Image caption A tax on sugary drinks - will it make any difference to Mexico's obesity levels?

A controversial tax on soft drinks is currently being debated in Mexico's Congress. But can a tax really reduce the nation's waistlines?

Sometimes coming top isn't such a good thing.

Over a third of Mexican adults are overweight, putting them way up the global league tables for obese nations.

A possible cause is found in another of Mexico's dubious honours - being the world's biggest consumer of soft drinks.

The health consequences are taking a hefty toll on the country's finances.

In 2011, 1.2% of the country's GDP was used to treat obesity-related diseases such as diabetes, which is the same percentage as Canada spent on its entire military.

In recent years Mexico's government has tried to combat this epidemic by reducing sugar and fat levels in school breakfasts, and removing processed foods from vending machines in schools.

But now the current administration is trying to take a much more controversial step: adding a tax of one peso (equivalent to 8 US cents) on to every litre of soft drink.

'Just like any Mexican'

One man who saw his life come crashing down after the highs of soda is 55-year-old teacher Armando Murillo Salgado.

He was diagnosed with type 2 diabetes 15 years ago and sugary drinks are now strictly off the menu.

But it was difficult to see the warning signs as he used to drink them "just like any Mexican does".

Media playback is unsupported on your device Media caption Can a tax make Mexico healthier?

However, he is currently in hospital being treated for heart complications related to his diabetes.

He says: "I think 50% of my condition is down to soft drinks because of the ingredients. You can see on any bottle the kind of stuff that goes into them."

But Armando is sceptical that higher prices would have stopped him drinking them.

I think 50% of my condition is down to soft drinks Armando Murillo Salgado, Type 2 diabetes patient, Mexico

"What you're looking for in soft drinks is the flavour, the taste. I might have stopped for a second to ask myself, 'Wow, how much?!' - but I would still have bought it."

Ines Sanchez, a nutritionist at Centro Medico Hospital 20 de Noviembre in Mexico City, where Armando is being treated, agrees that more is needed than a tax.

"We need other strategies to address health problems, before we even get to measures like taxes. It's better to teach healthy eating habits and give people alternatives."

Public health v manufacturers

The tax is being hotly debated around the country.

Those in favour say it is a significant first step, while opponents are certain it will not reduce obesity levels, and feel it is just a way of swelling the government's coffers.

Mexico's weight in numbers Seven out of 10 Mexicans have obesity-related health problems, according to a 2012 survey.

A third of Mexican adults are overweight making it the second fattest nation after the US.

One in 10 adults has diabetes, 90% of which is type 2.

13 out of every 100 deaths are caused by diabetes.

Mexico is third in the world for obesity-related deaths.

Jorge Ramo represents the National Association of Soft Drinks and Carbonated Water Manufacturers (ANPRAC).

Not surprisingly, he scorns the idea that a tax would do any good.

"Taxes are never important in beating health problems. The government has to create adequate programmes to improve the health of Mexicans - but not with a tax."

He argues that soft drinks make up only 5% of the average Mexican's daily calorie intake.

But Alejandro Calvillo, director of Consumer Power (Poder del Consumidor), one of the NGOs pushing for the tax to be introduced, points to Mexico's number one spot for global soda consumption - a yearly average of 163 litres per person.

"If you take out the people who don't drink them - about 5% - and those who only drink them once or twice a week, you're left with a percentage of the population who drink 880ml of soft drinks every day."

'No nutritional value'

But why tax soft drinks rather than other unhealthy culprits?

Kelly Brownell, professor of public policy at Duke University in the US, was the man who introduced the idea of a "soda tax" back in 1994.

There are a long list of reasons to target sugared beverages first, he told the BBC.

"Sugared drinks have no nutritional value. The caffeine and sugar may be triggering craving and withdrawal, setting up an addictive-like process.

"Sugar, rum, and tobacco are commodities which are nowhere necessaries of life, which are become objects of almost universal consumption, and which are therefore extremely proper subjects of taxation." Adam Smith, The Wealth of Nations, 1776

"Finally, the body does not register calories very well when they are delivered in beverages - you do not feel as full."

He believes such taxes will be routine in a few years, and the wisdom of using them will be taken for granted - much like taxes on tobacco.

Mexico isn't the first country to toy with the idea of so-called "fat taxes".

Hungary, France and Denmark have all introduced taxes on unhealthy comestibles in the past few years.

But last year Denmark repealed its tax on food with more than a 2.3% saturated fat content - a fact that many of those who oppose the Mexican bill bring into the debate.

"It failed because people looked for other options," says Jorge Ramo.

But fat taxes need to be hefty to work, according to a 2012 study published in the British Medical Journal.

It showed the benefits only kicked in when taxes increased prices by about 20% - and the proposed Mexican tax amounts to just a 10% price hike.

'Not enough'

Really steep increases are likely to leave consumers frothing at the mouth.

Indeed, in the United States - where some states have taxed fizzy drinks - there has been strong opposition to the government interfering with something as personal as diet.

But Prof Nicholas Finer, a specialist in obesity medicine at University College Hospital in London, believes that governments do have a role to play, not least because it is so difficult for individuals to lose weight of their own accord.

"Humans have evolved to survive famine, so our brains tend to tell us it's good to eat as much as we can while it's there, in order to survive the periods of food shortage.

"We haven't adapted to our appetites to live in a society with less food shortages. So it makes sense for governments to step in."

One thing that both those for and against the tax agree on is that by itself it will not be enough to significantly reduce the obesity epidemic in Mexico.

More support is needed, they say, for the health system as a whole and for healthy eating campaigns, especially those targeted at children.

And with or without the tax, Mexico has a long way to go before it slips down the global obesity charts.