By Jack Challoner

In the battle to lose weight, hunger is the dieter's worst enemy. But research has revealed a simple aid to taming the appetite: soup. It's dieting's best kept secret says one science writer.

Imagine a typical lunchtime meal - say, chicken and vegetables with a glass of water.

No snacks for them for a while

If you eat the food and drink the water, you will feel full for a couple of hours before hunger kicks in. But if you blend the food with the water - to make soup - you will stay hunger-free for much longer, and less likely to snack through the afternoon.

How can blending the food into soup make such a difference? The answer lies in the stomach. Scientists have used ultrasound and MRI scans of people's stomachs to investigate what happens after eating solid-food-plus-water meals compared with the same food made into soup.

After you eat a meal, the pyloric sphincter valve at the bottom of your stomach holds food back so that the digestive juices can get to work.

Water, however, passes straight through the sphincter to your intestines, so drinking water does not contribute to "filling you up".

FIND OUT MORE... 10 Things You Need to Know About Losing Weight is broadcast on BBC One on Wednesday, 27 May at 2000 BST Or catch up here using the iPlayer

When you eat the same meal as a soup, the whole mixture remains in the stomach, because the water and food are blended together. The scientists' scans confirm that the stomach stays fuller for longer, staving off those hunger pangs.

The key to this low-tech weapon against hunger is a hormone called ghrelin. It is one of the major players in the body's appetite system.

Discovered as recently as 1999, ghrelin is released by specialised cells in the stomach wall.

'Cupcake circuit'

These cells produce a constant stream of ghrelin whenever the stomach is empty. The ghrelin travels via the blood stream to the brain's appetite centre, an organ called the hypothalamus. As a result, the hypothalamus screams "You are hungry - find food."

But whenever the stomach wall is stretched - when the stomach is full - the cells stop producing ghrelin, and the hypothalamus responds accordingly, turning off the appetite signal. The longer the stomach remains full, the longer you feel satisfied and the less you are likely to eat.

WHICH VARIETY IS BEST? Vegetable soup is best It produces a more consistent blend And it generally has fewer calories than chicken or fish soup Find out more about sensible weight loss at BBC Health: Your weight

The stomach gradually empties, more slowly for the soup than the solid meal plus water. The BBC staged an experiment for the programme 10 Things You Need to Know About Losing Weight to test this theory. In this experiment, and in previous experiments, participants reported feeling full for up to an hour-and-a-half longer than their solid meal counterparts.

Although some researchers refer to appetite as "the cupcake circuit", the mechanism behind human appetite evolved long before cupcakes were invented - at a time when food was scarce.

As a result, we are hardwired to eat high-calorie foods, which are unfortunately so abundant in the modern world.

Finding ways to control the appetite signal is crucial if we are to stave off the meteoric rise in obesity. Food scientists and pharmaceutical companies alike are on a major quest to find ways to do just that.

Could soup help address obesity?

Appetite is one of the most researched areas of weight-loss science. Unfortunately, the appetite system is complex, and still poorly understood.

There are probably dozens of hormones that play a role in regulating appetite. Of those that have already been discovered, there is one that is released after eating protein-rich meals (called PYY), one that is released by fat cells (leptin) and several that respond to the presence of any kind of food.

But of all the hormones that make up the appetite system, it is ghrelin that has caused the most interest. In addition to its role in sending the "stomach empty" signal to the brain, ghrelin also promotes fat storage.

Even worse, it inhibits the breakdown of stored fat during times of weight loss. Inject ghrelin into the bloodstream of a rat and the animal eats insatiably - and quickly becomes obese.

In 2006, scientists at the Scripps Research Centre in the US developed a vaccine to counteract the influence of ghrelin, in an attempt to control appetite.

It is still undergoing clinical trials - so for now, the best and simplest way to keep hunger at bay is to reduce your stomach's release of ghrelin: blend your food into a healthy, voluminous soup. The best sort? Vegetable soup, as it produces a more consistent blend and is generally lower calorie than chicken or fish soup.

Jack Challoner is a science writer and author of the website explaining-science.co.uk.

Below is a selection of your comments.

It has been interesting to read a scientific article on what I held to be true, simply by feeding my family. We often have freshly made soup at home and it seems to satisfy well for hours.
Liza Moon, London

Is the same true of cereal with milk, or does milk act like water and head straight for the old pyloric sphincter? I love cereal, not so keen on soup though. What about porridge, I quite like that?
Ben, Norwich

While the science behind the fact has probably not been quite so clearly laid out before, the knowledge that soup makes one feel full for longer (than an equivalent solid meal consumed with water) is not new.. It is, however, always interesting to learn of such scientific studies as the weight-conscious amongst us continue to wade through the ever changing (and often contradictory) expert advice on the matter.
Tom, Hong Kong

I have been routinely having soup for lunch as part of a campaign to lose weight triggered by concerned comments from my wife and doctor. I have found it a very effective way of eating less without feeling hungry and it has helped me to lose around 10 kg in five months.
Stephen Murray, London, UK

A lot of people who overeat do not do so because they are hungry but for other reasons. A person can feel full to the point of sickness but will still eat more. However, if you're going to overeat, then low calorie foods are better, such as soup. But that still does not address the core issue which is the need to eat in the first place.
Jacquie, Yorkshire

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