If you're going low-carb on either the Atkins or South Beach diet, you'd better have a fat wallet, says Supermarket Guru columnist Phil Lempert, who helps ACNielsen track food trends.

LOW-CARB DIETS Menu Maker Lowdown On Low-Carb Diets Diary Of A Fat Man Discuss Atkins On Low-Carb Diet? How Low-Carbing Works How Atkins Works

Lempert said following a low-carbohydrate diet can raise a person's weekly food bill by as much as 69 percent.

He said they've calculated that if you stick to the recommended ingredients and portion sizes for a week, you'll pay close to $100 on the Atkins low-carbohydrate diet and a little over $91 on the South Beach diet. Compare that to the $59 a week that the Food Marketing Institute says the average single person spends on food.

Lempert said going low-carb means you'll be eating a lot of protein and a lot of fresh meats and fish, as opposed to rice, sugar, grains and other foods that cost less. Plus, there are special low-carb brands and packaged foods that also tend to be pricey.

The cost is an important issue since 17.2 percent of Americans told Nielsen they're currently on a low-carb regimen, and the numbers have been growing.

To be cost-conscious, as well as carb-conscious, Lempert recommends using the diet plans simply as a guide. He suggests substituting salmon and other high-priced fish with lower-priced fish, chicken breast or tofu. Instead of steak, use ground beef.

He also suggested buying frozen fish rather than fresh -- and the same holds true for expensive fruits. They're packed at the peak of freshness, so you might find that frozen blueberries, for example, are even tastier than fresh blueberries that might have been flown in from a different part of the world.

Wallets can also be spared by substituting canola oil or a less expensive store brand of olive oil for extra virgin olive oil and using less expensive herbs and spices.

SupermarketGuru.com: Lempert's Column
U.S. Department of Agriculture: Healthy, Thrifty Meals (Adobe Reader required)