Bittman's thesis is simple but sobering: What you choose to put on your plate has a direct impact on the environment, especially in terms of global warming. Especially if that something is beef, raised on a factory farm.
To produce one calorie of corn takes 2.2 calories of fossil fuel...but if you process that corn, and feed it to a steer, and take into account all the other needs that steer has through its lifetime—land use, chemical fertilizers (largely petroleum-based), pesticides, machinery, transport, drugs, water and so on—you're responsible for 40 calories of energy to get that same calories of protein.
Still don't get it? He puts it more bluntly:
Eating a typical family-of-four steak dinner is the rough equivalent, energy-wise, of driving around in an SUV for three hours while leaving all the lights on at home.
Calm down, carnivores! Bittman's not saying you have to become a vegetarian, and neither am I. He's simply pointing out that Americans eat far more meat than we need from a nutritional standpoint. Both our bodies and our planet would be a lot healthier if we would cut back even occasionally on our beloved burgers and buckets of fried chicken. Or, as Michael Pollan famously wrote: Eat food. Not too much. Mostly plants.
Bittman's personal approach to eating more consciously, he says, is to consume about one-third as much meat, dairy and fish as he used to. Refined carbohydrates, fast food, or junk food are only occasional indulgences, with the exception of pasta, which he still eats regularly. It's been a big change, but a "nearly painless" one, he says, and has brought down his weight, blood sugar and cholesterol. And interestingly, his appetite and food preferences have adjusted to match his new habits. While some diets grow tiresome in the long run, this one feels more natural with time.
As someone who made a similar shift about 10 years ago, I heartily agree. It's been so long since I considered McDonalds or Burger King as vendors of actual food that it doesn't even occur to me to stop there when I'm hungry; they might as well be selling office supplies. I don't have to force myself to eat vegetables—I crave them. (On a trip to Germany, after days of dining mostly at tourist cafes whose idea of a "salad" was a few scraps of cabbage slathered in mayonnaise, I literally dreamed about broccoli at night!)
On the other hand, I'm far from perfect. I still eat some processed foods, and several of the soy-based products in my fridge and freezer come from industrial-scale farms too many miles away. I don't have a garden (although this year I've invested in a CSA half-share which will supply me with a weekly bounty of locally grown, organic fruits and vegetables). And I'm not giving up coffee, wine, cheese, or chocolate, even though I don't technically "need" any of them in my diet. But I will be more thoughtful about the sources I support with my food dollars, both at the grocery store and in restaurants.
That's Bittman's point: Eat sanely. Eat consciously. And enjoy.