If you yearn for smooth skin that glows with youth, the chances are that at some point you will have heard the exhortation to drink lots of water in order to flush out those evil toxins and keep your skin healthy. The exact amount people suggest varies. US-based advice tends to recommend eight glasses a day, while in hotter climates people are advised to drink more to compensate for higher rates of sweating. But regardless of the exact volume of water suggested, the principle behind the advice remains the same – taking extra water on board will keep your skin hydrated. In other words, water acts like a moisturiser, but from the inside out. This is such a common idea you might be surprised at the lack of evidence to back this up. You might expect there to be countless studies where people are separated into two groups, one assigned to sip water all day, the other to drink a normal amount. Then the smoothness of the skin could be assessed a month or so later to establish whether sipping more led to smoother skin. In fact such studies are rare, partly because water can’t be patented, so it is hard to find anyone to fund such research when there will be no new medication or cosmetic to sell that could repay the costs. A review by the dermatologist Ronni Wolf at the Kaplan Medical Centre in Israel found just one study looking at the effect of long-term water intake on the skin. But the results were contradictory. After four weeks, the group who drank extra mineral water showed a decrease in skin density, which some believe suggests the skin is retaining more moisture, while those who drank tap water showed an increase in skin density. But regardless of the type of water they drank, it made no difference to their wrinkles or to the smoothness of their skin. That’s not to say that dehydration has no effect on skin. We can measure some effect through the assessment of skin turgor. This is a measure of how fast it takes the skin to return to normal if you pinch some skin and lift it up. If you are dehydrated your skin will take longer to get its shape back. But it doesn’t follow that because drinking too little water is bad for the skin, drinking above average quantities is good. It would be like saying that because a lack of food leads to malnutrition, overeating must be good for us. Or as Wolf puts it, it’s like saying a car needs petrol, therefore the more petrol the better. Mystery advice Another common belief is that if you drink extra water the body will somehow store it. But it depends on how fast you drink it. Drink several glasses within a fifteen-minute period and you will just pass extra urine. If you spend more than two hours sipping the same amount, more liquid is retained. There is one study suggesting that drinking 500ml of water increases the blood flow through the capillaries in the skin. But the skin was only evaluated thirty minutes after drinking the water, and what we don’t know is whether this in turn improves skin tone. One counterargument is that skin contains up to 30% water, and this helps it to look plump. This may be true, but the skin’s youthful appearance is affected more by factors such as genetics, exposure to the sun and damage from smoking. So the mystery is where the eight-glasses-a-day recommendation for good skin comes from. Few of the official guidelines even refer to the skin. Water is undoubtedly the most important nutrient for the body. Without it we die in a matter of days, and there are of course other health benefits from staying hydrated. A review in 2010 found good evidence that it reduces the recurrence of kidney stones in those who have already had them, but evidence for other specific benefits is weaker. Arguments rage over the eight glasses a day rule, with disputes over how much is needed to clear the kidneys of toxins and whether or not water helps curb the appetite. It depends on how high the ambient temperature is and how much you are exerting yourself. It’s also a myth that other liquids don’t count. It doesn’t have to be water. Even food contains more liquid than you might expect. Pizza is 40-49% water, for instance. The percentage of water we derive from food in the diet depends on where you live. In the US it’s 22%. In Greece, where people eat more fruit and vegetables it is much higher. So the problem is a general lack of evidence that drinking more water makes any difference to your skin. We can’t say it definitely doesn’t work, but there’s no evidence that it does. Which leaves the question of how much water you should drink. Since it depends on the weather and what you are doing, then there is a very good internal guideline we all have that can help. And that’s thirst. Note: All content within this column is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional.