Wednesday, July 30, 2008

Exercise and Weight Loss

A friend sent me a link to this just published article about weight loss and exercise. According to the abstract of the actual scientific paper (I couldn’t access the full text), overweight and obese women (201 subjects, aged 21–45 years with BMIs from 27–40) were assigned to 1 of 4 behavioral weight loss intervention groups. They were randomly assigned to groups based on physical activity energy expenditure (1000 vs. 2000 kcal/wk) and intensity (moderate vs. vigorous). Participants also were told to reduce food intake to 1200 to 1500 kcal/d. (Doesn’t sound to me like an easy caloric intake to achieve on any of the usually recommended weight loss diets.)

According to the abstract, weight loss did not differ among the randomized groups at 6 months (8–10% of initial body weight) or 24 months (5% of initial body weight) follow-up. (In other words, calorie restriction on whatever diet they were using was not sustainable; or starvation is not fun!) Post-hoc analysis showed that individuals sustaining a loss of 10% or more of initial body weight at 24 months reported performing more physical activity (1835 kcal/wk or 275 min/wk) compared with those sustaining a weight loss of less than 10% of initial body weight (P < .001).

The authors concluded that the addition of 275 mins/wk of physical activity, in combination with a reduction in energy intake, is important in allowing overweight women to sustain a weight loss of more than 10%. They also concluded that interventions to facilitate this level of physical activity are needed. In other words, the current recommendations for exercise to achieve and maintain weight loss are not nearly enough—at least not as currently practiced.

We couldn’t resist posting our own observations about weight loss and exercise. From the female perspective:

Back when I was a wee lass (in college), I recall weighing myself in the locker room after a tough workout for the crew team, and weighing in at 120 lbs. My usual weight was probably more like 125 lbs. (It sure would be easier to sprint up the stadium steps at 125 lbs than my current weight of 150 lbs!) I don’t know for sure whether there was any weight change as a result of crew training, but I remember eating enormous amounts during spring break when we worked out three times a day! Crew workouts can easily spend energy at the rate of 1000 kcal/hr (though few could keep up that pace for very long).

Then in my mid-twenties, I jogged a lot and ran several marathons. I didn’t train excessively—I probably logged 35+ miles per week, with occasional longer weeks when preparing for longer distance events. It helped me deal with stress and improved my confidence and self-image. Overall, jogging was a good thing for me, though I was never fast. (I almost qualified for the Boston marathon once, but got injured and had to slow down-- I could never be a world class athlete). I was thin then, but not skinny. I don’t recall even having a scale then, so I have no data on my actual weight (though I still have some clothes from that era, which currently do not fit). I remember, in particular, a friend asking if I was having any menstrual cycle disturbances from all this running, and I replied no. He felt my arm and said something like “of course not” in a derogatory voice, meaning I was way too fat for that. Did I say friend? I meant obnoxious jerk!

At any event, I was slim, healthy and strong (relatively), looked better than ever, and ate plenty of carbs (although I recall eating mayonnaise straight out of the jar once after a long run I was so starving!) I was also single, and that I think turns out to be the real kicker here. At least for me, I am less likely to eat well when eating alone. Conversely, I tend to overeat at times when eating with friends and family. I suspect this may be true for many others as well. So I can’t say for sure that my relative slenderness as a young runner and marathoner was only due to the exercise.

Twenty odd years later, running doesn’t do the same thing for me. I have a family—we cook great meals in our house, enjoy desserts, and generally relish living. Is it the inevitable middle-aged spread? Is it due to reductions in hormones related to the aging process? Not exercising enough? Is it due to excess calories in general? Or fat or carbs in particular?

I started using an ankle brace regularly on trail runs (see for example Monte Bello OSP or the Bay to Ridge Trail) about 2 years ago to prevent the perennial ankle sprains I am susceptible to (it has worked well!). I was able to train more or less consistently for longer periods of time. My weight stabilized, but I couldn’t seem to lose any weight, even with ever increasing mileage. An hour or so of jogging or uphill walking most days was typical, with occasional 3–4 hour outings, frequently covering a few thousand feet in elevation change. You’d think the weight would just fall off, but it didn’t. Any interruption in training saw the weight pile on quickly too (whether due to breaks from work demands, bad weather, holidays and holiday eating, illness or injuries). I think there is a tendency after long or hard exercising to believe that you are entitled to a few extra calories, and slightly larger serving sizes of food become habitual, and there goes any weight loss! It’s trivial to eat a few hundred calories extra, and not so trivial to burn them off. (See, for example, the chart at Also, there is evidence that exercise increases insulin secretion and growth hormone production (but that’s another post…). Certainly I have found myself famished after longer training events, and not satisfied with just one meal.

So why did it seem to work when I was in my twenties? I’m not sure—there are many factors that could have contributed. More importantly, why is it working now that I’m restricting carbs in my diet? I’m not even sure the exercise has anything to do with it. We’ve seen some of the fastest decreases in weight when exercising the least. For example, after an injury (I dislocated my elbow in early May) and cutting back on distances and speed, I spent a whole month with my weight consistently below the trend line on my weight loss graph. Afterwards, we started doing more distance again and pushing the pace, and the weight actually increased for a few weeks. (See our earlier posting about weight loss curves.)

From the male perspective:
My experience is, I think, quite typical of active males. I've always been a fast eater with a relatively high metabolism. Fast eaters tend to overeat a bit, and I tended to be 5–10 lbs above my ideal weight as I was growing up. I then went through periods when I was extremely active and periods when I was relatively sedentary. As a competitive varsity athlete in college, the weight naturally came off. It also came off when I spent summers mostly outdoors backpacking a lot and when I worked more physically active jobs organizing trail runs and doing construction work. I also ate more at those times. As I've gotten older, my weight has gradually gone up roughly along the typical pound-per-year curve. There have tended to be spikes when my level of activity decreased suddenly (when I stopped doing the physically demanding sport or job, and I had to relearn how to eat less and feel satisfied). The weight always seemed to have a local (in time) set point that was not very dependent on short-term variations in the amount I ate.

A few years ago, I switched once more from a period of high-activity work to more sedentary work, and my weight started drifting upward alarmingly even with significant efforts at reducing calorie intake. Essentially, I was starting to exhibit the typical set of symptoms known as metabolic syndrome: weight gain, increased waist circumference, moderate elevation of blood glucose levels, moderately elevated triglycerides, high blood pressure. If I paid close attention to my weight and caloric consumption, I could temporarily get some of the weight off, but it tended to come right back as soon as I stopped paying attention or went through a holiday period. Adding back in a strenuous exercise program also helped temporarily, but always tended to drive up my calorie consumption. Basically, I was confirming the usual experience that diet and exercise only sort of/maybe work to keep weight under control, and only if you pay close attention and keep paying attention. I've now lost all of the recent excess weight gain (about 25 pounds), though I'm still 30 pounds over what I weighed as a college athlete. The secret has really been exactly what my pediatrician told me 40 years ago before we all went low-fat, low-cholesterol: cut back on the simple starches and sugars! I don't make any attempt to calorie-restrict, or count calories—I'm rarely unusually hungry. I've just cut way back on the amount of sugars, potatoes, and grains I eat. End of story.

If anything, our relatively generous exercise schedule seems to lead to an ability to tolerate a few more carbs in our diet, but I’m sure if we restricted even more, we’d lose faster. Having adapted to a low carb diet, our metabolisms are acting in a carb-sparing capacity, using fat preferentially so that the glycogen is there for emergencies. It’s unclear whether we are even glycogen-depleted anymore. Certainly our blood glucose levels are stable—the body readily manufactures glucose from protein, and we eat sufficient protein so that we’ve never really been all that ketogenic (we’ve checked).

The body contains something like 400 g glycogen, stored in muscles and the liver. This glycogen binds 2.6 g water per gram of glycogen, and hence the total weighs 1440 g in total in the body—3.2 lb). Low carb critics often incorrectly state that the weight lost on a low carb diet is only “water weight,” due to the loss of glycogen stores. Those 400 g of glycogen theoretically contain about 4 kcal/g of metabolic energy (assuming efficient conversion), so that’s 1600 kcal worth of carbs that could theoretically be eaten each day if you burned all the stored glycogen for energy during exercise.

Of course, that’s not how it actually works in aerobic or mixed aerobic/anaerobic exercise. Unless you are exercising beyond your anaerobic threshold for long periods of time, or your aerobic fitness is very poor, fat is the preferred energy source for exercise. Just keeping to a pace where my heart rate is elevated (130–160 beats per minute) means I must cross the anaerobic threshold at least some of the time, but even so, there’s no way that I deplete all my body glycogen during normal exercise. One exception might have been a time about two weeks into our low carb diet when we went on a particularly difficult route, climbing and descending about 2000 ft and covering about 11 miles (in a cold rainstorm!). By the end, I was cramping and weak (might have been due to the cold too), and my blood glucose was down to 76 mg/dl. Every other time, even if exhausted to the point of feeling ill, my blood glucose was never below 90 mg/dl. I was never in any danger of completely running out of glucose for fuel (or fat!). (See this nice discussion of fat vs. glycogen burning in distance training.)

So if you are eating 2000 kcal at 60% carbs as recommended by many medical professionals, that's 1200 kcal from carbs consumed every day. But it's very unlikely that you will use up and need to replenish that much glycogen every day from normal exercise and living activities. Unfortunately, the body naturally stores at least some of the excess as fat.

So, our conclusion to date is that exercising is a great thing to do, but is not necessary or necessarily even helpful for weight loss at all (though of course it is very helpful for general health, increasing insulin sensitivity, raising HDL levels, etc.) On the other hand, if you eat large amounts of carbs, you'd better burn it off somehow or the likely destination for a lot of the excess is your fat stores!


Stephan said...

A Taubes-esque criticism of the study is that people with properly-functioning metabolisms tend to be more active, and so the causality might actually be reversed. The people whose metabolic profile changed in a more positive direction had more energy and motivation to exercise than those whose metabolisms improved less. That implies that whipping people to run on a treadmill all day might not actually be effective. I think that agrees with your experiences with diet and weight loss.

Drs. Cynthia and David said...

Thanks for your observations. That's entirely possible, and not controlled for in any of the experiments I am aware of. It's the usual problem of not understanding cause and effect, and confusing association with causality.

Tony Kenck said...

Nice blog. I just ran across it. I didn't see a button to subscribe to the posts only the comments. It's proably there and I'm missing it.

I did a post on exercise and weight loss a month or so ago. You might find it interesting.