Dieters, watch out: paying with plastic at the grocery store can make you more likely to load up your cart with donuts, ice cream, potato chips and other fattening foods.

Studies have shown that when shoppers pay with credit or debit cards, they buy about 40 percent more unhealthy foods than those who pay with cash.

The logic may not be very mysterious, but it's definitely worth being aware of if you want to keep both your spending and your weight in check. When you use cash, you usually have to plan the purchase, estimate how much money you'll need, then go to the ATM to withdraw it, says David Just, associate professor of behavioral economics at Cornell University, who has studied the use of plastic vs. cash in food purchasing decisions. "But a credit card is there whenever you have the urge for a snack."

When you use cash, you also have to calculate whether you'll have enough to pay, and that forces you to use your brain, Just says. "The act of counting gets you thinking, not just about money but about the long-term effects of the food," he says, and that makes cash-paying consumers more likely to opt for the apples but not the Apple Jacks. "With cards, you just put the stuff in the basket, walk over and swipe the card. It takes so little thought."

Worried your credit card might be plumping up your, uh, other bottom line? Here are six ways to make sure plastic doesn't derail your diet:

1. Pay for food with cash. Experts say the best thing you can do for your waistline, as well as your wallet, is to buy groceries with cash. "I know plastic is more convenient," says Kalpesh Desai, a professor at Binghamton University who did a study on the grocery shopping habits of 1,000 single-family households. "But using cash will help you fight the impulse to buy unhealthy foods."

The study, published in 2011 in the Journal of Consumer Research showed shoppers who paid with plastic bought at least 40 percent more junk foods, such as ice cream, donuts, cookies, gum and candy. However, 86 percent of consumers in the study paid with plastic on their shopping trips, while only 14 percent used cash.

Researchers looked at records of the study participants' purchases and payment methods over a six-month period. The purchasing method didn't affect the amount of healthy food -- including meat, vegetables, fruit, baby food and whole grains -- consumers bought. In the study, shoppers who used cards paid attention to prices and knew they'd regret buying foods laden with fat and sugar, but did it anyway, Desai says: "Unhealthy products have such a strong hold on us that we're unable to resist."

2. Or, just go cash-only for treats. If you prefer using plastic for your groceries, you could take a hybrid approach: buy only nutritious foods with a card and buy treats with cash. In a 2009 study at the Cornell Food and Brand Lab, which does research on how consumers relate to foods, researchers found that students who paid cash for school lunches, instead of using prepaid debit cards, spent 30 percent more on healthy items such as low-fat milk, bottled water, fruit and vegetables.

However, researchers saw students make similarly healthy food choices when they were given a debit card that could only be used to purchase foods that were good for them, and also were given additional cash they could use to buy anything they wanted, Just says.

3. Limit your funds. If you walk into a store with only $50 in your wallet, you'll probably spend less than if you go in with a credit card that has a $5,000 limit, experts say. "People who have credit have more money to spend, so they tend to spend more freely," Just says. So, you could nix the credit card for your food purchases and instead load up a debit card with a set amount each week or month. Once the money is gone, it's gone.

"That's a modern incarnation of the old budget trick where people would take cash out and put it in an envelope," Just says. While it likely would not be as effective as using cash, it's still a way to trick yourself into staying on track with your purchases, experts say.

4. Make a grocery list, and stick to it. If you make a grocery list just so you won't forget anything, it probably won't help you in your resolution to avoid unhealthy foods, Just says. In other words, you'll probably remember the cabbage you need to make your veggie soup, but you might also grab those cream puffs you don't need on your hips. However, if you make a list of nutritious items and decide ahead of time not to buy anything else, Just says it probably will help a lot.

"You need to make your decisions before you even walk in the door," says Lisa Galper, a Phoenix psychologist and expert in the psychology of weight loss. That can mean making a grocery list, looking at a menu online and deciding what to order before a business lunch or even vowing to order only one pastry before you walk into a donut shop, she says.

5. Put a lock on temptation. First it was fast food restaurants and now more vending machines are starting to take credit cards, according to Capital Processing Network. So experts say consumers need to think ahead about ways to avoid temptation or make it harder for themselves to pull out a card, swipe and munch.

One tactic is to avoid the temptation. For example, Just says he tries not to walk past a vending machine that takes credit cards right down the hall from his office.

Another option? Lock your wallet in your desk drawer, he suggests. Similarly, Galper recommends locking your purse in your car trunk if driving past fast food restaurants makes you crave a greasy burger. "No one wants to pull into the drive through, put the car in park, go around to the trunk and get their purse out," Galper says.

For the same reasons that plastic can make it easy for consumers to get into debt, credit and debit cards also can promote unhealthy eating habits, Galper says: "People tend to spend and eat mindlessly, so it's important to be mindful."