It may not surprise you to learn that the old formula of cutting 500 calories a day to lose a pound a week does not work for every person. Now scientists have come up with a new formula — all you have to do is convert your BMI into furlongs per fortnight, divide it by the circumference of the Earth in centimeters, and then add the total number of hairs on your scalp. Easy!

Okay, not quite. According to LiveScience, the new formula, unveiled at this year’s American Association for the Advancement of Science meeting, goes like this: for every 10 calories you cut per day, expect to lose one pound over the course of three years. So if you eat 100 calories less every day, you should weigh 10 pounds less three years later. Weight loss researcher Kevin Hall says this new formula takes into account the fact that weight loss tends to slow down over time. If you want to lose more weight after three years, he says, you’d need to cut more calories. Also, he acknowledges that weight loss isn’t the same for everyone — he has a more complicated model for doctors and researchers that also takes into account factors like age, height, and start weight.

Perhaps this will put to rest the idea that losing weight is a simple subtraction problem. Also challenging this notion: doctors aren’t especially good at predicting whether their patients will lose weight. LiveScience reports on another study, in which 40 doctors predicted that 55% of their patients would follow their weight-related recommendations. But after three months, just 16% of the patients they thought would lose weight did so. Only 19% improved their diets, and 6% added an hour or more of walking to their weekly schedule.

Given that there are tons of incentives for people to lose weight, not least the ever-present stigma against fat people, I’d bet that most of these patients wanted to follow their doctors’ recommendations. What these dismal numbers speak to is that losing weight is hard, and it takes more than a doctor’s say-so to change eating and exercise habits. And of course, even changing those habits might not actually result in weight loss. The study didn’t seem to include doctors who followed a Health at Every Size approach, so we don’t know if this would’ve been more effective. What we do know is that just telling people to lose weight doesn’t make them do it — especially since even the supposedly simple science of calorie-counting is actually pretty convoluted.

New Weight-Loss Equation: Researchers Determine Key Calorie Cutoff [LiveScience]
Who Will Lose Weight? MDs Predictions Often Wrong [LiveScience]