Introduction
The reduced bioactivity of arterial endothelial-derived nitric oxide (NO) has been demonstrated to be involved in the pathogenesis of atherosclerosis, which leads to hypertension (1). Persistent hypertension is acutely harmful to highly vascularized organs, in particular, the brain. Endothelial dysfunction with the gradual loss of NO bioavailability in the brain is accelerated by progressive atherosclerosis, which impairs general cognitive function, but also decreases the specific cognitive domains, such as memory, visual or spatial perception and reaction time to stimuli.
An epidemiological link between hypertension induced by NO-deficiency and Alzheimer's disease has been established in preclinical models (2). Reportedly, hypertension exacerbates the Alzheimer-like pathology in amyloid protein mouse models (2), suggesting that impaired NO bioavailability contributes to the pathophysiology of Alzheimer's disease and that cognitive impairment appears to be dependent on hypertension-associated NO-deficiency.
The link between cognitive impairment and low NO bioavailability is further exacerbated by an increase in natural endothelial nitric oxide synthase (eNOS) inhibitors in pathological conditions such as hypertension, resulting in a decrease in the production of NO in the brain (3). However, the reversal of this condition by dietary NO donors has been shown. A series of studies have reported that almost 1,000 healthy seniors who had a daily helping of NO-potent foods had a slower rate of cognitive decline (4). Brain maps have revealed a higher cerebral blood flow to areas of the brain involved with working or short-term memory following the consumption NO-potent natural foods (5).
A growing number of preclinical and human studies have suggested that aged-garlic extract (AGE) possesses anti-hyperlipidemic, anti-platelet, anti-oxidative, anti-inflammatory, anti-atherosclerotic and cardio-brain protective activity, which are all likely mediated by reversing endothelial dysfunction with concomitant increase in restoring cNOS and NO bioavailability (6).
AGE appears to attenuate the progression of atherosclerotic and, independently, stimulate eNOS expression in endothelial cells, without triggering inducible NO synthase (iNOS), a potentially inflammatory response (7).
AGE may stimulate eNOS in advance of reversing atherosclerotic plaques, which, in turn, increases NO bioavailability. Therefore, AGE may restore NO bioavailability by targeting multiple levels, as follows: i) Attenuating plaque formation, thereby, reversing endothelial dysfunction, resulting in eNOS expression; ii) directly stimulating eNOS, independent of plaque regression; iii) inhibiting or not augmenting a NO-associated inflammatory response associated with progressive atherosclerotic lesions, i.e., AGE selectively upregulating eNOS, not iNOS.
Previous research has demonstrated an anti-hypertensive effect on both systolic and diastolic lowering with daily AGE consumption (6). The antihypertensive properties of AGE are likely the result of restoring endothelial function and stimulating eNOS, which in turn promotes vasodilation and thus, reduces blood pressure (7).
Nitric oxide generated by L-arginine metabolism by eNOS is a critical regulator of vascular function. Recent research has also shown that NO can regulate intracellular heme allocation in a concentration-dependent manner, impacting redox-sensitive proteins involved in immunity and cellular metabolism (8).
As previously mentioned, AGE may be unique among orally active ‘NO boosters’ since it may target both the lesion by slowing it's progression and independently triggering eNOS in the endothelium despite the damage incurred by the advancing atheroma. Furthermore, it has been demonstrated in vivo that AGE selectively upregulates eNOS without eliciting a potentially inflammatory iNOS response (8).
A growing number of studies have demonstrated the beneficial effects of AGE on plaque progression, including increases in NO bioavailability, plaque stability, lumen diameter and fibrous cap thickness with a corresponding decrease in cholesterol crystalline, oxidized low-density lipoprotein, arterial stiffness, platelet activation among other nitric oxide-mediated outcomes (7-10). Arterial stiffness is an increasingly used metric to assess cardiovascular risk (11-15).
A growing body of evidence suggests that garlic may improve cognitive health by increasing NO bioavailability (6,16-18). Increases in NO bioavailability have been shown in multiple studies to enhance cerebral blood flow, particularly to those areas in the brain associated with working memory. A previous preclinical study suggested that garlic may improve spatial memory (18).
Results
A total of 80 subjects were enrolled in a prospective, randomized, placebo-controlled clinical trial, with 72 completing both baseline and the 12-week follow-up visit. The study was conducted according to the CONSORT (Consolidate Standards of Reporting Trials) guidelines and statement. The mean age of the participants was 53.0±12.3 years with 45 (63%) subjects being female. The baseline characteristics, stratified by arm (AGE group, n=37; and the placebo group, n=35) of the study participants were well balanced (Table I).
There was a significant effect of AGE on triglyceride levels. The triglyceride levels in the placebo group at baseline were 116.8±59.7 mg/dl vs. 124.9±74.0 mg/dl at the study endpoint, whereas in the AGE group, baseline values were 128.6±52.6 mg/dl, which decreased to 117.6±46.7 mg/dl. The AGE group exhibited a mean decrease of 14.7±46.4 vs. an increase in the placebo group of 8.6±41.8, for a between-group difference of 23.4±44.2 mg/dl (P=0.032) (Fig. 2).
In those with baseline triglyceride levels of ≤150 mg/dl [AGE group, n=27 (92.2±26.8 mg/dl) and the placebo group, n=25 (119.7±64.4 mg/dl)], the AGE group exhibited a decrease of -1.9±37.8 mg/dl vs. an increase in the placebo group of 3.6±23.3 mg/dl. In those with baseline triglyceride levels of 150-200 mg/dl [AGE group, n=5 (166.2±12.8 mg/dl) and the placebo group, n=4 (172.3±15.5 mg/dl)], the AGE group exhibited a decrease of -31.4±26.3 mg/dl vs. increase in the placebo group.
For assessing cognition, the MOCA tool was utilized. At the conclusion of the trial, there were more patients with no cognitive impairment in the AGE arm (92 vs. 82%), similar numbers in the mild cognition arm (8 vs. 9%) and the AGE group had no patients with moderate cognitive impairment (0 vs. 9% for tge placebo). Of note, more patients had no cognitive impairment and 0 patients had moderate cognitive impairment in the AGE group. From visit 1 to the 12-week visit, MOCA increased by 3 points in the AGE group from 27 to 30, while it did not increase in the placebo group (28 to 28) (Fig. 1). A breakdown by scores of the test is presented in Table II.
Discussion
Garlic (Allium sativum L.) is deemed to have a variety of therapeutic applications, including platelet aggregation inhibition, the reduction of cholesterol and the control of blood pressure. These therapeutic actions of garlic parallel NO physiological effects (18). The present study demonstrated that two potential actions of NO, brachial artery reactivity and brain function, which are improved with use of AGE, substantiating earlier research on garlic with brain function and garlic with endothelial function. The present study demonstrated that AGE led to an improvement in cognitive function, as demonstrated by improvements in MOCA, as well as independent improvements in triglyceride levels. In the present small study, AGE improved cognition and triglycerides, consistent with previous research (1). AGE imparts benefits through multiple mechanisms. It has previously been demonstrated that AGE leads to improvements in lipids, inflammation and triglycerides, all of which have been shown to impact cardiovascular outcomes, including flow mediated dilation and lipid values, similar to the present study (1,19-23). AGE improved cognitive function in the present short-term (12-week) study. However, longer studies, including studies performed on individuals with more baseline cognitive impairment, are warranted.
The limitations of the present study are the small sample size, short duration (12 weeks), the use of a single cognitive tool and the lack of biomarker assessment for NO directly. Given the known benefits of AGE on hypertension, the present study opted to only include those patients with pre-hypertension. However, generalizability is limited due to the inclusion of mainly pre-hypertensive subjects. Further larger and longer studies are warranted to fully assess the physiological changes that explain cognitive improvements with AGE.