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A small study, but interesting to see if it holds up. Note that watermelon extract is not actual watermelon. From Science Daily:

Chowing down on watermelon could lower blood pressure, study suggests

A new study by Florida State University Associate Professor Arturo Figueroa, published in the American Journal of Hypertension, found that watermelon could significantly reduce blood pressure in overweight individuals both at rest and while under stress.

The study started with a simple concept. More people die of heart attacks in cold weather because the stress of the cold temperatures causes blood pressure to increase and the heart has to work harder to pump blood into the aorta. That often leads to less blood flow to the heart.

Thus, people with obesity and high blood pressure face a higher risk for stroke or heart attack when exposed to the cold either during the winter or in rooms with low temperatures.

So, what might help their hearts? It turned out that watermelon may be part of the answer.

Figueroa's 12-week study focused on 13 middle-aged, obese men and women who also suffered from high blood pressure. To simulate cold weather conditions, one hand of the subject was dipped into 39 degree water (or 4 degrees Celsius) while Figueroa's team took their blood pressure and other vital measurements.

Meanwhile, the group was divided into two. For the first six weeks, one group was given four grams of the amino acid L-citrulline and two grams of L-arginine per day, both from watermelon extract. The other group was given a placebo for 6 weeks. Then, they switched for the second six weeks. Participants also had to refrain from taking any medication for blood pressure or making any significant changes in their lifestyle, particularly related to diet and exercise, during the study.

The results showed that consuming watermelon had a positive impact on aortic blood pressure and other vascular parametersNotably, study participants showed improvements in blood pressure and cardiac stress while both at rest and while they were exposed to the cold water.

Vegetarian diet lowers blood pressure without medications. From Medscape:

Vegetarian Diet Cuts Blood Pressure in Meta-Analysis

Eating a vegetarian diet is associated with reductions in blood pressure (BP) on par with adopting the DASH (low-sodium) diet, and roughly half that of starting pharmaceutical treatment, a new meta-analysis suggests .

"These findings establish the value of nonpharmacologic means for reducing BP," lead author on the study, Dr Yoko Yokoyama (National Cerebral and  Cardiovascular Center, Osaka, Japan). "Unlike drugs, there is no cost to a diet adjustment of this type, and all the 'side effects' of a plant-based diet are desirable: weight loss, lower cholesterol, and better blood sugar control, among others. I would encourage physicians to prescribe plant-based diets as a matter of routine and to rely on medications only when diet changes do not do the job."

Yokoyama et al's meta-analysis is published February 24, 2014 in JAMA Internal Medicine. The study is the latest to examine the health impact of a vegetarian-style eating pattern on cardiovascular disease risk factors.

The authors reviewed over 250 studies addressing vegetarian diets, ultimately including seven clinical trials (six of which were randomized) and 32 observational studies that included blood-pressure findings. Diets ranged from vegan to lacto-ovo vegetarian, with one study including fish, but no meat).

The authors found that reductions in both systolic and diastolic blood pressure were significantly greater for the vegetarian diets than for the comparator (omnivorous) diets, both in the pooled clinical trials and in the pooled observational studies, although drops were greater in the observational studies.

"This issue was examined by nearly 40 independent studies, some of which had hundreds or even thousands of participants, and the findings are strikingly consistent," Yokoyama said. "A vegetarian diet is clearly associated with lower blood pressure. Or, put another way, a meat-based diet is associated with higher blood pressure."

As with the DASH diet, the effect of switching to a vegetarian diet appears to be fairly rapid, and that's likely the result of two factors.