Eat whole foods; limit processed products. Energy-intensive processed foods, clustered in the center of the grocery store, are high in calories and low in nutritional benefit, says Anna Lappé, author of Grub: Ideas for an Urban Organic Kitchen (Tarcher/Penguin, 2006). “To take an apple and turn it into a frozen apple turnover requires a tremendous amount of energy,” adds Geagan. Freezing, drying, canning, and refining — as well as shrink-wrapping and boxing — take energy.

Lappé also points to the thorny issue of palm oil, an ingredient used in place of unhealthful trans fats in some “healthy” processed foods. In Malaysia and Indonesia, where nearly 90 percent of the world's palm oil is produced, carbon-sequestering rainforest and peat bogs are being destroyed to make way for palm-oil plantations.

Make a balanced choice: Michael Pollan, author of In Defense of Food (Penguin, 2008), recommends shopping the periphery of the supermarket, where you find energy-efficient whole foods — fruits, vegetables, and fish. If you must buy processed products, Lappé suggests her “rule of thumb”: Avoid those with ingredient lists longer than the width of your thumb, particularly ones filled with unpronounceable ingredients.

Next: Eat locally and seasonally