Effect of Soy Nuts on Adhesion Molecules and Markers of Inflammation in Hypertensive and Normotensive Postmenopausal Women

Abstract

Recently, we showed that substituting soy nuts for non-soy protein in a therapeutic lifestyle change (TLC) diet lowered systolic and diastolic blood pressure 9.9% and 6.8%, respectively, in hypertensive postmenopausal women and 5.2% and 2.9%, respectively, in normotensive postmenopausal women. To examine mechanisms for these reductions, we measured markers of inflammation including soluble vascular cell adhesion molecule (sVCAM-1), soluble intercellular adhesion molecule (sICAM-1), C-reactive protein (CRP), interleukin-6 (IL-6) and matrix metalloproteinase-9 (MMP-9). Sixty healthy postmenopausal women (48 normotensive and 12 hypertensive) were randomized in a crossover design to a TLC diet alone or a TLC diet in which one-half cup soy nuts (25 g soy protein and 101 mg aglycone isoflavones) replaced 25 g of non-soy protein daily. Each diet was followed for 8 weeks. Compared to the TLC diet alone, levels of sVCAM-1 were significantly lower on the soy diet in hypertensive women (623.6±153.8 ng/ml versus 553.8±114.4 ng/ml, respectively, p=0.003) whereas no significant differences were observed in normotensive women. Soy nuts were associated with a trend toward reduction in CRP in normotensive women. No effect on levels of sICAM-1, IL-6 or MMP-9 was observed. In conclusion, the reduction in sVCAM-1 with soy nuts in hypertensive women suggests an improvement in endothelial function which may reflect an overall improvement in the underlying inflammatory process underlying atherosclerosis.

Recently, we showed that substituting soy nuts for non-soy protein in a therapeutic lifestyle change (TLC) diet lowered systolic and diastolic blood pressure (BP) 9.9% (average reduction 15 mm Hg) and 6.8% (average reduction 6 mm Hg), respectively, in hypertensive postmenopausal women and 5.2% and 2.9%, respectively, in normotensive postmenopausal women.¹ To examine mechanisms, we measured markers of inflammation including levels of soluble vascular cell adhesion molecule-1 (sVCAM-1), soluble intercellular adhesion molecule-1 (sICAM-1), C-reactive protein (CRP), interleukin (IL)-6 and matrix metalloproteinase (MMP)-9.

Methods

Briefly, 60 healthy postmenopausal women without clinical atherosclerosis or diabetes (48 normotensive and 12 hypertensive) were randomized in a crossover design to a TLC diet alone or a TLC diet in which one-half cup soy nuts (25 g soy protein and 101 mg aglycone isoflavones) replaced 25 g of non-soy protein daily as previously described.¹ Each diet was followed for 8 weeks. At the end of each 8 week period, the women crossed over to the other diet arm. BP measurements were performed with cycling dynamaps at the end of each diet period with women seated quietly for at least 5 minutes in a chair at 60–85 degrees with feet on the floor and right arm supported at heart level. Two readings were taken at the beginning of each visit at least 30 seconds apart. If there was more than a 5 mm Hg difference in systolic BP between the two readings, a third reading was taken. Hypertension was defined by systolic BP ≥ 140 mm Hg and/or diastolic BP ≥ 90 mm Hg. Study subjects were counseled to adhere to their current exercise regimen (if active exercisers) or to walk 30 minutes daily (if sedentary) and not change their exercise throughout the study to prevent a confounding effect of weight change or exercise on study results. Study subjects recorded the number of minutes of exercise performed daily throughout the study.

Serum sVCAM-1, sICAM-1, IL-6 and MMP-9 concentrations were measured using enzyme-linked immunosorbent assays (R & D Systems, Minneapolis, MN) as previously described² and CRP concentrations were measured with a validated high-sensitivity immunoturbidimetric assay on the Hitachi 917 analyzer (Roche Diagnostics Indianapolis, IN), using reagents and calibrators from DiaSorin (Stillwater, MN) as previously described in Dr. Nader Rifai’s laboratory.³

Data are expressed as mean ± SD. Continuous variables were compared using 2-tailed paired t tests (within BP group) and unpaired t tests (normotensive to hypertensive). P ≤ 0.05 was considered statistically significant.

Results

At baseline, hypertensive women (n=12) were significantly older (58.3±6.5 years vs. 53.5±5.3 years, p=0.01), heavier (BMI of 28.0 ±4.3 kg/m² vs. 25.4±4.9 kg/m², p=0.008) and experienced menopause for a longer time (7.4±6.1 years vs. 4.5±4.3 years, p=0.01) than normotensive women (n=48).¹ Caloric intake and percent intake of saturated fat, monounsaturated fat, polyunsaturated fat, protein and carbohydrates in the soy and control arms were shown previously.¹ Exercise was not significantly different between normotensive and hypertensive women at baseline (156±118 vs. 127±120 min/wk, respectively, p=0.25). No change in exercise or weight occurred in either group of women during the study.

Baseline levels of sVCAM-1, sICAM-1, CRP, IL-6 and MMP-9 were not significantly different between hypertensive and normotensive women (Table). Compared with the TLC diet alone, levels of sVCAM-1 were significantly lower on the soy diet in hypertensive women (623.6±153.8 ng/ml vs 553.8±114.4 ng/ml, respectively, p=0.003) whereas no significant differences were observed in normotensive women. Soy nuts were associated with a trend toward reduction in CRP in normotensive women. No effect on levels of sICAM-1, IL-6 or MMP-9 were observed.

Table.

Levels of Soluble Adhesion Molecules and Markers of Inflammation at the End of each Diet Period in Normotensive and Hypertensive Women

	Normotensive women (n=48)				Hypertensive women (n=12)
Variable (unit)	Control diet	Soy diet	% Change	p value*	Control diet	Soy diet	% Change	p value†	p value‡
Systolic Blood Pressure (mmHg)	116±10	110±11	5.2↓	<0.001	152±12	137±15	9.9↓	0.003	<0.001
Diastolic Blood Pressure (mmHg)	69±8	67±7	2.9↓	0.02	88±7	82±8	6.8↓	<0.001	<0.001
Vascular Cell Adhesion Molecule (ng/mL)	586±108	577±100	1.5↓	0.35	624±154	554±114	11.2↓	0.003	0.33
Intercellular Adhesion Molecule (ng/mL)	290±59	291±54	0.6↑	0.73	269±63	262±52	2.3↓	0.41	0.28
C-reactive Protein (mg/dL)	0.22±0.45	0.16±0.39	27.3↓	0.07	0.20±0.20	0.19±0.19	5.0↓	0.79	0.90
Interleukin-6 (ng/ml)	2.7±1.7	2.8±2.1	3.7↑	0.38	2.1±1.1	2.5±1.2	19.0↑	0.18	0.34
Matrix Metallo- proteinase-9 (ng/ml)	35.2±13.3	32.9±9.6	6.5↓	0.13	32.9±11.1	33.2±10.2	0.9↑	0.75	0.58

Values are mean ± SD;

^↓% decrease;

^↑% increase;

^*soy compared to control in normotensive women;

^†soy compared to control in hypertensive women;

^‡baseline values in hypertensive compared to normotensive women

Discussion

In the current study, soy nuts were associated with a significant reduction in levels of sVCAM-1 in hypertensive women compared to the control diet. This reduction in sVCAM-1 may reflect an overall improvement in the underlying inflammatory process which is now known to underlie the atherosclerotic process.⁴ Diets high in saturated fat or glucose increase inflammation and adhesion molecules and lead to insulin resistance, findings proposed to increase BP.⁵ Therefore, we hypothesized that a whole soy food may lower BP by improving inflammation and thereby, improving endothelial function. Our results provide support for this hypothesis.

The sole difference in dietary content in the hypertensive women was higher isoflavone levels with soy nuts whereas normotensive women had differences in isoflavones and several additional dietary components when consuming the soy nuts.¹ Since both groups of women, hypertensive and normotensive, had reductions in BP, these findings suggest that the isoflavone content may be responsible for BP reductions in both groups. Genistein, the main soy isoflavone, has been shown to facilitate nitric oxide-dependent endothelial vasodilation in humans.⁶ Although we did not measure plasma nitric oxide levels, soy nuts have been shown to increase levels of nitric oxide in normotensive postmenopausal women who had all 5 components of the metabolic syndrome.⁷ Therefore, it is reasonable to predict that increased levels of nitric oxide may be responsible for the BP reductions in both hypertensive and normotensive women.

The trend toward reduction in CRP, a marker of inflammation, in normotensive women provides support for a beneficial effect on inflammation. Soy had no significant effect on levels of IL-6 in the current study in normotensive women, a finding similar to a previous study of postmenopausal women receiving 36 oz soy milk containing 112 mg isoflavones daily for 16 weeks.⁸

The lack of effect of soy nuts on levels of sVCAM-1 and sICAM-1 in normotensive postmenopausal women in the current study is consistent with 2 previous studies, the first in normotensive, mildly hypercholesterolemic postmenopausal women undergoing a 6 week, 25 g soy protein/day dietary intervention.⁹ A second study in healthy, normotensive postmenopausal women following an 8 week, 50 mg soy isoflavone/day-enriched diet found no effect in the total group; however, those with the AA genotype of the estrogen receptor β, ERβ AluI polymorphism had lower sVCAM-1 levels.¹⁰