It is a rare thing – in this job, in film, in life – to be left literally shaken by something; to have it hit you with the force of a blow; to be left astonished by true courage, true pain, true compassion.

But that's exactly how I feel after seeing Thin, the debut documentary from photographer Lauren Greenfield. There's an easy, old joke that "'Sundance' spelled backwards is 'Major Depression,'" that many films at the Festival are harsh and harrowing, that after an entire day of press screenings here you go back to wherever you're staying and flick on the TV in desperate need of Bruckheimer action films, of laugh tracks, of banal talk shows, of pop culture as a soothing narcotic. Thin is about tough stuff; it follows four women at The Renfrew Center, a Florida private treatment facility for eating disorders – and yet it is also an amazing portrait of strength, of compassion, of community, of love.

Too many documentaries about social issues fall into two traps: They either bombard you with statistics like a Very Special episode of Beverly Hills 90210 ("Brenda, don't you know that one out of eight college kids binge drinks?") or they're smothered by voice-over, a thick blanket of dispassionate omniscience insulating you from what's on-screen. Thin features only two statistics – that one in five women will struggle with eating disorders, and that one in seven women with an eating disorder will die from it – and then concerns itself with facts about individuals: Weights, ages, suicide attempts, prior attempts at treatment. And there's no cool, all-seeing narrator: Greenfield's camera is constantly present, unblinking but never imposing, capturing events but rarely altering them.

And so we meet Shelly, a 25-year old with a feeding tube surgically installed in her stomach because otherwise she won't eat; Brittany, a 15-year old girl whose own mother has battled with eating disorders; Polly, a 29-year old with a rebellious streak and lightning-bolt charisma; and Alisa, a 30-year old mother of two children who has been binging and purging for over a decade. They've all come to get better, and we see the regimen the center puts them through, beginning with the mandatory morning check of vital signs as they stumble into an exam room with their spines and shoulder blades jutting through their skin under their gowns, skin blue from poor circulation, skin and hair ruined by years of deprivation, starvation, calorie control and willful self-neglect.

One of the things Greenfield finds at Renfrew is a certain gallows humor; as Alisa recounts an epic binge session, she's laughing at herself as she enumerates every fast-food stop and piece of food she bought along the way. Or as Polly stares at the icing-choked cupcake she's been given for her birthday and, according to the center's policies, has to eat: "I wanted a bran muffin with a candle." Or after the surgery to remove her feeding tube – because she's been using it to purge herself directly – Shelly's first words coming up from anesthesia are a slurred, delighted "Does this shit last all day?"

But Greenfield also finds sincerity and compassion and care, as the staff of nurses, therapists, nutritionists and financial administrators works carefully and competently to help their patients, and as the patients strive to help each other. When Brittany – whose insurance has run out – is about to leave the center and already longing to lose weight until she feels better, the group gathers for a community meeting to tell her how concerned they all – staff and patents – are. A fellow patient, Jen, is asked how she, at 28, feels her life might have been better if she had been able to get treatment and stick with it at 15, and her rage and tears flow out of her like fire: Jen talks about taking pre-packaged controlled meals to family Thanksgiving dinners, about shame and guilt and self-hate, and how now she's a 28-year old woman but  " … I don’t have periods anymore because I've got the body of a little girl. …" as she's wracked by sobs.

Great documentaries go from micro to macro – they find the large story in a small one, enlighten the audience about a big issue with human-scaled stories of real people. And that's exactly what Thin does. (Thin is produced by HBO; wiser heads will comment how HBO has slowly, carefully become the largest financial backer of documentary film in America, and the glib will note how something like Thin makes up for a lot of Taxicab Confessions, and both are right.) Thin is a film worthy of praise – one that shows real pain but also real healing, that shows people trying to change their lives every numb, aching step of the way instead of dissolving to a Hollywood-style montage that makes effort and agony into a music video.  Thin is heart-wrenchingly good: A film that knows exactly how much pain there is in the world, and doesn't flinch from showing us the hard work and challenges of trying to heal some of it.

Others on Thin
: David Poland at MCN calls it his "first great film" of this year's festival, and uses his review to explore our own obsessions, and how the modern growth of technology and media affect them. Also impressed was Flavorpill's Lisa Rosman, who praised the film for its "tremendous, unblinking candor, [which] invites us to question what else lives hidden all around us.'