People talk about orthorexia, an eating disorder that takes the form of an obsession with healthy food, as if it were a new thing – or, at least, an illness that has broken cover recently, encouraged by those who spend their days posting pictures of their fantastical beet- and cashew-based diets on Instagram. But as Laura Shapiro reveals in her new book, What She Ate: Six Remarkable Women and the Food that Tells Their Stories, the condition has been with us for decades – sometimes in plain sight. It was back in 1959 that Helen Gurley Brown, future editor of Cosmopolitan and bestselling author of Sex and the Single Girl, first walked into the Los Angeles health-food store on whose shelves she saw the (terrifying) future.

Gurley Brown was then feeling rather glum: David Brown, her movie executive boyfriend, was refusing to marry her, and she had just finished an assignment at the Miss Universe Pageant at Long Beach, which had done her ego no good at all (all those younger, prettier girls). Needing a pick-me-up, she swung by a place she’d heard people raving about, Lindberg Nutrition, and by the time she left, she was a convert: to vitamin supplements, to soy-flour pancakes and to the Serenity Cocktail, which comprised, among other things, pineapple chunks, calcium lactate, vanilla, powdered milk and brewer’s yeast.

At this point, she was not interested in dieting so much as in wanting to improve her physical and emotional health (Gurley Brown had always been rake thin). But this soon changed. Although David did finally propose, her new status couldn’t alter the fact that she was nearing 40, a number which filled her with horror – and so the starving began. Except that she didn’t quite let on that she was starving, not at first. She always cooked for David, a hot breakfast and dinner every night, and she liked to tell people how “scrumptious” she thought this or that dish was; once her writing career took off, she even published a (ghostwritten) cook book. Thus, she maintained the illusion that her appetite was perfectly healthy.

In fact, her diet was extremely restricted. Sure, she was a great one for tins and packets: the times were different then. (She was devoted to Jell-O, though it had to be sugar-free, of course.) But much of what she ate will sound weirdly familiar to anyone acquainted with so-called clean eating (as will her conviction that anyone who dared to criticise it was simply “jealous” of her figure). She liked to feast, for instance, on such “satisfying” and “delicious” treats as prunes, dried apricots and unsalted almonds. Meanwhile, many normal foods began to seem actively dangerous, and they had to be avoided at all costs. Other people were welcome to eat brownies, but if someone at a party happened to hand her one, straight into her handbag it would go (or, in an emergency, behind the nearest cushion).

All this sounds desperately miserable and unpleasant. Combine the aspirations of a 60s hostess with a mortal fear of food and you get some pretty weird stuff; the mind boggles at the thought of avocados stuffed with orange ice (even as I’m typing this, I’m wondering whether some pouting blogger isn’t about to try it: “so amazingly refreshing, and lactose-free, too!”). Ill and exhausted as she must have been, maintaining the fiction it was just wonderfully good luck that those foods which made a girl “sexy, exuberant [and] full of the [sic] joie de vivre” were also the ones that happened to keep her “slender” was too much even for her. Slowly and surely, the smokescreen fell away.

If the “incredible salad bar” in which she “simply rolled around” every night of her holiday sounds exactly the kind of thing you’d read on Instagram, her frank admission of a weekday diet consisting only of tuna, cottage cheese and an apple has a veracity that, however pathetic and troubling, seems almost touching in the age of the online chickpea mountain (“I eat so much,” bloggers insist, waving a loaded plate you feel certain they will never empty).

Of all the anecdotes Shapiro dishes up, my favourite is the one with which she ends. Once, she writes, Gloria Steinem – Gurley Brown’s favourite feminist – begged her to say something positive about herself, something that reflected the serious, complicated person she really was. “Helen tried her best,” writes Shapiro. “She really did.” But in the end, all she could up with was: ‘I’m skinny! I’m skinny!’”