When you think of it, a healthy diet starts within the supermarket, where you shop for your week's supply of groceries. After all, when you buy healthy, you eat healthy.

And how you pay for your groceries (in cash, credit or debit) actually influences your buying decision -- i.e. whether or not you'll succumb to impulsive 'unhealthy' food choices, according to a new study in the Journal of Consumer Research.

The study's authors analyzed the shopping behavior of 1,000 single-member households and their grocery purchases over six months, and found that there were larger proportions of food items rated as impulsive and unhealthy (think cookies, cakes and pies) in the shopping carts when shoppers pay using credit or debit versus cash.

Why? "Cash payments are psychologically more painful than card payments, and this pain of payment can curb the impulsive responses to buy unhealthy food items," explain authors Manoj Thomas (Cornell University), Kalpesh Kaushik Desai (State University of New York, Binghamton), and Satheeshkumar Seenivasan (State University of New York, Buffalo).

The authors also suggest that there may be a connection between rising obesity and changing modes of payment: More than one-third of U.S. adults are classed as obese. And nearly 40 percent of all purchases in 2006 were paid by credit and debit cards.

"The notion that mode of payment can curb impulsive purchase of unhealthy food products is substantially important," note the authors. Bottom-line: If you're watching your weight, making your payment in cash may help you make healthier decisions. Here are some more ways to shop healthy at the supermarket:

Don't shop hungry

You're more likely to make impulse purchases when you shop on an empty stomach, so eat a snack or meal before you shop.

Make a shopping list

Plan ahead what you need to buy -- and stick to the list. You'll be less likely to buy what you don't need.

Shop on the outer perimeter

That's where the fresh produce, fruits and vegetables, meat and fish usually are. To avoid temptation, steer clear of the center aisles where the unhealthy junk foods are shelved.

Look for healthier choices

Go for low-fat and low-sodium versions of food products.