Most beef sold in supermarkets today is grain-fed, meaning that after the animals are 6 months old, they are fed a diet of corn and other grains. These grains are cheap and fatten the animals quickly, lowering production costs for large-scale farms and increasing the marbling in the meat. But since cows’ bodies are not designed to digest corn, this type of diet makes them sick, often requiring antibiotics. Grass-fed animals, on the other hand, eat grass and hay, are not fed antibiotics or growth hormones, and are not confined. Grass-fed meat is generally leaner than grain-fed, but after some testing, we discovered that the differences are minimal. Note that because it has less fat, grass-fed beef is less forgiving when it comes to overcooking, so be sure to check the temperature of the meat at the beginning of the time range. The best way to source grass-fed beef, lamb, and bison is to seek out a high-end grocery store or a local butcher shop.

Unlike cows, pigs and chickens are naturally omnivorous and have no trouble digesting grain. But conventionally raised pigs and chickens are often confined to tiny cages in unhygienic conditions with no room to roam. So when shopping for pork, poultry, and eggs, try to purchase pasture-raised. Note that the term “pasture-raised” is not government regulated, so you’ll have to look for third-party certifications, like Certified Humane, Animal Welfare Approved, or American Humane Certified. (“Free-range” means that animals must have access to the outdoors, but the amount, duration, and quality of access are not defined.) If you can’t find pasture-raised chickens, go for organic: Certified organic producers must follow stricter guidelines than conventional producers. Birds must be fed organic  (and therefore non-GMO) feed, be raised without antibiotics, and have access to the outdoors (though how much is not regulated).

In addition, be sure to avoid pork or chicken that has been “enhanced,” meaning it has been injected with flavorings. It’s not paleo, and it compromises the flavor and texture of the meat. We also recommend that you avoid water-chilled chicken, which is chilled after slaughter in a water bath that may contain chemicals like chlorine. Instead, look for labels that say “air-chilled.” Besides not containing added chemicals, air-chilled birds don’t absorb extra water (which you would pay for at the market).

Read Our Guide to Buying Produce