Some people swear smoking cigarettes keeps them slim.

But being exposed to cigarette smoke can actually make people pile on the pounds - the surprise is that secondhand smoke is the biggest culprit.

Innocent victims of secondhand cigarette smoke are the most likely to gain weight, researchers found.

'For people who are in a home with a smoker, particularly children, the increased risk of cardiovascular or metabolic problems is massive,' said lead author Professor Benjamin Bikman, of Brigham Young University in Utah.

Passive smoking damages the body because second-hand smoke contains more than 4,000 chemicals, many of which are irritants and toxins, and some of which are known to cause cancer.

Data shows half of the U.S. population is exposed at least once daily to secondhand cigarette smoke and approximately 20 per cent of young children live with someone who smokes in the home.

In the UK it's estimated that second-hand smoke kills over 12,000 people every year from lung cancer, heart disease, stroke and the lung disease Chronic Obstructive Pulmonary Disease (COPD).

Professor Bikman, and his co-author Professor Paul Reynolds wanted to investigate how cigarette smoke is tied to metabolism.

They wanted to pinpoint the mechanism behind why smokers become insulin resistant, which leads to weight gain.

To carry out their study, they exposed mice to or second-hand smoke and analysed their metabolic progression.

Sure enough, those exposed to smoke put on weight.

When they drilled down to the cellular level, they found the smoke triggered a tiny lipid called ceramide to alter mitochondria in the cells, causing disruption to normal cell function and inhibiting the cells' ability to respond to insulin.

The lungs provide a vast interface with the environment, and the research shows that passive smoking alters a body's sensitivity to insulin, Professor Reynolds said.

'Once someone becomes insulin resistant, their body needs more insulin. And any time you have insulin go up, you have fat being made in the body.'

The key to reversing the effects of cigarette smoke, they discovered, is to inhibit ceramide.

The researchers found the mice treated with myriocin - a known ceramide blocker - didn't gain weight or experience metabolic problems, regardless of their exposure to the smoke.

However, when the mice exposed to smoke were also fed a high-sugar diet, the metabolic disruption could not be fixed.

Now Professor Bikman and his team are in a race with other researchers to find a ceramide inhibitor that is safe for humans.

'The idea that there might be some therapy we could give to innocent bystanders to help protect them from the consequences of being raised in a home with a smoker is quite gratifying,' he said.

But for the smokers themselves, there is no 'protective therapy' available.

'They just have to quit,' he said. 'Perhaps our research can provide added motivation as they learn about the additional harmful effects to loved ones.'

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