Scratch the surface of virtually any liberal political initiative and you’ll find a sick need to mind other peoples’ business. Perhaps some day they’ll discover a “busy body” gene and we’ll indeed enjoy a new politics.

Alas:

As someone who commutes by bicycle into Manhattan, I would normally applaud any scientific rationale for more bike lanes. But some calculations in the new issue of the Lancet make me uncomfortable. The authors argue that policies promoting cycling and walking are good for the planet because they could reduce obesity — and obesity, the authors calculate, contributes to global warming.

Do we really need to give fat people one more reason to feel guilty?

The Lancet authors, Dr. Phil Edwards and Dr. Ian Roberts of the London School of Hygiene and Tropical Medicine, crunch the numbers and conclude:

Compared with the normal weight population, the obese population consumes 18% more food energy. Additionally, more transportation fuel energy will be used to transport the increased mass of the obese population, which will increase even further if, as is likely, the overweight people in response to their increased body mass choose to walk less and drive more.

Urban transport policies that promote walking and cycling would reduce food prices by reducing the global demand for oil, and promotion of a normal distribution of B.M.I. [Body Mass Index] would reduce the global demand for, and thus the price of, food. Decreased car use would reduce greenhouse gas emissions and thus the need for biofuels, and increased physical activity levels, would reduce injury risk and air pollution, improving population health.

Sure, exercise has lots of benefits. But would more exercise really lead to less obesity, or would the exercisers just get hungrier and eat more? Check out this article by Gina Kolata, my Times colleague and the author of “Rethinking Thin.”