Some recent reports in the media claimed that consuming Watermelon regularly can be the answer to controlling one's blood pressure and probably this may give an impression to those suffering some form of hypertension that by eating this fruit all dietary precautions can be thrown to winds. There are thousands of such reports flashed across popular media for "creating" some sort of sensation to increase their viewership. The general tendency amongst some scientists to approach popular media with such sensational news can cause considerable harm to the cause of science as well as to consumers many of whom believe them blindly. Frequently results of ill-designed animal studies are extrapolated to human system and without peer review, same is passed on to journalist eagerly looking forward for such "explosive" news items. The need for restraint is all the more important when one realizes that most consumers are ignorant about many aspects of food, nutrition and health and unnecessary hope is raised amongst desperate consumers looking for solace to their health related problems.
Taking the case of Watermelon, the claim of controlling blood pressure (BP) is based on the fact that it contains L-Citrulline and in limited trials an extract of this chemical obtained from the fruit was able to reduce BP after 6 weeks, though to what extent, is not clear. The authors qualified their claim by indicating that the results were obtained with just 9 subjects suffering from "border line" BP, with a systolic pressure of 120-139 and a diastolic pressure of 80-89. The quantity administered was 6 gm of L-Citrulline/Arginine extracted from watermelon juice. Imagine the quantity of fruit required to get 6 gm of the amino acid considering that maximum content reported was 28.5 mg per gm dry weight! Similarly in another study some scientists administered Watermelon juice to the extent of almost two liters to their volunteers for eliciting its effect on Arginine levels in the blood.
It is true that Citrulline, an alpha amino acid, considered non-essential because human body can make it given the precursors, is a key intermediate in Urea cycle by which humans excrete Ammonia from the body. Citrulline was also found to relax blood vessels and its malate derivative is sold as a performance enhancing athletic dietary supplement. There is an association between Watermelon and Citrulline as the very name is derived from the Latin name for the fruit, Citrullus. It was isolated in pure form in 1930 and several scientific studies have implicated its role in optimizing blood flow through conversion to L-Arginine and then Nitric Oxide which is involved in Vasodilation. Low levels in the body are manifested in mental and physical fatigue and other problems well documented. On positive side Citrulline is reported to increase energy mobilization and stimulation of immune system.
Of course no one should have any quarrel with consuming Watermelon regularly every day as a part of the diet but claiming that a slice of this fruit can bring down BP is some what far-fetched. Water melon contains mainly water, to the extent of 92% besides nutrients like proteins, Vitamins A, B6 and C, fiber, Potassium and Carotenoids like Lycopene, Phytofluene, Phytoene, beta-Carotene, Lutein and Neurosporene. Though presence of Citrulline has attracted much attention, its concentration varies over a wide range of 3.9 mg to 28.5 mg per gm of dry weight of the pulp and possibly this poses a challenge regarding selection of varieties with high Citrulline content, if BP lowering benefits are to be derived, as claimed. Incidentally the rind of the fruit which is not normally consumed contains more Citrulline than the juice!
As many eminent doctors have said consuming Watermelon can be good for health as it contains many desirable nutrients but expecting marvels to happen through this fruit while leading an undisciplined and careless life style is foolhardy. Watermelon or for that matter any single food cannot be a silver bullet for shooting down inconvenient health disorders like BP, CVD, diabetes etc which are widely prevalent to day. Reasonable dietary control, avoiding over weight, consumption of diverse foods with more stress on whole grains, fruits and vegetables of different types, control of intake of fat, refined sugar and sodium combined with an active physical activity regime only can confer the benefits of good health.