Dr. Roberts on Exercise and Weight Loss

I don’t generally comment on epidemiological studies — the results are so influenced by what participants choose to do and how willing and able they are to accurately report, that they often come up with false conclusions. However, a study published this week on exercise in identical twins received such crazy media interpretations that I thought it would be useful to review it again and introduce some reality.

First, the study

For many years, researchers in Finland have tracked a cohort of twins to understand genetic and non-genetic influences on health. The new report was on identical twin pairs who had similar levels of exercise early in life but for at least the past 3 years had exercised at very different levels. So they had identical genes, but different amounts of time in the gym, which is a very nice scientific comparison for teasing out the effects of exercise independent of our genetic makeup.

The sedentary twins were found to have higher body weight, higher body fat and increased risk of diabetes compared to the more active twins. This is what newspapers like the New York Times emphasized, and clearly it was easy to interpret the research as yet another strong confirmation that being active is essential for staying slim.

Now, what the study actually showed

First, out of a database of 202 identical twins, there were just 10 twin pairs who had very different activity levels. Can there be any clearer demonstration that genetics plays an important role in whether our idea of a good time is to go for a run or see a good movie?

How much exercise exactly?

The study used an activity scoring system called METs (for Metabolic Equivalent for Task), which is a way to combine the different activities people do into a single number. Sedentary twins had a MET level of 1.2 on average, which basically meant they were sitting around all day, whereas the active twins had a MET level of 5.0, which is astonishingly high and means that they were doing something like 90 minutes of brisk walking daily, or 45 minutes of jogging.

So 45-90 minutes of moderate to vigorous activity. Every. Day. 365 days per year. In return for this truly enormous effort, the active twins weighed just 5.5 pounds less — they had 7 pounds less fat and 1.5 pounds more muscle — hardly a confirmation that working out more is the solution to weight problems. So although the difference between the twins was statistically significant, in biological terms it was depressingly small for the amount of work it required. This interpretation was conveniently ignored in the media hype over the benefits of exercise, but in fact is totally consistent with numerous previous studies showing that huge amounts of exercise are needed for very tiny amounts of weight loss.

It is also useful to know that there are individual differences in responsiveness to exercise. A nice study done in the UK a few years ago showed that while average weight loss was low in participants in an exercise program for weight loss, some people did lose more weight than average, while at the other end of the spectrum some people actually gained weight. Unfortunately, there is no test we can take that will tell us whether exercise helps or harms our personal weight loss journey, which means that being super-aware of how exercise affects us personally is important.

Real exercise benefits

Exercise has also been proven to be very useful for helping prevent weight regain. Partly this is a question of calories — if you work out regularly you can eat more — and partly because the metabolic effects of exercise help regulate blood glucose, which means you can control your hunger better. This is why iDiet continues to recommend increasing exercise after you have lost some weight.

Looking beyond the headlines

Please don’t get me wrong here, I totally believe in the importance of regular exercise. Although my brain tells me I’m a couch potato, I try to do something vigorous 3 times a week. I feel more energetic when I’m on track, and also enjoy knowing that I’m keeping chronic diseases at bay. Understanding that exercise isn’t a magic bullet for everything means that we can embrace exercise for the benefits it does give, and not get frustrated about it failing to help us in ways that have been misrepresented.