Bradley Wiggins looks painfully thin, as if one big gust of Alpine wind on a Tour de France stage could knock him over.

When the 29-year-old takes off his top, he reveals a protruding ribcage, his cheeks are noticeably concave and his veins stick out from his arms - in short there is not a single centimetre of fat on his body.

The leading Briton in this year's Tour admits he has a weight problem, even going so far as to suggest that his bid for race glory has driven him to the verge of anorexia.

Wiggins, who lies a remarkable third behind Alberto Contador and Lance Armstrong in cycling's most prestigious event, has shed 6kg since when he last rode the Tour in 2007.

The 6ft 3in Londoner tips the scales at a relatively meagre 11st. And he admitted his bid to lose weight to make life easier on the Tour's tough climbs had led both he and his wife, Cath, in particular, to seriously worry about his eating habits.

"I think my wife has struggled a bit because of how obsessive I get with what I eat and stuff," he said. "I'm borderline on being unhealthy at the moment. It's difficult and it's an incredibly fine balance between getting your weight right down and being anorexic."

His body fat is just four per cent and, should that get any lower, his Garmin Team doctors have warned his body will start eating into his muscle.

And Wiggins added: "It's quite scary and I've been told I must put on weight after the race. I know my wife will want to fatten me up when I get home."

His pre-Tour diet included going gluten free for two months earlier in the year, a refusal to have breakfast before his morning training rides and scrapping beer completely.

However, during the race he has had to massively increase his food intake just to survive. On a regular day, he consumes about 5,000 to 8,000 calories. Breakfast entails cereal, fruit and a plate of pasta and eggs. This is followed by snacks en route to the start, two meals on the road, a quick bite to eat at the finish and a full dinner in the evening back at the team hotel.

On average, Wiggins will eat five eggs a day, 200 grams of meat or fish, 10g of nuts, three yoghurts, all manner of energy bars, four fruit jellies, 200g of oats, a similar serving of pasta, seven rice cakes, a tablespoon of honey, a quarter of a baguette and four pieces of fruit.

Despite the apparent dangers of his pre-Tour weight-loss programme, there is no denying it has helped his cause during this year's race. While the headline act has been the battle between Armstrong and Contador, which is heavily tipped in the Spaniard's favour after his victory on the climb to Verbier on Sunday, Wiggins has provided a surprise sideshow.

The Briton (pictured right in this year's race) was unable to go with Contador up to Verbier but was in a select group of riders, including seven-time Tour winner Armstrong, just over a minute behind.

It goes without saying that the lighter a rider is, the easier it is to cope with the Tour's Alpine and Pyrenean ascents. As Wiggins explained: "Compared to the 2007 Tour, the weight loss means I'm carrying the equivalent of six bags of sugar less up a mountain. Shedding that weight is all that I can do to give myself the best chance on the climbs other than taking drugs and I'm not going to do that."

Wiggins's efforts have understandably raised eyebrows among the sceptics, understandable in a sport littered with positive drugs tests at virtual every Tour in recent memory.

But he is adamant he is 100 per cent clean and rides for a team that pride themselves on his anti-doping message. He is bored of defending himself against the accusers but recently wrote on his Twitter page, "keep the faith people, I ain't no Bernhard Kohl", in reference to the Austrian who finished third at last year's Tour only to later fail a drug test.

Wiggins, who grew up in Maida Vale but now lives in Spain, has no idea what lies in store for him when the Tour resumes after a rest day today. The final week of the race is brutal, with three hideous mountain stages, including the penultimate stage climb up Mont Ventoux, and an individual time trial.

"I don't make predictions," is his assessment. "I know what I can do and I try not to think too far ahead."

The time trial should play to Wiggins' strengths. As a double Beijing Olympic gold medallist - he won the individual and team pursuit titles - he is one of the best riders against the clock.

But his new-found love for the Tour means he may not even defend those titles despite the fact the Olympics are being hosted in his home city.

"I haven't decided what I'll do," he said. "I may never get back to the track. The problem was that I was dominating my event and the winning became slightly boring. I wanted new challenges and I've got that on the road."

Should he hold on to his podium place come Sunday's finish in Paris, Wiggins will become the highest-placed British rider in Tour history. Robert Millar's fourth place at the 1984 Tour is the current best.

Physically, Wiggins is already feeling the rigours of cycling's most arduous event but said that the mental exhaustion far outweighed the physical impact over the first 15 stages. He admits: "The Tour's pretty stressful as you have to concentrate for so much of every day making sure you don't get caught in a crash or get caught out, and I don't plan to switch off for a minute."

And surely there would be nothing like a spot on the podium at the end of the Champs-Elysees to focus the mind.