Last Night's TV: Horizon: The Nine Months That Made You/BBC2<br />Jamie Cooks Summer/Channel 4

Exercise daily? Eat your fruit and veg? Stopped smoking? Bad news. You could be doomed, nonetheless. David Barker's theory sounds pretty hokey when you first hear about it. The idea is that, irrespective of that £100 gym membership, those five-a-day and that kicked habit, much of our health has been determined by the time we get out of the womb. You can see why mothers don't believe it when, on Horizon: The Nine Months That Made You, they are asked about Barker's theory, at random in the street. It seems less a scientific prescription, more a kind of fortune-teller's ruse. I'm not sure I want to believe it, either. After all, life would have been a lot more fun if I'd known I had a free pass all along (for this was one enormous baby, back in the day).

Of course, diet and exercise and all those other things do come into it, too. But they are at their most significant where birth weights are particularly low. The lighter you are when you are born, the more you will be affected by your lifestyle. That is why, or so the theory goes, people like my grandfather live to their late eighties while smoking like chimneys. It's also why Dr Ranjan Yajnik has discovered a new body type, the thin-fat Indian – considerably slimmer than their English counterpart, but carrying significantly more body fat. It explains, he argues, India's epidemic of obesity-related ailments: heart disease, type 2 diabetes and so on.

It is not just our health that may have been pre-determined in this way. In the US, Dr Janet DiPietro has been testing foetuses for their reactions to disturbance, sounds and movement, then monitoring babies to see if the traits persist. She is adamant that they do. Elsewhere, studies indicate that testosterone levels in the womb are related to the kinds of activities we enjoy later in life. It's convincing – though the real test is still to come.

Back in India, one of Barker's colleagues is leading a drive to boost birth weight, to see if his theory provides dividends. Potential mothers are put on a diet of specially-prepared snacks, to ensure maximum nutrition and boost baby weight. This, of course, is the ironic part. Our own lifestyle might not be quite the self-imposed sentence we tend to assume. Instead, our mother's behaviour is what shapes us – because diet and lifestyle determine birth weight.

During the Dutch famine of 1944, babies tended to be tiny, because their mothers' diets were poor. However, even more amazingly the lifestyle of your maternal grandmother comes into play, too, since women are born with all their eggs. So the debate is less one of nature versus nurture, but of who was nurtured when. If Barker is right his work represents, as several interviewees noted, a paradigm shift. Either way, it made for top-notch head-scratching from Horizon.

Summer food needn't just be about a sausage on a barbecue, said Jamie Oliver in Jamie Cooks Summer. It's a bit late now, though – I've spent the past four months buying nothing but Jamie Oliver Cumberland. It is more than midway through August and we are only just getting to hear about Jamie's portable chilli brisket. It wasn't quite clear what is so portable about the brisket, by the way – put any old stew in an artfully battered Le Creuset, cover it with foil and plonk it in the back of a VW van and it becomes portable. Doesn't it?

Despite the faint whiff of parody – the artfully erected campsites, the happy families bonding, and the fact that Jamie Cooks Autumn might be a better idea at this stage of the year – Oliver was on fine form here, showing us genuinely interesting things, like how to smoke your own trout or make your own ice lollies, and cooking burgers with the indie band Athlete (still going, it would appear, despite all appearances to the contrary).

He was at his best when cooking a pancake layer cake with a bunch of kids. "So are you married?" he asked one three-year-old girl, to squeals of laughter. Oliver was at his worst when banging on about the manly skill of smoking and how pleased he is to finally have a son he can share it with. Don't girls like smoked trout, too?

There were the usual special guests: Gennaro Contaldo rocked up to demonstrate three new ways to cook with fire – on a spit, on a stick and on a hot rock – and Levi Roots demonstrated the way to make jerk chicken. There was nothing that is going to set the world alight, but plenty of useful things to try out next summer. If we remember them.

a.jarvis@independent.co.uk; twitter.com/aliceazania