There was a time when if you wanted custom nutrition or exercise advice, your only choice was to see a dietitian or personal trainer.

Now, thanks to a growing demand for quick, convenient personalized health and fitness information, all you’ve got to do is pull out your cellphone or personal digital assistant (PDA). Within seconds you can access everything from a complete abdominal workout guide to the number of grams of fat in a McDonald’s bacon, egg and cheese biscuit.

The idea is that if we turn to these devices, which we always tend to have on hand, for healthy behavior tips, we might have a better shot at adopting them.

A study published in the American Journal of Preventive Medicine shows that programmed PDAs encouraged middle-aged and older Americans, a group that’s known to be sedentary, to get moving. Researchers found that participants who were assigned to use the PDAs were more than twice as active as those in the control group, exercising five hours week versus two.

Study co-author and professor of medicine and health research and policy at the Stanford Prevention Research Center, says another study that will be published this summer in the same journal has shown these devices can increase people’s vegetable and fiber intake by reminding them of its importance.

A major way these services appear to help users is by eliminating their excuses for falling off the health wagon. The Sensei for Weight Loss program, which delivers customized meal recommendations, weekly shopping lists, fitness tips and motivational messages to a mobile phone or personal Web page, aims to get involved in as many of your daily decisions as possible.

Have a tendency to snack at night? For a weekly fee, Sensei will send you a variety of messages reminding you to brush your teeth after dinner to quell your sweet tooth. Get invited to a hastily arranged business lunch at a steakhouse? Alert the program and you’ll get advice on healthy choices the restaurant offers.

If you’re on the road frequently and can’t stand hotel gyms, you can pay a small fee to have PumpOne Mobile send your cellphone traveling workouts that use hotel furniture, water bottles and luggage as props. IPhone or iPod touch users also can access the just launched Pump10, which offers weekly, 10-minute video workouts, fitness tips and personal training advice for free.

And if your downfall is eating out, try texting DIET1 for free instant nutritional information. Part of Diet com’s Nutrition on the Go program, the service can tell you calorie, fat, carbohydrate and protein counts for items on more than 1,700 restaurant menus nationwide.

“It’s not that easy to eat healthy,” says Carol Davies, a partner at Stamford, Conn.-based consultant group Fletcher Knight. “Anything that can help consumers do that on the spot is very on target.”

You can’t discount the fun factor either.

For instance, for a subscription fee Weight Watchers Online for Men shares customized advice, including which beers are the most diet-friendly and whether you’re better off snacking on wings or nachos at the bar. It also lets users constantly manage their food intake and monitor their weight loss progress with charts and statistics sheets, not unlike tracking the stats of an athlete for fantasy baseball, says Jason Carpenter, site editor.

While you can’t discount the benefits of face time with a dietitian or a personal trainer, who can give you advice with a human touch, experts say these services are a good option for those who are time-crunched and have enough discipline to consistently log in to them.

“These things aren’t going to give you willpower or determination,” Davies says. “They’re tools to help you make good decisions.”