Want to burn more fat while exercising? Take a look at your breakfast.

A recent study conducted in the U.K. measured the amount of fat burned when previously sedentary women exercised for 60 minutes about three hours after eating breakfast. The researchers found that the women who ate a breakfast that didn’t lead to a rapid spike in blood sugar burned more fat after breakfast, while exercising, and even after lunch.

The researchers randomly assigned sedentary, overweight women to eat one of two calorie-matched breakfasts for the study. Half ate a low-glycemic breakfast — consisting of muesli, yogurt and fruit — while others ate a high-glycemic index meal of cornflakes, white bread and jam.

The glycemic index ranks carbohydrates according to how quickly they increase blood glucose levels. High-glycemic foods are typically more refined food products, such as white bread, white rice or sugar, while low-glycemic foods, such as whole wheat bread or brown rice, have more fiber, which slows their breakdown.

The study

The experiment was set up to resemble a schedule that many working women might adopt: going to work after breakfast, then exercising over their lunch hour before eating lunch. The researchers took blood samples at various times during the day to see how much fat was burned.

As expected, blood sugar levels rose after breakfast for both groups and fat burning dropped immediately after the meal. Women eating the low-glycemic index breakfast saw less effect on blood sugar and fat burn than those eating the high-glycemic index meal.

But when the women began their exercise — walking at a moderate pace on the treadmill — those eating the low-glycemic breakfast burned dramatically more fat.

When the women in the high-glycemic group began their exercise, they had high levels of blood sugar, allowing muscles to rely primarily on those carbohydrates for energy. The women in the low-glycemic group, however, had lower levels of blood sugar, prompting their bodies to tap into fat stores for energy.

After the workout, the women in the low-glycemic group reported feeling less hungry at their post-workout lunch than those in the high-glycemic group. The study also showed the women in both groups continued to burn fat at an increased rate for the two hours after their post-workout lunch.

In a statement about the study, lead researcher Emma Stevenson, a nutrition professor at Nottingham University, said long-term studies were needed to determine whether a low-glycemic diet and regular exercise could increase fat-loss over time.

“A low (-glycemic) breakfast also had an impact on appetite, with test subjects feeling fuller for longer after they’d eaten these types of foods,” Stevenson said. “As such, individuals trying to shed fat may consider choosing low(-glycemic index) foods before they exercise.”

‘Work out longer’

While the study did not measure the difference in calories burned, Cathy Sassin, a Bend nutritionist, said relying on fat stores for energy during exercise has benefits for both the individual trying to lose weight and the endurance athlete.

“If your body is utilizing sugar as its main (energy source), you basically have a limited supply. It doesn’t give you much energy,” she said. “If you can access stored fat and use that while you’re doing your exercise, you can go farther, longer, faster.”

A high-glycemic breakfast, however, causes a rapid spike in blood sugar, prompting the body to pump out large quantities of insulin. The insulin allows muscle cells to absorb that sugar, but leaves blood sugar levels very low. Sensing the lack of sugar, the brain shifts into a defensive, energy-storing mode that reduces the amount of fat burned. It’s part of the reason why people feel so sluggish after a “sugar-high” wears off.

A better strategy is to keep a slow and steady supply of blood sugar that will tap into fat stores, instead of prompting a shift to energy-storing mode. If the body runs out of glucose in the blood stream or stored in muscles and the liver, it hits the proverbial wall, and the person lacks energy to go further.

By tapping into fat stores, however, the glucose supply is stretched further. For someone trying to lose weight that could mean the difference between lasting 20 minutes on the treadmill or an hour. For a marathoner, it could mean the difference between dropping out after mile 20 or finishing the race.

“The concept behind fitness training and getting in better shape is to be able to work out longer to have that cardiovascular adaptation,” Sassin said. “It’s way better to go for 20 minutes, 40 minutes, an hour. You can’t go that long on sugar stores alone.”

Sassin said a healthy diet that relies on low-glycemic index foods, balanced with an intake of fat and protein, can help train the body to tap into its fat stores more efficiently. (The Nottingham researchers previously conducted a similar study with active females, finding that women eating a low-glycemic breakfast burned 55 percent more fat than those eating a high-glycemic breakfast.)

But Sassin warned against popular advice to work out before breakfast to increase the amount of fat burned even more.

“You might be using a hair more fat at the moment, but it becomes counterproductive because it shifts your body into defense mode for the rest of the day,” she said. “When people do that, it forces the brain to be telling them the rest of the day they’re missing something. So they start to crave sugar, carbs, high-fat foods, just to get the calories and the sugars up.”

These two breakfasts

were matched for calories, fat, protein and carbohydrate content but had vastly different glycemic index scores. Fiber helps to slow the breakdown of foods, minimizing the speed at which carbs are converted into blood sugar.