By Tanner Garrity Thru-hiking has a weight-loss problem. We don’t talk about it enough, but why should we, when completing 2,190 grueling miles (the length of the Appalachian Trail, for instance) in a matter of months is such a resounding physical achievement? The reality, though, is that of the some 3,000 men and women who attempt the A.T. each year, an overwhelming majority of them will lose weight they can’t afford to lose. And as a hiker named Julia Sheehan conveyed to The Trek for an article two years ago, this weight loss can get out of control well before people reach their final test o…