Abstract
Both obesity and hypertension are associated with endothelial dysfunction. The purpose of this study was to investigate the effects of a low-calorie diet on endothelial function in obese patients with essential hypertension.
We measured forearm blood flow (FBF) during intra-arterial infusion of acetylcholine (ACh; 7.5, 15, 30 μg/min), an index of endothelium-dependent vasodilation, and isosorbide dinitrate (ISDN; 0.75, 1.5, 3.0 μg/min), an index of endothelium-independent vasodilation, in obese patients with essential hypertension before and after 2 weeks on a low-calorie diet (800 kcal/d). The study included 11 obese hypertensive Japanese patients (mean body mass index, 30.8 ± 3.6 kg/m2). Fifteen healthy Japanese normotensive individuals were recruited as a control group.
In obese patients with hypertension, the response of FBF to ACh was attenuated compared to healthy individuals (P < .001). Caloric restriction reduced body weight from 77.5 ± 15.0 to 73.2 ± 13.5 kg (P < .01), the mean blood pressure from 118.4 ± 8.7 to 105.7 ± 8.5 mm Hg (P < .01), fasting plasma insulin from 85.8 ± 22.8 to 64.8 ± 27.0 pmol/L (P < .05), serum total cholesterol from 5.30 ± 0.76 to 4.67 ± 0.58 mmol/L (P < .05), and low density lipoprotein cholesterol from 3.80 ± 0.48 to 3.29 ± 0.44 mmol/L (P < .05). Basal FBF was similar before and after weight reduction. Caloric restriction enhanced the response of FBF to ACh (P < .05), but did not alter the response to ISDN. The intra-arterial infusion of NG-monomethyl-L-arginine (8 μmol/min), a nitric oxide synthase inhibitor, decreased the enhanced ACh-induced blood flow response induced by caloric restriction.
The present findings suggest that the caloric restriction improves endothelial-dependent vasodilation through an increased release of nitric oxide in obese hypertensive patients.
Am J Hypertens 2002;15:302–309 © 2002 American Journal of Hypertension, Ltd.
It is well established that lifestyle modification plays an important role in reducing arterial blood pressure (BP). Specifically, weight reduction is recommended as a nonpharmacologic therapy for the treatment of essential hypertension in obese patients.1 However, the sole mechanism responsible for the decrease in BP with weight reduction is unknown. Possible mechanisms including a decrease in cardiac output or total circulating blood volume, suppression of the renin angiotensin system, decreased sympathetic activity, and improved insulin sensitivity have been proposed.2,3
Previous investigators have reported impaired endothelium-dependent vasodilation in the forearm,4 coronary,5 and renal vasculature6 in patients with essential hypertension. Obesity is also associated with endothelial dysfunction.7 Although it has been demonstrated that endothelial function of forearm circulation in patients with essential hypertension is improved by angiotensin converting enzyme inhibitors,8 lifestyle modification, including exercise,9 estrogen replacement in postmenopausal women,10 and novel properties of vitamin C11 and tetrahydrobiopterine,12 there is little information on the effect of the nonpharmacologic antihypertensive therapy of a low-calorie diet on endothelial function. It is clinically important to determine whether a low-calorie diet is effective in improving endothelial dysfunction in obese hypertensive patients.
This study was designed to examine the effects of a low-calorie diet on endothelial function in obese patients with hypertension by measuring forearm blood flow (FBF).
Methods
Subjects
Eleven Japanese patients with essential mild to moderate hypertension who were overweight (4 men and 7 women; mean age, 49.7 ± 11.2 years) were studied. Hypertension was defined as a systolic BP more than 160 mm Hg or a diastolic BP more than 95 mm Hg in a sitting position on at least three different occasions in the outpatient clinic of Hiroshima University Faculty of Medicine. Obesity was defined as a body mass index more than 27 kg/m2 (range, 27.7 to 36.7 kg/m2). Patients who had cardiovascular or cerebrovascular disease, renal failure, diabetes mellitus, or hypercholesterolemia were excluded. No patients had any antihypertensive agents before the study. Fifteen healthy Japanese normotensive individuals (6 men and 9 women; mean age, 54.1 ± 12.6 years) were recruited from members of the medical staff and people undergoing annual examinations as a control group. Normal BP was defined as a systolic BP <130 mm Hg and a diastolic BP <80 mm Hg. The normotensive control individuals had normal findings on physical and laboratory examinations. The study protocol was approved by the ethics committee of the Hiroshima University Faculty of Medicine. Informed consent was obtained before participation.
Study protocol
Eleven obese hypertensive patients were admitted to the First Department of Internal Medicine of Hiroshima University Faculty of Medicine for the study. After hospitalization, all patients were placed on a standard diet (2400 kcal, 80 g of protein, 55 g of fat, 400 g of carbohydrate per day). One week later, a low-calorie diet (800 kcal, 55 g of protein, 25 g of fat, 90 g of carbohydrate per day) was instituted for 2 weeks. The daily dietary contents of sodium (170 mmol), potassium (100 mmol), and calcium (40 mmol) were kept constant throughout the study. All patients were sedentary and did not exercise regularly, and did not exercise during the study.
Blood pressure was measured in a sitting position with a standard mercury sphygmomanometer at 7:30 AM, 2 PM, and 6 PM. The mean BP was calculated by adding one-third of the pulse pressure to the diastolic pressure. Body weight was measured at 7:00 AM after voiding. The percentage of body fat was measured by determining the bioelectric impedance using a body composition analyzer (HBF 300, Omuron Co., Tokyo, Japan). National Institutes of Health technology Assessment Conference stated that this analysis was useful to assess body composition.13 A fasting blood sample was collected at 7:30 AM after the patient had rested for 30 min in the supine position. Blood pressure, body weight, and the percent body fat are expressed as average values for the last 2 days of the standard and low-calorie diets.
The forearm vascular response to acetylcholine (ACh) or isosorbide dinitrate (ISDN) in the absence and presence of NG-monomethyl-L-arginine (L-NMMA) preinfusion was evaluated on the last day of the standard and low-calorie diets. The study began at 8:30 AM. Patients fasted over night for at least 12 h. They were kept in a supine position in a quiet, dark, air-conditioned room (temperature, 22°C to 25°C) throughout the study. A 23-gauge polyethylene catheter (Hakkow Co., Okayama, Japan) was inserted into the left brachial artery for the infusion of ACh, ISDN, and L-NMMA and for recording arterial pressure using an AP-641G pressure transducer (Nihon Kohden Co., Tokyo, Japan) under local anesthesia (1% lidocaine). Another catheter was inserted into the left deep antecubital vein to obtain blood samples.
After 30 min, basal FBF and arterial BP were measured. The effects of endothelium-dependent vasodilation with ACh and endothelium-independent vasodilation with ISDN on forearm hemodynamics were determined. ACh (7.5, 15, and 30 μg/min) and ISDN (0.75, 1.5, and 3.0 μg/min) were infused intra-arterially at each dose for 5 min using a constant rate infusion pump (Terfusion STG-523, Termo Co., Tokyo, Japan). The FBF was measured during the last 2 min of the infusion. The infusions of ACh and ISDN were carried out in a random order. The subsequent study was performed when FBF returned to the baseline value. In a preliminary study, FBF returned to the baseline value within 30 min after the infusion of a maximal dose of ACh (30 μg/min) or ISDN (3.0 μg/min). Therefore, the infusions of ACh or ISDN were followed by a 30-min recovery period. After the 30-min recovery period, L-NMMA, an inhibitor of nitric oxide (NO) synthase, was infused intra-arterially for 5 min at a dose of 8 μmol/min and the basal FBF and arterial BP were recorded. The FBF was measured during the last 2 min of the drug infusion. In a preliminary study, we examined the effects of four doses of L-NMMA (1, 4, 8, and 16 μmol/min) on forearm hemodynamics. L-NMMA at doses of 1, 4, and 8 μmol/min significantly decreased basal FBF (5.6% ± 1.9%, 30.2% ± 9.4%, and 47.1% ± 12.0%, respectively) and the ACh-induced increase in FBF in a dose-dependent manner, without affecting arterial BP. In contrast, 16 μmol/min of L-NMMA increased the mean arterial BP from 108.7 ± 7.8 to 114.5 ± 8.5 mm Hg (P < .05). Therefore, we used a dose of 8 μmol/min of L-NMMA in the present study.
No significant change was observed in arterial BP or heart rate by intra-arterial infusion of either ACh and ISDN alone and after L-NMMA infusion in any groups.
Measurements of FBF
FBF was measured using a mercury-filled Silastic strain gauge plethysmograph (EC-5R, D.E. Hokanson, Inc., Bellevue, WA) as previously described.9 Briefly, the strain gauge was attached to the upper part of the left forearm and connected to a plethysmography device, and was placed above the level of the right atrium. A wrist cuff was inflated to a pressure 50 mm Hg greater than the systolic BP to exclude the effect of hand circulation during the measurement of FBF. An upper arm cuff (EC-20, D.E. Hokanson, Inc.) was inflated to 40 mm Hg for 7 sec during 15-sec cycles to occlude venous outflow from the arm. The FBF output signal was transmitted to a recorder (U-228, Advance Co., Nagoya, Japan). FBF is expressed as milliliter per minute per 100 mL of forearm tissue volume. The averages of four plethysmographic measurements were used for the analysis of FBF at baseline and during the administration of drugs. Forearm vascular resistance (FVR) was calculated as the mean arterial BP divided by FBF. The intraobserver coefficient of variation was 3.0% ± 1.8% for the measurements.
Drugs
The following drugs were used in this study: ACh chloride (Daiichi Pharmaceutical Co., Tokyo, Japan), ISDN (Eisai Pharmaceutical Co., Tokyo, Japan), and L-NMMA (Sigma Chemical Co., St. Louis, MO). All drugs were obtained from commercially available sources and were dissolved in saline (0.9% NaCl; Ohtsuka Pharmaceutical Co., Tokushima, Japan) immediately before use.
Analytical methods
Samples of venous blood were placed in tubes containing sodium EDTA (1 mg/mL) and in polystyrene tubes. The EDTA-containing tubes were chilled promptly in an ice bath. Plasma was immediately separated by centrifugation at 3100 g for 10 min at 4°C, and serum was separated by centrifugation at 1000 g for 10 min at room temperature. Samples were stored at −80°C until the time of assay. Serum concentrations of total cholesterol, triglycerides, high density lipoprotein (HDL) cholesterol, glucose, and electrolytes were determined by routine chemical methods. Serum insulin was measured using an automated radioimmunoassay technique. Fasting concentrations of glucose and insulin were used to determine homeostatic model assessment (HOMA) parameters of insulin resistance using a program based on the HOMA algorithm.14 Plasma norepinephrine concentrations were measured by high-performance liquid chromatography. The serum low density lipoprotein (LDL) concentration was estimated using Friedewald's method. Plasma renin activity (Gamma Coat PRA, SRL Co., Atsugi, Japan), and the concentration of plasma leptin (Human Leptin RIA kit, SRL Co.) were determined by radioimmunoassay. The nitrite/nitrate (NOx) concentration was measured by a colorimetric assay based on the Griess reaction.
Statistical methods
Values are expressed as the mean ± SD. The Mann-Whitney U test was used to evaluate differences between obese hypertensive patients and normotensive individuals with respect to baseline parameters. Two-tailed Student paired t test was used to evaluate differences before and after the low-calorie diet in the obese hypertensive patients. The FBF responses to ACh and ISDN were analyzed by two-way ANOVA for repeated measures followed by Scheffe's F test. Results were considered significant at P < .05.
Results
Clinical characteristics and FBF of normotensive individuals and hypertensive obese patients
The clinical characteristics of normotensive individuals and hypertensive obese patients are summarized in Table 1. Before starting the low-calorie diet, the obese hypertensive patients had a significantly greater body weight, body fat ratio, arterial BP, fasting plasma glucose concentration, fasting insulin concentration, serum triglyceride concentration, and plasma leptin concentration compared with normotensive individuals.
Clinical characteristics of normotensive control individuals and hypertensive obese patients
| Obese Hypertensive Patients | |||
|---|---|---|---|
| Parameter | Normotensive Individuals | Before Diet | After Diet |
| Body weight (kg) | 64.1 ± 11.3 | 77.5 ± 15.0† | 73.2 ± 13.5*† |
| Body fat (%) | 24.4 ± 4.2 | 35.5 ± 5.8† | 33.9 ± 5.4*† |
| Systolic blood pressure (mm Hg) | 114.6 ± 9.1 | 158.2 ± 28.5† | 140.1 ± 16.0*† |
| Diastolic blood pressure (mm Hg) | 68.8 ± 7.3 | 98.6 ± 7.8† | 89.0 ± 6.9*† |
| Heart rate (beats/min) | 68.1 ± 6.2 | 71.1 ± 6.3 | 67.4 ± 9.1 |
| Fasting plasma glucose (mmol/L) | 4.8 ± 0.3 | 6.0 ± 1.7 | 5.6 ± 1.1† |
| Fasting plasma insulin (pmol/L) | 56.1 ± 10.9 | 85.8 ± 22.8† | 64.8 ± 27.0* |
| Insulin resistance index (HOMA) | 1.99 ± 0.24 | 3.92 ± 1.51† | 2.52 ± 1.24*† |
| Total-cholesterol (mmol/L) | 5.18 ± 0.82 | 5.30 ± 0.76 | 4.67 ± 0.58* |
| Triglyceride (mmol/L) | 1.19 ± 0.52 | 2.03 ± 1.13 | 1.41 ± 0.55* |
| HDL cholesterol (mmol/L) | 1.40 ± 0.30 | 1.10 ± 0.35 | 1.09 ± 0.35 |
| LDL cholesterol (mmol/L) | 3.48 ± 0.52 | 3.80 ± 0.48 | 3.29 ± 0.44* |
| Plasma renin activity (ng/L·s) | 0.38 ± 0.14 | 0.53 ± 0.76 | 0.36 ± 0.44 |
| Plasma norepinephrine (pmol/L) | 1655.1 ± 470.1 | 1812.1 ± 822.1 | 1350.7 ± 708.2 |
| Plasma NOx (μmol/L) | 35.6 ± 9.0 | 34.8 ± 8.9 | 33.3 ± 11.0 |
| Plasma leptin (ng/mL) | 5.2 ± 4.1 | 14.8 ± 12.2† | 8.2 ± 4.2*† |
| Obese Hypertensive Patients | |||
|---|---|---|---|
| Parameter | Normotensive Individuals | Before Diet | After Diet |
| Body weight (kg) | 64.1 ± 11.3 | 77.5 ± 15.0† | 73.2 ± 13.5*† |
| Body fat (%) | 24.4 ± 4.2 | 35.5 ± 5.8† | 33.9 ± 5.4*† |
| Systolic blood pressure (mm Hg) | 114.6 ± 9.1 | 158.2 ± 28.5† | 140.1 ± 16.0*† |
| Diastolic blood pressure (mm Hg) | 68.8 ± 7.3 | 98.6 ± 7.8† | 89.0 ± 6.9*† |
| Heart rate (beats/min) | 68.1 ± 6.2 | 71.1 ± 6.3 | 67.4 ± 9.1 |
| Fasting plasma glucose (mmol/L) | 4.8 ± 0.3 | 6.0 ± 1.7 | 5.6 ± 1.1† |
| Fasting plasma insulin (pmol/L) | 56.1 ± 10.9 | 85.8 ± 22.8† | 64.8 ± 27.0* |
| Insulin resistance index (HOMA) | 1.99 ± 0.24 | 3.92 ± 1.51† | 2.52 ± 1.24*† |
| Total-cholesterol (mmol/L) | 5.18 ± 0.82 | 5.30 ± 0.76 | 4.67 ± 0.58* |
| Triglyceride (mmol/L) | 1.19 ± 0.52 | 2.03 ± 1.13 | 1.41 ± 0.55* |
| HDL cholesterol (mmol/L) | 1.40 ± 0.30 | 1.10 ± 0.35 | 1.09 ± 0.35 |
| LDL cholesterol (mmol/L) | 3.48 ± 0.52 | 3.80 ± 0.48 | 3.29 ± 0.44* |
| Plasma renin activity (ng/L·s) | 0.38 ± 0.14 | 0.53 ± 0.76 | 0.36 ± 0.44 |
| Plasma norepinephrine (pmol/L) | 1655.1 ± 470.1 | 1812.1 ± 822.1 | 1350.7 ± 708.2 |
| Plasma NOx (μmol/L) | 35.6 ± 9.0 | 34.8 ± 8.9 | 33.3 ± 11.0 |
| Plasma leptin (ng/mL) | 5.2 ± 4.1 | 14.8 ± 12.2† | 8.2 ± 4.2*† |
HOMA = homeostatic model assessment; HDL = high-density lipoprotein; LDL = low-density lipoprotein; NOx = nitrite/nitrate.
All results are presented as mean ± SD.
P < .05 versus before diet;
P < .05 versus normotensive individuals.
Clinical characteristics of normotensive control individuals and hypertensive obese patients
| Obese Hypertensive Patients | |||
|---|---|---|---|
| Parameter | Normotensive Individuals | Before Diet | After Diet |
| Body weight (kg) | 64.1 ± 11.3 | 77.5 ± 15.0† | 73.2 ± 13.5*† |
| Body fat (%) | 24.4 ± 4.2 | 35.5 ± 5.8† | 33.9 ± 5.4*† |
| Systolic blood pressure (mm Hg) | 114.6 ± 9.1 | 158.2 ± 28.5† | 140.1 ± 16.0*† |
| Diastolic blood pressure (mm Hg) | 68.8 ± 7.3 | 98.6 ± 7.8† | 89.0 ± 6.9*† |
| Heart rate (beats/min) | 68.1 ± 6.2 | 71.1 ± 6.3 | 67.4 ± 9.1 |
| Fasting plasma glucose (mmol/L) | 4.8 ± 0.3 | 6.0 ± 1.7 | 5.6 ± 1.1† |
| Fasting plasma insulin (pmol/L) | 56.1 ± 10.9 | 85.8 ± 22.8† | 64.8 ± 27.0* |
| Insulin resistance index (HOMA) | 1.99 ± 0.24 | 3.92 ± 1.51† | 2.52 ± 1.24*† |
| Total-cholesterol (mmol/L) | 5.18 ± 0.82 | 5.30 ± 0.76 | 4.67 ± 0.58* |
| Triglyceride (mmol/L) | 1.19 ± 0.52 | 2.03 ± 1.13 | 1.41 ± 0.55* |
| HDL cholesterol (mmol/L) | 1.40 ± 0.30 | 1.10 ± 0.35 | 1.09 ± 0.35 |
| LDL cholesterol (mmol/L) | 3.48 ± 0.52 | 3.80 ± 0.48 | 3.29 ± 0.44* |
| Plasma renin activity (ng/L·s) | 0.38 ± 0.14 | 0.53 ± 0.76 | 0.36 ± 0.44 |
| Plasma norepinephrine (pmol/L) | 1655.1 ± 470.1 | 1812.1 ± 822.1 | 1350.7 ± 708.2 |
| Plasma NOx (μmol/L) | 35.6 ± 9.0 | 34.8 ± 8.9 | 33.3 ± 11.0 |
| Plasma leptin (ng/mL) | 5.2 ± 4.1 | 14.8 ± 12.2† | 8.2 ± 4.2*† |
| Obese Hypertensive Patients | |||
|---|---|---|---|
| Parameter | Normotensive Individuals | Before Diet | After Diet |
| Body weight (kg) | 64.1 ± 11.3 | 77.5 ± 15.0† | 73.2 ± 13.5*† |
| Body fat (%) | 24.4 ± 4.2 | 35.5 ± 5.8† | 33.9 ± 5.4*† |
| Systolic blood pressure (mm Hg) | 114.6 ± 9.1 | 158.2 ± 28.5† | 140.1 ± 16.0*† |
| Diastolic blood pressure (mm Hg) | 68.8 ± 7.3 | 98.6 ± 7.8† | 89.0 ± 6.9*† |
| Heart rate (beats/min) | 68.1 ± 6.2 | 71.1 ± 6.3 | 67.4 ± 9.1 |
| Fasting plasma glucose (mmol/L) | 4.8 ± 0.3 | 6.0 ± 1.7 | 5.6 ± 1.1† |
| Fasting plasma insulin (pmol/L) | 56.1 ± 10.9 | 85.8 ± 22.8† | 64.8 ± 27.0* |
| Insulin resistance index (HOMA) | 1.99 ± 0.24 | 3.92 ± 1.51† | 2.52 ± 1.24*† |
| Total-cholesterol (mmol/L) | 5.18 ± 0.82 | 5.30 ± 0.76 | 4.67 ± 0.58* |
| Triglyceride (mmol/L) | 1.19 ± 0.52 | 2.03 ± 1.13 | 1.41 ± 0.55* |
| HDL cholesterol (mmol/L) | 1.40 ± 0.30 | 1.10 ± 0.35 | 1.09 ± 0.35 |
| LDL cholesterol (mmol/L) | 3.48 ± 0.52 | 3.80 ± 0.48 | 3.29 ± 0.44* |
| Plasma renin activity (ng/L·s) | 0.38 ± 0.14 | 0.53 ± 0.76 | 0.36 ± 0.44 |
| Plasma norepinephrine (pmol/L) | 1655.1 ± 470.1 | 1812.1 ± 822.1 | 1350.7 ± 708.2 |
| Plasma NOx (μmol/L) | 35.6 ± 9.0 | 34.8 ± 8.9 | 33.3 ± 11.0 |
| Plasma leptin (ng/mL) | 5.2 ± 4.1 | 14.8 ± 12.2† | 8.2 ± 4.2*† |
HOMA = homeostatic model assessment; HDL = high-density lipoprotein; LDL = low-density lipoprotein; NOx = nitrite/nitrate.
All results are presented as mean ± SD.
P < .05 versus before diet;
P < .05 versus normotensive individuals.
The responses of FBF to intra-arterial infusion of ACh and ISDN in the normotensive control groups and the obese hypertensive patients are shown in Fig. 1. Basal FBF was similar in the two groups (4.7 ± 1.2 v 4.8 ± 1.0 mL/min/100 mL tissue). The response of FBF to ACh infusion was greatly attenuated in the obese hypertensive patients when compared to normotensive individuals (P < .001). The response of FBF to ISDN infusion was similar in the two groups.
Basal forearm blood flow (FBF) and FBF during intra-arterial acetylcholine (A) and isosorbide dinitrate (ISDN) (B) infusion in normotensive individuals and before and after low-calorie diet in obese hypertensive patients. Results are expressed as the mean ± SD. The probability value refers to the comparison between normotensive individuals and obese hypertensive patients and before and after the diet. N.S. = not significant.
Clinical characteristics of hypertensive obese patients before and after low-calorie diets
Two weeks of the low-calorie diet significantly decreased body weight by 4.3 ± 2.3 kg (P < .01), the percent of body fat by 1.7% ± 1.4% (P < .05), and BP by 18.2 ± 5.9/9.6 ± 4.7 mm Hg (P < .01). Systolic BP began to decrease at the start of the low-calorie diet treatment, and diastolic BP began to decrease at 1 week after the start of the low-calorie diet treatment. Both systolic and diastolic BP reached plateau levels a few days before the end of low-calorie diet treatment. The plasma fasting glucose concentration did not change with the low-calorie diet, but the insulin concentration (21.2 ± 12.8 pmol/L, P < .05) and the insulin resistance index (1.40 ± 1.2, P < .05), which was calculated by HOMA, were decreased significantly. The serum total cholesterol concentration (0.63 ± 0.08 mmol/L, P < .05), triglyceride concentration (0.90 ± 0.65 mmol/L, P < .05), LDL cholesterol concentration (0.51 ± 0.22 mmol/L, P < .05) and plasma leptin concentration (6.1 ± 5.9 ng/mL, P < .05) were also decreased significantly. The plasma renin activity, the plasma norepinephrine concentrations, and NOx concentration did not change with the low-calorie diet.
FBF and FVR in hypertensive obese patients before and after low-calorie diet
The responses of FBF before and after low-calorie diet are shown in Fig. 1. A low-calorie diet increased significantly the response of FBF to ACh infusion (P < .05). The response to ISDN infusion did not change with a low-calorie diet. Furthermore, the change in FVR in response to ACh decreased (P < .05) with the low-calorie diet. But the change in FVR in response to ISDN infusion did not change.
Effects of L-NMMA on the forearm vascular response to ACh in hypertensive obese patients before and after low-calorie diet
Intra-arterial infusion of L-NMMA decreased significantly basal FBF before and after low-calorie diet (4.7 ± 1.0 to 3.1 ± 0.5 mL/min/100 mL tissue and 5.9 ± 1.5 to 3.8 ± 0.9 mL/min/100 mL tissue, respectively, P < .05). L-NMMA decreased the augmented FBF response to ACh caused by the low-calorie diet in hypertensive obese patients (Fig. 2). There were no significant correlations between the increase in the maximal FBF response to ACh with the low-calorie diet and changes in parameters such as BP, body weight, percent body fat, insulin resistance index, and lipid profiles.
Basal FBF and FBF during intra-arterial acetylcholine infusion in obese hypertensive patients receiving the nitric oxide synthase inhibitor (NG-monomethyl-L-arginine [L-NMMA]) before and after a low-calorie diet. Other abbreviations as in Fig. 1.
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Discussion
It has been suggested that endothelium-derived NO is one of the most potent endogenous vasodilators and plays an important role in the regulation of vascular tone and BP.15 Because NO inhibits platelet aggregation and adhesion as well as smooth muscle cell proliferation, impaired endothelial function may result in an increased risk of atherosclerosis. Abnormalities in endothelium-dependent vasodilation have been observed in the setting of essential hypertension. In addition, obesity is also associated with the development of atherosclerosis.16 Impaired endothelium-dependent vasodilation has been shown to correlate with body fat content.7 On the basis of these findings, hypertension in the setting of obesity may be due, in part, to impaired endothelial function. In the present study, endothelial-dependent vasodilation was attenuated in obese hypertensive patients. Therefore, improving endothelial dysfunction may be a novel therapeutic goal in treating essential hypertension in obese patients.
There is a strong association between hypertension and obesity. Our findings are in keeping with previous studies showing that weight reduction with a low-calorie diet in obese hypertensive patients represents an effective antihypertensive therapy. In addition, a low-calorie diet improved endothelium-dependent vasodilation in response to ACh but not endothelium-independent vasodilation in response to ISDN. The infusion of L-NMMA, an NO synthase inhibitor, decreased the augmentation of the FBF response to ACh after the low-calorie diet. These findings suggest that a low-calorie diet restores vascular endothelial function. Some possible mechanisms by which low-calorie diet augments acetylcholine-induced NO release are postulated in obese hypertensive patients.
It is possible that reduction in BP per se is a cause of improvement of endothelial function. However, we found no significant correlation between the reduction in BP by a low-calorie diet and the increase in FBF response to ACh. Our laboratory has reported that treatment with an angiotensin-converting enzyme inhibitor, but not a calcium antagonist, improves impaired endothelium-dependent vasorelaxation in patients with essential hypertension, whereas both agents equally normalized BP,7,17 and that reduction in BP after exercise did not correlate with exercise-induced augmentation of endothelium-dependent vasodilation of forearm circulation in patients with essential hypertension.9 These results are consistent with previous studies in that reduction in BP does not contribute to improve endothelial function.8,18 Therefore, it is unlikely that a reduction in BP directly causes improvement of endothelial function.
Plasma insulin concentration frequently increase in obese individuals due to resistance to insulin-mediated glucose disposal. Furthermore, the relationship between essential hypertension and insulin resistance or hyperinsulinemia is well documented.19 Several mechanisms by which insulin contributes to BP regulation have been proposed: stimulation of sympathetic activity, promotion of vascular smooth muscle cell growth, and increased renal sodium absorption. Insulin has been shown to modulate endothelial function in vitro. Specifically, insulin stimulates NO production in a dose-dependent manner in human umbilical vein endothelial cells.20 Petrie et al21 demonstrated a positive correlation between endothelial NO synthesis in the forearm circulation and insulin sensitivity in healthy humans. Decreased insulin sensitivity has been reported to impair endothelial function through disturbances in NO production. Steinberg et al7 suggested that obese individuals are characterized by endothelial dysfunction caused by insulin resistance. Therefore, decreasing insulin resistance through the use of a low-calorie diet may improve endothelial dysfunction in obese hypertensive patients.
Hypercholesterolemia is a recognized cause of cardiovascular disease. Specifically, an increased LDL concentration plays an important part in the development of atherosclerosis. It has been suggested that LDL increases the vascular production of superoxide anions, which attenuates endothelium-dependent vasodilation by reducing the production or release of NO. It is believed that the superoxide anions may alter membrane receptors and intracellular signal transduction or cause direct NO degradation.22 Improved endothelial function has been reported with the initiation of lipid-lowering therapy. Stroes et al23 reported an improvement in endothelium-dependent vascular relaxation in patients with hypercholesterolemia who were treated with lipid-lowering agents for 12 weeks. Tamai et al24 demonstrated an acute effect of the reduction of total and oxidized LDL by LDL apheresis improving endothelial function. In our study, the total and LDL cholesterol concentrations were normal. Therefore, it is unlikely that hypercholesterolemia is a primary cause of impaired endothelial function in the present study. However, Steinberg et al25 reported that impairment of endothelial-dependent vasodilation can occur when total and LDL cholesterol concentrations are at the upper end of the normal range. Although we did not determine the oxidized LDL cholesterol concentration, the total LDL cholesterol concentration decreased with a low-calorie diet. Based on the findings, we cannot deny the possibility that an improved FBF response to ACh with a low-calorie diet may, at least in part, result from changes in the lipid profile.
The present study documented a decrease in the plasma leptin concentration, which correlates with the whole body fat mass. Recently, much attention has focused on adipocytes, which represent not only a passive fat storage organ but also a secretory gland. Leptin increases sympathetic nerve activity.26 In addition, tumor necrosis factor-α and plasminogen activator inhibitor type 1, which are also adipose tissue-derived peptides, may play roles in increases of insulin resistance,27 the downregulation of constitutive NO synthase mRNA,28 or the impairment of endothelial function.29 Therefore, it is possible that improved endothelial function may be due to decreased hormonal influences by adipose tissue.
Although our results demonstrated the beneficial effects of a low-calorie diet on endothelial function in obese hypertensive patients, we cannot distinguish in the present study the effects of the low calorie diets per se from effects on weight loss. The magnitude of improvement in endothelium-dependent vasodilation may be similarly affected by caloric balance at the same level of weight loss.
In the present study, we were unable to find any significant relationship between changes in FBF response to ACh and changes in body weight, lipids profile, insulin resistance, and BP after a low-calorie diet. A low-calorie diet did not alter the FBF response to ISDN, an endothelium-independent vasodilator. We consider that although the FBF response to endothelium-independent vasodilator ISDN, an index of smooth muscle function, at least in part, reflects the arterial stiffness, there is no close relationship between the FBF response to the endothelium-dependent vasodilator ACh, an index of endothelial cell function, and arterial stiffness.
Study limitation
A low-calorie diet modifies many biological variables, such as the plasma lipid profiles, the insulin sensitivity, and the sympathetic activation. Thus, the mechanism of improvement in endothelium-dependent vasodilation is highly complex. Hence, it would have been preferable to perform a logistic regression to determine the separate contributions of these variables to the improvement of endothelial function. However, for such a model a larger number of patients would be required.
A comparison of the effects of a low-calorie diet on ACh-induced vasodilation in obese nonhypertensives, nonobese hypertensives and obese hypertensives would allow us to obtain more specific conclusions regarding the relationship between caloric restriction and endothelial function. However, it would be ethically difficult to have control subjects participate in a study protocol that requires very low-calorie diets with hospitalization.
A plateau was not seen with a similar low-calorie diet over a long period of time in a larger study reported previously,2 in which BP continued to decline for the duration of the low-calorie diet. In the present study, both systolic and diastolic BP reached plateau levels a few days before the end of the low-calorie diet. Most subjects showed similar changes in BP during a low-calorie diet. The reason for this discrepancy concerning changes in BP during a low-calorie diet is not known.
In general, the euglycemic clamp technique is a gold standard for the assessment of insulin sensitivity. Matthews et al14 reported that the estimate of insulin resistance obtained by the HOMA index correlated with estimates obtained by use of the euglycemic clamp and the hyperglycemic clamp, suggesting that the HOMA index is also useful for assessing insulin resistance. Several investigators have used the HOMA index for assessment of insulin resistance and have confirmed the usefulness of this method.30
In conclusion, the present study is the first to demonstrate that rapid weight reduction with a low-calorie diet not only reduces BP but also improves endothelial-dependent vasodilation through increased NO release. This lifestyle modification may be beneficial in decreasing the risk of atherosclerosis in obese hypertensive patients.
Acknowledgments
We thank Dr. Hiroaki Ikeda for the preparation of the L-NMMA, Fumiko Nakamura for preparing the diets, and Yuko Omura for her secretarial assistance.
References
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Author notes
This study was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, a Japan Heart Foundation grant for research on hypertension and vascular metabolism, and a grant from the Research Foundation for Community Medicine.
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