Burn, Baby, Burn!

“What exercise intensity is best for fat burning? I’ve heard that more fat is used when I’m going easier—so is low intensity exercise best for weight loss?”

This is one exercise myth I’d like to put to rest permanently. Like most myths, it’s a mixture of partial truths that just don’t add up. Before I delve into it, here’s the bottom line: You’ll burn more fat by exercising intensely than by exercising at less intensity. Here’s why.

Fat Burned While Exercising
Exercise is primarily fueled by carbohydrate and fat and the percentages used differ with exercise intensity. The lower the intensity, say if you are just walking, the greater your body relies on calories from fat. (Actual percentages vary with fitness level—if two people run at the same speed, the fitter person uses a higher percent energy as fat.)

But percentages aren’t the whole story. But to understand the facts, we’ll need to use some numbers. For those math-phobes among you, we’ll illustrate by examining fuel use during exercise of ‘Leisurely Larry’ and ‘Frantic Fred.’

Larry, walking at a moderate pace, uses mainly fat to fuel his activity. In contrast, while out on a brisk run, Fred uses mainly carbohydrate. This might lead you to think that following in Larry’s footsteps would be better for burning fat. NOT TRUE! Let’s look at the actual amounts of fat and carbohydrate used rather than the percentages.

• Leisurely Larry: In 30 minutes of walking, Larry, who weighs 150 pounds, covers 1.5 miles and uses about 130 calories. Fat might provide 75% of the energy for his walk, or almost 100 calories. Because each gram of fat provides about 9 calories, Larry burned about 11 grams of fat (100/9 = 11).
  
  
• Frantic Fred: In 30 minutes of running at 7 minutes per mile, Fred, who also weighs 150 pounds, uses about 450 calories. In his case, 75% of calories might be provided by carbohydrate, and only 25% from fat. But that smaller percentage translates to 113 calories, or between 12 and 13 grams of fat (113/9 = 12.5).
  
  

1. Larry and Fred need 2,200 calories per day, without adding exercise, to maintain their weight of 150 pounds.
2. Their usual diets provide 2,200 calories. On days they don’t exercise, they burn all of these calories to sustain life—to keep their hearts beating, brains ticking, livers and kidneys functioning, as well as to go to work, buy groceries, and so on. Over time, their weight is stable, since they eat the same number of calories they expend.
3. This isn’t an assumption, it’s a ‘fact’: The body doesn’t store much carbohydrate, but, as many of us know, stores fat with ease. Normally, any carbohydrate eaten is used during the day because not much can be stored. So when calories eaten exceed calories used—someone eats 2,400 calories and uses 2,200—all the carbohydrate and any excess protein are used for fuel. The ‘leftover’ calories are stored as energy reserves (a euphemism for body fat). Conversely, when more calories are used than are eaten, the deficit is taken from body fat.

• Larry used 130 calories while walking, so he’s in a deficit situation—he ate 2,200 calories and expended 2,330. The deficit of 130 calories comes from fat. He used 100 calories of fat during his walk, and he’d use another 30 (or about 3 grams fat) from stored body fat during the rest of the day. His total daily fat loss is 12 grams (the 9 he used in exercise, plus the additional 3 from fat stores).
• In contrast, Fred used 450 calories while running, for a daily energy expenditure of 2,650 calories, and ate only 2,200. In addition to the 113 fat calories he used while running, the remaining 337 calories his body needs throughout the day would come from body fat. That’s about 38 grams of fat (337/9), so his daily total fat loss is 50 grams (38 + 12 used during exercise).