Last night's viewing - Horizon: Eat, Fast and Live Longer, BBC2; Jimmy's Forest, More4

Michael Mosley has been making quite a nice living out of trying to live longer just recently. Not very long ago, he did a Horizon on exercise, exploring recent research that suggested that you could reduce the amount of exercise you need to do to stay fit and healthy to just three minutes of flat-out effort a week. He rather hinted that he would be taking up this new regime, but here he was in Horizon: Eat, Fast and Live Longer presenting himself as a man still in need of some kind of lifestyle miracle.

What happened to the high-intensity training, Michael? Anyway, the good news is that he seems convinced he's found an alternative shortcut to rejuvenation. "This is the beginning of something that I think could be huge," he said at the beginning of the programme. "This could be genuinely revolutionary."

The bad news is that it involves fairly serious self-denial, though it was one encouraging feature of last night's film that the badness of the news gradually diminished as it went on. The heart of the matter is that we're digging our graves with our knives and forks, a truth so unrevolutionary that it's had proverbial expression for centuries. It's long been known that a restricted calorie diet will probably extend your lifespan, the only problem being that you spend most of your extra time on Earth fantasising about your next meagre meal. What Mosley was claiming is that there might be a way for us to eat as much as we want most of the time as long as we take a break from the gourmandising now and then.

The problem, it seems, is a hormone called IGF1, produced by the body when it thinks it has the resources to put on a growth spurt. Since most affluent Western types always have enough resources for a growth spurt, our levels of IGF1 are consistently high, which isn't great because it's also associated with cancer and diabetes. Mosley's first stab at tinkering with his blood chemistry involved a three-and-half-day fast, during which he was allowed only water, black tea and 50-calorie instant soup per day. It worked, sort of. "Just a few days' fasting has made my body decades younger," said Mosley, ridiculously. But his glucose and IGF1 levels were much healthier. Unfortunately, four-day fasts once a month require levels of willpower not often found outside Olympians.

Fortunately, someone in Chicago had worked out that you can get a similar benefit by alternating feast and famine in what they call intermittent energy restriction. On one day, you restrict yourself to a single 700-calorie meal. The next day, it's cheeseburgers à gogo. Even better, it seems to work on a 5:2 split so that only two days out of every week are spent gnawing your fingernails and thinking obsessively about buttered toast. Mosley tried it for five weeks and was startled by the improvements in his blood chemistry. Scientifically, this proved what I have long suspected – that an attendant television crew is one of the most powerful aides to willpower in existence. As for the fasting regime, I'd like a bit more evidence. But Mosley himself was certainly persuaded: "The most interesting journey/film/whatever you call it that I have been on," he concluded pensively. "And I've never said that before."

Jimmy Doherty is pursuing a different route to rejuvenation in Jimmy's Forest, by behaving like a big kid, something he does with a fair degree of charm. "All of a sudden, I'm 12 again," he said happily, showing us round the treehouse laboratory in which he does a lot of purposeful messing around, dissecting woodpecker skulls to reveal why they don't knock themselves unconscious or extracting carotenoids from mashed-up caterpillars to make woodland cupcakes. The badger ham wasn't a success ("There's a funny aftertaste"), but the programme itself is just what your inner child ordered.