechanisms may be associated with increased fat oxidation in obese rats receiving olive oil and its phenolic components. It is well known that absorption and delivery of dietary compounds by intestinal cells are part of a complex process, which is influenced by the physiological state of enterocytes. A persistent situation of redox imbalance, due to hypercaloric diet intake has been associated with gastrointestinal alterations [37
]. Under these conditions, it might be expected an imbalance in the absorption of dietary compounds and the metabolic utilization for energy generation [38
]. Olive oil and its compounds modulating cellular signal [39
] and the activity of peptidases [3
] would allow the adequate balance between uptake and metabolism of dietary compounds.
Judging from our experimental results, it was evident that the beneficial effects of olive oil and its minor constituents enhancing fat oxidation were reflected in cardiac tissue of obese rats. Note that there was no antioxidant activity of olive oil and its phenolic compounds in cardiac tissue of animals fed with hypercaloric diet.
The enhanced fat oxidation was demonstrated in cardiac tissue by higher OHADH and CS activities in Ob-Olive rats (Figure and ). OHADH is a key enzyme for fatty acid oxidation, and CS is the key enzyme for the control of the flux of metabolites through tricarboxylic acid cycle. A clear link between triacylglycerol accumulation and the cardiomyopathy was established in experimental models in which the rate of fatty acid uptake by the heart was increased, or the capacity for fatty acid oxidation was reduced in the mitochondria [41
]. Myocardial OHADH activity was also significantly enhanced by oleuropein and cafeic acid in obese animals, despite the maintenance of CS activity in these animals. It has been shown that hypercaloric diet induces adverse effects on cardiac function through changes in fatty acid metabolism, by inappropriate activation and expression of PPARα (peroxisome-proliferator-activated receptor α) as a result of glucotoxicity. The decrease in the transcription of PPARα may be a regulatory event for the reduced use of fatty acids [42
]. Thus, cafeic acid and oleuropein could allow adequate PPARα activation and fatty acid utilization, increasing the OHADH activity under obesity conditions, reducing triacylglycerol accumulation in cardiac muscle. Optimizing cardiac energy metabolism in obese conditions may be one approach to prevent and treat cardiac dysfunction [11
Curiously, no significant changes were found in myocardial metabolic enzymes of C-Olive rats, whereas, CS was significantly reduced in C-Oleuropein rats, despite no changes in OHADH (Figure and ). This indicated enhanced glycolytic pathway relative to aerobic metabolism, or delayed flux of metabolites through tricarboxylic acid cycle in cardiac tissue. Glycolysis preferentially serves energy channeling to sarcolemmal membranes, were glucose transport into cells occurs, by providing this readily available substrate for glycolytic enzymes bound to sarcolemmal molecular complexes [13
]. Glycolytic pathway thus represents low capacity, but high specificity modules of the integrated metabolic network of a cardiac myocite [11
]. On the other hand, the reduced OHADH, and the maintenance of CS activity, clearly indicated that C-Cafeic rats had depressed fatty acid degradation, relative to aerobic metabolism in cardiac tissue, as compared to C, C-Olive and C-Oleuropein rats. Therefore, in standard fed conditions, dietary supplementation with olive oil phenolic compounds induced changes in the substrate used for energy generation in cardiac tissue. Further studies may be considered to clear this open question, and to show the importance of these changes on cardiac function.
Anyhow, the beneficial effect of olive oil in standard-fed condition was evidenced from the reduced myocardial LH in these animals (Table ). Lipids accumulation within the myocardium induces cardiac lipotoxicity [12
]. Oxidative stress and lipotoxicity with increased LH may affect myocardial function, in a fashion that mimics reperfusion injury including persistent cellular loss of K+
, depletion of energy phosphates and decreased metabolic function [43
]. Note that despite the effects of cafeic acid reducing myocardial SOD, no significant changes were found in LH and TAS concentrations as compared with C rats.
Although both oleuropein and olive oil have reduced LH, the antioxidant responses in both groups occurred by different ways. The antioxidant defense system includes SOD that catalyzes the destruction of superoxide radical (O2_
) by dismutation and hydrogen peroxide formation, catalase and GSH-peroxidase that catalyzes the conversion of hydrogen peroxide to water. The total antioxidant substances (TAS) include non-enzymatic, lipophilic and aqueous antioxidants [11
]. Enhanced TAS was associated with direct antioxidant activity of olive oil and oleuropein, because of the free radical scavenger ability [44
]. This property depends on the number of hydroxyl radicals in the phenolic molecule [21
]. Oleuropein and hydroxytyrosol, free radical scavengers, have antioxidant activity as found in other antioxidants, such as vitamin E and butylated hydroxytoluene (BHT) [20
]. On the other hand, it has been demonstrated that olive oil may interfere in iron absorption [24
]. Considering the importance of iron in the oxidative stress induction, the reduced iron uptake and diminished serum concentration of iron may be alternative mechanism for reduced myocardial LH in C-Olive animals.