Health met physics in last week's New England Journal of Medicine with a study revealing that the type of diet doesn't matter so much for weight loss — be it low-fat, low-carb, 70-percent yak meat or whatever — as long as one simple requirement is met: Consume fewer calories than you burn. The discovery harkens back to the Age of Enlightenment and the law of conservation of energy, a concept lost in the fad-diet world for the last 200 years. That said, not all diets are created equal. This latest health study, led by Frank Sacks of Harvard School of Public Health, does not exonerate any particular diet, as some news reports have implied. The reason is that diets should not be simply about losing weight.