Last Night's Viewing - The Undateables, Channel 4; All You Can Eat, ITV1

I tied myself in liberal knots of anxiety the last time I reviewed The Undateables, in which Channel 4 paired up the dating show with the disability documentary in the hope that they'd get on famously. I take it everything went well after that first date because here we are nine months on for another series, just as I'm ironing the final kinks out of my ethical consciousness. And within minutes, all the patient work of untangling has gone for nothing.

The question of why we are watching is at the core of the problem. If it's because we want to understand more about some of the conditions portrayed on the show, there's no difficulty at all, I would have thought. Empathy is a pretty uncomplicated good. But television viewers are rarely quite as selfless as that. We seek entertainment too, and the fact that The Undateables can be entertaining makes things much trickier. Amusement and curiosity – both of which you're likely to experience in any given episode – aren't uncomplicated at all. They slide all too easily into mockery and prurience, and it isn't always easy to tell when it's happened.

Take Michael, for example, an autistic 26-year-old who yearns to have a girlfriend but who finds it tricky to negotiate that awkward first meeting. To help him rehearse for a blind date with a girl called Helen, his mother took him to a café for a bit of role-playing. Cue Michael, who marched up to the table and plonked himself down without an introduction. You didn't even check that I was the right person, his mother pointed out gently. Michael went back to his starting position to give it another go. "Are you sure you're Helen?" was his second stab at an insouciant opening gambit. Funny, no? Even his mother thought so and she didn't feel any anxiety about laughing, though Michael couldn't see the joke. But then it's all right for her to laugh because she loves and cares for him. We're just passing by and gawping and our laughter might be a different thing.

There is a sympathy short of love though, and it may be that The Undateable's ability to elicit that is what finally gets it off the hook. You'd have to be a sociopath not to have felt for Brent, who has Tourette's, or Sarah, a pretty 22-year-old who'd been left with aphasia after suffering a stroke. For both of them small talk was a nightmare, in Sarah's case because even the smallest talk is sometimes beyond her and for Brent because he couldn't be sure that it wouldn't come out blue and abusive: "It's basically like going on a date with a live grenade... and hoping it doesn't go off," he said. We accompanied both of them on blind dates, persisting through the early agonies of first encounter to a point at which it was at least clear that the occasion wasn't a humiliating failure. And the sweetness of that didn't seem entirely discreditable.

At least you feel The Undateables has some kind of ethical conscience, not something that can be said of All You Can Eat, an ITV celebration – there really is no other word – of decadent gluttony. While other channels address the obesity crisis with programmes such as Embarrassing Fat Bodies or Weight-Loss Ward, ITV gives us this cheery documentary about supersize food challenges and eating contests. "Today is about a lot of crazy people doing something that's really stupid," said the organiser of a British nettle-eating contest. "We love that sort of thing in this country." They love it even more in America, where Joey "Jaws" Chestnut later took a Nathan's Hot Dog trophy by consuming 68 of the things in just 10 minutes. The only health warning offered in a narration of jaunty witlessness concerned choking hazards. Alongside this, The Undateables was Spinoza.