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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.

From the Medical Daily:

High Blood Pressure In Teens, Young Adults A Sign Of Hardened Arteries Down The Road

Your blood pressure during your teens and early twenties, though often naturally low due to youth, may have something to do with your cardiovascular health in later years, according to new research from Northwestern University Feinberg School of Medicine. In the new study, which was published in JAMA, researchers found that having higher blood pressure during your teens and twenties was actually linked to hardened arteries at age 40.

The study, led by epidemiologist Norrina Allen, points out the significance of maintaining cardiovascular health at a young age.

The study reviewed 4,600 men and women throughout several different states and followed them for 25 years. They found that 19 percent of them had blood pressure that was much higher than their peers, and that another 5 percent started with high blood pressure that gradually rose. Though these blood pressure readings fell within “normal” range for their age, it was higher than average and thus they were more likely to develop hypertension by age 40. Hypertension is also known as high blood pressure or arterial hypertension.

“While you wouldn’t prescribe medications for this group, you might have conversations with those individuals about ways they can improve their diet or increase physical activity,” Allen told NPR. She notes that “many of these cardiovascular risk factors are cumulative,” meaning they often occur over a long period of time and are a combination of things, from smoking to living a sedentary lifestyle.

Another inadvertent way to kill off good bacteria. From the January 27, 2014 Medical Daily:

Antiseptic Mouthwash Raises Heart Attack Risk, Blood Pressure: Chlorhexidine Kills Off 'Good' Bacteria That Helps Blood Vessels Relax

Antiseptic mouthwash is commonly used to quickly refresh and clean the mouth after brushing your teeth in the morning, after eating, and before going to bed. According to a recent study published in the journal Free Radical Biology and Medicine, using a mouthwash twice daily — such as Corsodyl — may increase blood pressure up to 3.5 millimeters of mercury (mmHg), raising your heart attack risk.

Now a team of researchers at Queen Mary University of London believe using antiseptic mouthwash daily could increase the odds of HBP (high blood pressure) due to a chemical that kills the “good” bacteria responsible for helping the blood vessels relax.

Professor Amrita Ahluwalia, lead author of the study, and her team of researchers, observed the effects of a chlorhexidine-based antiseptic mouthwash — Corsodyl — by measuring the blood pressure of a small cohort of healthy participants during a two-week period. Chlorhexidine is an antiseptic that treats gingivitis and others problems of the mouth and gums. A total of 19 participants were recruited for the study during an initial seven-day control period followed by a seven-day treatment period with the antiseptic mouthwash. 

The findings revealed Cordosyl use retracts the oral bacterial conversion from nitrate to nitrate which reduces the plasma nitrate levels that are associated with increases in blood pressure. The mouthwash led the participants’ blood pressure to rise between 2 to 3.5 mmHg, with a noticeable effect found within one day of using the mouthwash twice.

The study authors believe killing off “good” oral bacteria plays a vital role in determining the plasma nitrate levels, and the bodily control of blood pressure. “Killing off all these bugs each day is a disaster, when small rises in blood pressure have significant impact on morbidity and mortality from heart disease and stroke,” said Ahluwalia, the Daily Mail reported. However, she adds, “We are not telling people to stop using antiseptic mouthwashes if they have a gum or tooth infection — but we would ask why anyone else would want to.”

The findings of the study do not apply to all mouthwashes because not all mouthwashes contain the chemical chlorhexidine, such as the popular Listerine. The study authors caution other mouthwashes could still produce the same effects as Corsodyl by damaging the mouth’s healthy bacteria.

Exposing skin to sunlight may help to reduce blood pressure and thus cut the risk of heart attack and stroke.From the January 17, 2014 Science Daily:

Here Comes the Sun to Lower Your Blood Pressure

Exposing skin to sunlight may help to reduce blood pressure and thus cut the risk of heart attack and stroke, a study published in the Journal of Investigative Dermatology suggests.

Research carried out at the Universities of Southampton and Edinburgh shows that sunlight alters levels of the small messenger molecule, nitric oxide (NO) in the skin and blood, reducing blood pressure... "NO along with its breakdown products, known to be abundant in skin, is involved in the regulation of blood pressure. When exposed to sunlight, small amounts of NO are transferred from the skin to the circulation, lowering blood vessel tone; as blood pressure drops, so does the risk of heart attack and stroke."

While limiting sunlight exposure is important to prevent skin cancer, the authors of the study, including Dr Richard Weller of the University of Edinburgh, suggest that minimising exposure may be disadvantageous by increasing the risk of prevalent conditions related to cardiovascular disease.

Cardiovascular disease, often associated with high blood pressure, accounts for 30 per cent of deaths globally each year. Blood pressure and cardiovascular disease are known to vary according to season and latitude, with higher levels observed in winter and in countries further from the equator, where ultraviolet radiation from the sun is lower.