Certain carbs boost fat burning

Former couch potatoes take note.  You don’t have to forsake all of your beloved breakfast carbohydrates. Switching from those that burn up quickly to others that break down slowly could do more than offer satiety. They could actually increase the rate at which your body stokes its furnace with body fat. Or so conclude researchers from the University of Nottingham, England, in a new report.

There is a catch. The women who took part in the new study obtained their benefit when they coupled the slowly digesting carbs with the E-word (exercise).

Earlier research has shown that people tend to burn the most body fat by exercising on an empty stomach (like before breakfast). The real take-home message of the new study is that by tailoring meals carefully, people can boost the fat-burning impact of any exercise that they engage in several hours after eating.

Emma Stevenson and her colleagues studied eight healthy — and non-overweight — but sedentary women. Each arrived for a pair of test sessions several days apart which started with a prepared breakfast, then a rest period of three hours, followed by a 60 minute walk on a treadmill. The pace of each woman’s walk was set to make her really work, but not to the point of exhaustion. Then each woman was offered lunch and the rate at which she burned energy was measured for another two hours.

When the day’s breakfast was muesli, skimmed milk, an apple, apple juice, canned peaches and yogurt, each woman burned more fat during and after the exercise session than when she had instead dined on cornflakes, skimmed milk, white bread and jam, margarine and a carbonated sugary drink. This difference emerged even though each breakfast had contained the same number of calories and same proportion of those calories from carbs, fat and protein.

The sole difference was the type of carbs eaten for breakfast — ones that broke down relatively quickly or ones that didn’t.

The findings weren’t a total surprise, Stevenson and her colleagues note in the May Journal of Nutrition. Several earlier studies showed much the same trend. But they included trained endurance athletes, who tend to process foods differently than the rest of us. By contrast, women in the new trial did not regularly engage in “structured exercise.”  That would be things like playing tennis, running and swimming — as opposed to chasing after a toddler, walking the dog, changing a tire on the car, carrying eight bags of groceries into the house or hauling three loads of laundry down to the basement washer and then upstairs to be ironed and put away.

The good news for would-be dieters: Women burned fat at roughly two to three times the rate in the first hours after exercise if they had eaten a breakfast deriving 40 percent of its calories from slowly digesting carbs as opposed to quickly burning ones. Researchers tend to refer to the slow-burning carbs as having a low-glycemic index. In layman’s language, these are the carbs that don’t tend to elevate blood sugar very much after a meal. So they should be the dietary staples of people with diabetes.

What’s a low- versus high-glycemic index food? G.I. values turn out not to be nearly as intuitive as we’d like. Jennie Brand-Miller of the University of Sydney, Australia, has measured values for a range of foods. Those for regular ice cream and chocolate milk tend to fall under 50, she’s found, while the G.I. for baked potatoes can range from 75 to 100, depending on the spud’s variety. Even sweet fruits pose a conundrum. Cherries tend to weigh in with a G.I. of about 22 where apples and pears can be 38.

I did a big feature story on G.I. values for foods and how diets that lower them might pay dividends for people at risk of heart disease. That was several years back. But there’s no shortage of websites, books and other references to help people sort out which foods qualify as low G.I. fare.

The new study reflects the type of research I expect to encounter each year at the Experimental Biology meeting (which I leave for, in New Orleans, tomorrow morning). I’m expecting I’ll run into more of such wiser-eating news at this conference, which typically draws some 12,000-plus attendees.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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