A member of my health club was talking about marathon runners and how skinny they all are. I wondered if he’d ever gone down to watch the Chicago Marathon in person. I doubt it, because there you see all kinds across a broad spectrum of weight. His misguided assumption, though, is a common one.

That if you’re exercising significantly, you’re certain to achieve and maintain a lean and trim body. Let me use an example of a client from work to illustrate.

A client came to me two years ago to discuss her goal to lose weight. After going through her exercise and nutrition history it was very clear that she was dropping the ball where most people do: a poor diet.

Her body composition hovered near 35 percent (healthy for her would be 18-23 percent), in spite of six days a week of exercise, which included 60-90 minutes of cardio and a spare and sporadic strength routine. Nutritionally, however, she skipped breakfast, ate lunch at her workplace cafeteria and dined out or took out dinner every night, typically eating large portions of entrees such as ribs. At 5-foot-3-inches and 185 pounds, we calculated her caloric needs to be about 1,800-1,900/day, while still achieving a two-lbs/week weight loss (this is a healthy rate of loss). In reality she was taking in over 3,000 calories, most of them nutrient poor and dangerously high in saturated fat and sodium.

When I asked her to recall when her weight was last stable and lower, she said it was two years and 25 pounds ago. Her habits at that time included less exercise but a better diet as she ate a small breakfast, brought her lunch to work and cooked dinner for herself each night. She was textbook weight gain, yet she was befuddled. Why had she gained 25 pounds in two years? It’s the diet!

Many seeking weight loss believe that a steady exercise regime will make up for all the ills of a poor diet. Exercise does give some flexibility when it comes to discretionary calories, but not nearly as much as you’d think. A nutrient poor/calorie rich diet is very difficult to outrun.

Have you seen a fat marathoner? They exist in droves. On the other hand, I challenge you to find anyone on a reduced calorie (about 1,200-1,600 calories) who’s not losing weight. Aside from the small number of medically challenged among us, you won’t find one. Before you tell me your metabolism is slow, let me tell you that most adults fall into a range of resting metabolism that’s very normal. Mine, for instance, is only 1,400 calories. As active as I am, you’d think I could eat anything in any quantity, which is completely untrue.

To further explain, a marathoner averaging 25-35 miles/week is burning approximately an extra 2,500-3,500 calories/week through that training.

Subtract the calories they’re getting from Gatorade/Gu and the like, and you’ve got about 2,000-3,000 extra calories per week. Divide that into seven days and you’ve got about 300-400 extra calories/day. That’s a large bagel and a little schmear. Guess what many marathoners think? Outta my way! I’m training for a marathon and I gotta EAT! Go to the message board of any long distance event during the 2-3 week taper before the event, and that’s all anyone’s talking about.

Six months ago my client decided to tackle her nutrition in a structured manner after a year and a half of trying to cutback on her own. She’s been on the Seattle Sutton’s 1,200 calorie/day plan and has lost about 25 pounds. Her exercise routine remains the same.

Fran Scott is an exercise physiologist. She is giving a free weight loss lecture at the Oak Park Public Library on Tuesday, June 7 at 7 p.m.. Please RSVP at franscottfitness@hotmail.com.

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