BODY WATCH : Thinking Small : Tired of diet failures and exercise burnout? Researchers say it’s the little things that count when it comes to weight control.

Mitchell Walker doesn’t especially love oatmeal, but he eats it every day--instead of a high-fat cereal--because he’s trying to reduce fat in his diet wherever he can.

During coffee breaks from his sedentary job as a feature film animator, he burns off a few more calories. “I walk up and down a hilly street near work just about every day for 20 minutes,” says Walker, 42, who lives in West Los Angeles.

“I also drink lots of water, about 1 1/2 liters a day,” he says. It gives him a feeling of fullness and helps in his weight-control efforts.

Walker has discovered what researchers wish the rest of us would start believing: Making small changes in diet and physical activity can make a difference.

Trouble is, many of us think big--way too big--when it comes to losing weight, working out and eating better.

“Americans dichotomize,” says Alicia Moag-Stahlberg, a Chicago dietitian and spokeswoman for the American Dietetic Assn. “It’s all or none.”

Once in major overhaul mode, we plot a dramatic self-improvement campaign with an impossible deadline. Why not lose 10 pounds, start exercising and slash fat intake--all by month’s end?

The problem with such ambition, of course, is the poop-out factor.

Now, the growing number of experts who suggest less is more say that one small change leads to others. This slow, steady approach is considered a much more predictable route to long-term success in maintaining a healthy diet and a healthy weight, they say.

Several recent studies back up this approach. Among them:

* Walking at a moderately fast pace for 30 minutes, three times a week can lower mildly elevated blood pressures, says Dr. Jeffrey L. Tanji, a University of California, Davis researcher who presented his findings recently at the Canadian Academy of Sport Medicine meeting.

* Losing as little as three pounds can start to reduce blood cholesterol, according to a review of studies published in the American Journal of Clinical Nutrition.

* Engaging in just one session of high-intensity physical activity a week helps control weight, according to a study published last year in the International Journal of Obesity by University of Minnesota researchers Simone French, Robert Jeffery and others. Their two-year follow-up of more than 3,500 subjects found that the men who exercised once a week weighed an average of four pounds less than the sedentary subjects.

* Fitting in short exercise sessions--even three 10-minute sessions a day--can yield training effects similar to one 30-minute session, according to a study by Dr. Robert F. DeBusk of Stanford University and published in the American Journal of Cardiology.

Once convinced that small changes do make a difference, how do you decide on the first step?

When it comes to diet, evaluate an entire week’s food intake to uncover problem spots, suggests Moag-Stahlberg. For one woman, it was her nighttime snack: “She had popcorn with oil and butter five times a week,” Moag-Stahlberg groans. But she didn’t insist that the woman give it up. Rather, she suggested either having it twice a week or switching to low-fat microwave popcorn.

One overweight teen-ager who drank three sugary sodas a day reduced calories by 450 just switching to diet sodas. Then, she was ready for other small changes that didn’t leave her feeling deprived. She ate a pork chop but trimmed the fat, substituted fruit-juice bars for ice cream, and ate fig bars rather than butter cookies.

“Most people eat about 15 or 20 different foods a day,” Moag-Stahlberg says. “Change four to seven of those (to more nutritious choices) and you are going to have a huge impact on calories and fat.”

Barbara Schiffman did exactly that. Instead of reaching for her usual doughnut or croissant at 4 p.m., the 44-year-old Burbank script consultant and screenwriter now has a baked sweet potato. Then, she gave up putting milk in her coffee and switched from cappuccino to espresso.

Rich St. Clair, 51, a Los Angeles attorney, has discovered the fat-saving strategy of having breakfast for lunch. “I often have nonfat cereal and nonfat milk and fruit instead of a sandwich,” he says.

Allow six months of phasing in small diet changes to slash fat intake to the recommended 30% or less of calories, advises G. Ken Goodrick, an assistant professor of medicine at Baylor College of Medicine who co-wrote “Living Without Dieting” (Warner Books, 1994). Those who find low-fat and nonfat dairy products and the “nutrient-modified” foods palatable can move more quickly.

Even the conservative guideline to keep weight loss at one or two pounds a week is overkill, Goodrick says. “Give yourself (a goal of) 12 ounces a week,” he says. “And don’t weigh yourself more than once a month.”

The briefest exercise session, whether it’s the only activity or added to a more intense workout routine, can also help.

“Take whatever opportunity you have during the day to exercise,” says DeBusk, a cardiologist and professor of medicine at Stanford University School of Medicine. Even if the only chance for exercise is to climb stairs or walk from the parking lot to work, “it will still have effects on weight maintenance,” DeBusk says.

In his view, no exercise session is too short. “I would like to do a study, comparing seven five-minute bouts of exercise with one 35-minute bout,” he says. “My suspicion is, there would be quite a comparable effect.”

St. Clair buys into that theory. Besides his five-day-a-week routine of lifting weights and running on a treadmill, he takes advantage of a few minutes stuck in traffic to do his “gut flexes” to strengthen abdominals.

“If I can’t do an hour exercise class or walk, I go for a power walk around the mall,” Schiffman says. “I make it into an exercise situation for the short term.”

“You might not think walking once or twice a week will make a difference (in weight control),” adds French of the University of Minnesota. But maintain that activity for two years, and she predicts weight maintenance will be less of a struggle, based on her research.

“We feel bouts of exercise can be thought of as medication,” says Tanji of UC Davis, who recommends exercise for patients with mildly elevated blood pressures. “If you have high blood pressure, the amount of exercise needed to improve blood pressure is not the same quantity as needed to improve physical fitness. It’s less.” In his eight-week study of 10 men, blood pressures dropped 10%-20% when they walked for half an hour three days a week at a three-mile-per-hour pace.

When clients tell Moag-Stahlberg that a short bout of exercise or a small diet change is a waste of time, she tells them: “Log it.” After a week or so, she says, they’ll see how one small change leads to others and--faster than they had believed--visible differences.