We often wish we could see less of our politicians - and now that wish is being granted.

Last year I was convinced Alex Salmond would not live to see the referendum. He seemed to enter rooms several paces behind his stomach. If a man's life expectancy can be measured by his waistline (and doctors say it can) I thought Scotland could be leaderless in a finger-snap.

Then, just when he looked likely to explode, a slimmer, rejuvenated First Minister emerged. He now resembles the living embodiment of his political dream: lighter, fitter, younger and looking to a prosperous future.

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And just as I was thinking "if he can do it, so can I", I registered a trimmer Johann Lamont facing Salmond across the debating chamber. The Scottish Labour leader says she approached her weight loss in small steps - cutting out sweets, biscuits, crisps and cakes and going to the gym. She has dropped four stones and reduced her dress size from an 18 to a 10.

Between them these leaders of their parties have shed around 100 pounds. They are going to be in great shape for the referendum in September.

Whether by coincidence or design, the two most powerful occupants of Downing Street are doing likewise.

George Osborne is reported to be following the 5:2 eating regime - just like Mr Salmond. The Chancellor was prematurely aged with close cropped hair and a wobbly belly. The crop remains but the wobble is no more. He too looks fitter. Now I read that David Cameron ordered low-fat options on his flight back from Israel.

Has fat become a political issue?

It has every right to be. We know that carrying excess weight is bad for our health. Almost one-third of Scots are obese while twice that number is overweight. It is the one European league table we top. Meanwhile, the French languish smugly at 15.6%.

It hasn't always been this way. As recently as 1980 obesity here was measured in single figures: 8% for women and only 6% for men. Now statisticians tell us that by 2045 half of us will be obese.

It's an appalling thought. Already the annual cost to the NHS is £50 billion a year (about £5bn for Scotland alone) and that's just the financial cost.How can we begin to quantify the misery? Obesity accounts for almost half of all type-two diabetes cases with potential side effects like declining eyesight and even limb amputation. Obesity causes more than one-third of hypertension, one-fifth of heart attacks, one in every six cases of angina and more than one-tenth of osteoarthritis.

It triples a man's chances of developing colon cancer and is blamed for 30,000 deaths a year, 9000 of which are before retirement. And we are looking at doubling the numbers.

Already many hospitals have had to reinforce operating tables. Some crematoriums have had to install larger ovens because super-sized coffins just wouldn't fit inside the old ones.

It doesn't have an upside.

As one former surgeon, now member of the House of Lords said last year: "It is killing millions, costing billions and the cure is free."

So while the sight of leading politicians losing weight in concert is probably coincidence, the coincidence is interesting. I wonder if something subliminal is going on, if it's an acknowledgement of the major policy challenge that obesity is becoming.

Will it be possible in future for politicians to speak with authority on the subject if they are walking examples of the problem, especially politicians who harbour ambitions to lead their countries in future, as Mr Salmond, Mr Osborne and Ms Lamont do?

I wonder also, given the scale of the looming crisis, whether it might even become impossible for an obese politician to lead a government in either London on Edinburgh?

The policy challenges are already difficult. They will get tougher as the health service struggles with the financial consequences of a double whammy - obesity and dementia. One example of the choices that might have to be made emerged last weekend. Scottish Conservative leader Ruth Davidson's proposed doing away with free prescriptions for those who can afford to pay. She says the money saved will pay for 1000 more nurses. Just look what more could be saved if we returned to our 1980s lifestyle and, by so doing, cut our obesity levels to single figures.

We have been fortunate in Britain that the American-style looks-based politics never took hold. We still pick our representatives for the content of their character and their manifesto.

They come in all shapes and sizes. In fact, if a candidate was too smooth or too good looking, it might go against him or her.

As the financial and human cost of obesity grows ever larger, as the money required to treat the extremely over-weight competes with money needed for Alzheimer's sufferers or for advances in cancer treatments, will that change? Is this weight shift a pointer to what may lie ahead?

Will we expect our politicians to look healthy and to have healthy lifestyles? I ask in the full understanding that weight is a complex issue. I am entirely capable of demolishing a packet of chocolate biscuits in moments of stress without even registering what I'm doing.

I am that woman who 20 minutes before a party discovers all her decent clothes have mysteriously shrunk. I have jeans in three sizes and several shrouds for when nothing fits.

A fortnight ago, while discussing this epidemic of obesity with a nutritionist, I suggested depression causes people to overeat. She believes the opposite. She thinks being fat makes us depressed.

On reflection I saw her point. I've always thought it unfair that being thin doesn't make me happy but being overweight is dispiriting. It makes us feel self-conscious and robs us of self- esteem.

It is hard to stay slim with sedentary jobs, television for entertainment and supermarkets filled with temptation. But that nutritionist passed on some facts that had escaped me, despite a lifetime of diets.

The first is that eating 3000 excess calories equals a pound of weight gain. The second is that if you Google BMR you can calculate your Basal Metabolic Weight and see how many calories you can eat in a day (without exercising) to maintain your present weight. To lose you need to eat fewer. Messrs Salmond and Osborne chose to do it by cutting down to 500 calories for two days each week. Either way, it works.

Losing weight is just as hard, if not harder, for leading politicians as it is for the rest of us. They live unhealthily, going from one meeting to the next, snatching food in between, being driven. We can bring a homemade lunch. We can walk to a bus-stop further away.

Has fat just become a political issue? You know, I think it has. If it was for vanity's sake I would be concerned. But since it is for physical and mental good health, I think that what started as a personal choice for our leaders has a positive political message for us all.