When food writer Lisë Stern needs fresh vegetables to roast with a chicken, she bicycles to the green market near her Cambridge, Mass., home where local farmers sell organically grown produce. Once back in her kitchen, she prepares the meal using knives, bowls, utensils, a cutting board and a roasting pan dedicated solely to cooking with meat, and serves it to her two teenage sons (her 11-year-old daughter is a vegetarian) on glass plates never touched by milk, cheese or other dairy foods.

Stern, the author of How to Keep Kosher: A Comprehensive Guide to Understanding Jewish Dietary Laws, is one of a million or so American Jews (out of around six million total) who keeps her kitchen year-round according to the laws of kashruth, or kosher. She's also interested in the environment. So how does keeping kosher contribute to—or undermine—her efforts to go a little lighter on the planet?

In 2007 kosher foods were worth $12.5 billion of the $500-billion retail food market, according to market research firm Mintel. It isn't only Jews: According to marketing company Lubicom, the 10.2 million Americans who eat kosher foods include around three million Muslims, whose halal dietary rules overlap with kosher ones.

Kosher rules state: those who keep kosher eat traditionally domestic fowl like chicken and turkey; most fish with fins and scales—that means no shrimp, crab or lobster; and mammals that both chew their cuds and have split hooves, which includes cows and sheep, but not pigs.

What would the environment look like if everyone kept kosher? Per capita, Americans consume about 63.5 pounds (29 kilograms) of beef, 48.2 pounds (22 kilograms) of pork and 59 pounds (27 kilograms) of chicken per year, according to the U.S. Department of Agriculture. They also down 54 pounds (25 kilograms) of fish and shellfish, including about four pounds (two kilograms) of shrimp (the U.S.'s most popular seafood), according to the National Oceanic and Atmospheric Administration's Marine Fisheries Service.

So how does a kosher diet fare as one that is ecofriendly? Time for some calculations: first, let's assume that kosher vegetarians would still steer clear of meat in any quantity, even if they did not keep kosher, meaning that observing the rules would have no impact. Let's also assume that kosher omnivores consume the same average weight of meat per capita as other Americans, but replace pork with either beef or chicken. That would have an impact. Solely in terms of how much grain livestock consume, producing a pound (0.45 kilogram) of beef releases 13.67 pounds (6.2 kilograms) of greenhouse gases, compared with around 6.75 pounds (3.1 kilograms) to produce a pound of pork, and 3.37 pounds (1.5 kilograms) for every pound of chicken—and this does not even take into account the other factors in meat's carbon footprint, from deforestation for pasturage to shipping it to market. Globally, meat production generates 18 percent of the world's man-made greenhouse gases, according to the United Nations Food and Agriculture Organization.

That means replacing nonkosher pork with an equivalent 48 pounds (22 kilograms) of beef releases about 1,504 pounds (682 kilograms) of greenhouse gases annually, compared with 1,378 pounds (625 kilograms) of carbon a year for the pork-friendly eater.

Of course, you could go the other way: If the kosher-only omnivore replaced all the pork with chicken, their greenhouse emissions would drop to 1,216 pounds (552 kilograms) per annum. But if the "pork difference" were split equally between beef and chicken, the kosher-only meat diet would yield 1,460 pounds (662 kilograms) of emissions—about 6 percent more than the nonkosher diet.

What about shrimp? It takes 243 gallons (920 liters) of diesel fuel to trawl about 1.1 tons (one metric ton) of the shellfish, according to Dalhousie University in Halifax, Nova Scotia, making shrimp one of the most energy-intensive wild seafood harvests, with a footprint of 5,395 pounds (2,447 kilograms) of carbon dioxide per metric ton even before processing and transportation are taken into account. And shrimp farming (which provides well over one million metric tons of shrimp annually, about 25 percent of all shrimp consumed) has been linked to the destruction of almost half of the world's mangroves: coastal forests that absorb carbon dioxide and provide essential habitat for wild fish species. Crab, meanwhile, was among the least energy-intensive species to catch in the Dalhousie study, whereas the fuel needed to collect a ton of lobster swung wildly—ranging from 5.3 gallons (20 liters) per metric ton in Iceland to about 38 gallons (144 liters) in Maine to 271 gallons (1,025 liters) in Norway.

Just as with livestock, the ultimate green boost from kosher law's taboo on shrimp and other shellfish depends on what you eat in its place. Assuming that the kosher consumer replaces the average American's four pounds of shrimp a year (and its 9.79 pounds, or 4.4 kilograms, of carbon dioxide emissions) with another fish, Canadian North Atlantic herring is a good choice: it takes around 5.28 gallons (20 liters) of fuel to purse seine (net using two trawlers) a metric ton of these small fish, according to Dalhousie, releasing about 117 pounds (53 kilograms) of carbon dioxide—meaning four pounds of herring have a carbon footprint of a mere 0.21 pound (0.09 kilogram). Wild U.S. or Canadian salmon take an average of just over six gallons (23 liters) of fuel per metric ton to catch, releasing about 133 pounds (60 kilograms) of carbon dioxide. So eating four pounds of salmon a year would account for 0.24 pound (0.1 kilogram) of carbon dioxide. Both of these are obviously just a fraction of the 9.79 pounds of carbon dioxide for the shrimp eater.

Tuna are energy hogs by comparison, needing about 460 gallons (1,740 liters)—twice the fuel of trawling for shrimp—to harvest the same single metric ton of tuna. That adds up to a massive 10,212 pounds (4,632 kilograms) of carbon dioxide per catch. So eating four pounds a year would have a footprint of 18.5 pounds (8.4 kilograms) of carbon dioxide, almost twice the shrimp eater's footprint.

Kosher rules do remove some overfished wild species from your plate—such as sharks, which are in serious decline worldwide, according to the Monterey Aquarium's Seafood Watch program. On the other hand, some popular fish that are kosher, such as bluefin tuna and Chilean sea bass are also in peril.

Kosher rules also forbid mixing meat and dairy foods: No cheeseburgers, please. "The idea repeated three times in the Bible is, 'you shouldn't boil a kid in its mother's milk'," says Stern. This has evolved over the centuries into complex rules and practices to keep the two apart in the kitchen and the stomach as well as in the cooking pot. That means two sets of dishes. Doubling one's kitchenware would seem to run counter to the "less is more" mantra of contemporary environmentalism but, as Stern notes, because both sets are never used simultaneously, the useful life of each is likely extended over time.

Even though keeping kosher is not inherently more or less ecofriendly than a conventional diet, Stern notes that the small but growing kosher organic meat offerings, along with the overall boom in organic foods, make it easier to suffuse keeping kosher with her green values. And, of course, there are benefits that can't be counted by the numbers. "For me, keeping kosher is a spiritual commitment," Stern says. "It imbues the mundane with the sublime."