Stepping on the Wii Balance Board for the first time, you may be shocked by being told that you are, in fact, overweight. We examined the potential ramifications of Wii Fit's approach to weight management and assessment in our previous article, Wii Fit: Bruised Boards and Balanced Egos. On the eve of the U.S. launch of Wii Fit, we decided to get an expert opinion on BMI measurement and the viability of Wii Fit as a platform for promoting exercise in our overweight nation.

To get an informed take on Wii Fit and the merits of Body Mass Index measurement (BMI), we spoke to Dr. Judith S. Stern, a distinguished professor of Nutrition and Internal Medicine at the University of California, Davis. An expert on diet and nutrition, Stern has published extensively on nutrition, obesity, and the effect of exercise on appetite and metabolism. She is co-founder of The American Obesity Association, a lay advocacy organization dedicated to advancing understanding of the condition of obesity.

Dr. Judith S. Stern
GameSpy: Nintendo's Wii Fit is launching tomorrow in the United States. The idea is that with Wii Fit, people who would normally play games on their couch will get up, get active, and use the Balance Board to improve their personal fitness. What do you think of the premise behind Wii Fit, where using the game will help reduce your BMI?
Dr. Judith Stern: My understanding is that Nintendo is saying that if you track BMI, and by using this program, that you'll improve your BMI, or lower your BMI. I say baloney to that. In fact, I'm sure you'll see very little change in BMI. It's not just BMI you want to look at, but how fat you are.

When you are physically fit, you tend to replace fat with muscle, and your BMI probably won't change. It will change when you do extreme things, like if you exercise for two or three hours a day, I'm pretty sure your BMI will change. If you've just lost weight, you can maintain your BMI more readily with exercise, but I don't see anything that shows me that BMI will change, and I really think it's false and misleading.
Are you overweight? Or just athletic? How can Wii Fit know?

GameSpy: Is this a problem due to them using the Body Mass Index as a measurement and assessment tool? What exactly does it measure, and is it an appropriate way for doctors and health professionals to make a physical assessment?
Dr. Judith Stern: BMI measures indirectly. You're looking at your weight relative to your height. And so a normal BMI goes up to 24.9. If you're overweight, by this definition, it goes 25-29.9. Obese is 30 and above, very obese is 40 and above. For example, if you're 5'5", and have a BMI of 30, you weigh about 180 pounds. So clearly 180 pounds for someone who's 5'5" is obese, and probably body fat is increased. If you are very muscular and very active, you can have a high BMI but not be unfit or too fat. So when you look at BMI in a range of 25 and 29.9, you're not sure what that means.

In fact, there are different BMI criteria for people of different ethnicities. For example, for people who are Asian-American, Type 2 diabetes begins to increase with BMIs of about 23. If you're African-American, probably an overweight BMI shouldn't start until a BMI of 28, because you are more muscular if you're African-American, and also your body density is higher.
GameSpy: So is it a serious misstep that Wii Fit doesn't take your ethnicity into account?
Dr. Judith Stern: Well, it's different, not a misstep. What is a misstep here is using BMI as an index of fitness. There's no basis for that. And it promises you something that it can't deliver.
GameSpy: If BMI isn't a valid index of fitness, what is?
Dr. Judith Stern: How long it would take you to be exhausted, time to exhaustion, say if you're riding a bike or running...
GameSpy: Your heart rate?
Dr. Judith Stern: Right. You'll hit a maximum heart rate and then you won't be able to run anymore. That does aerobic capacity. If you want to look at strength training, clearly it's the amount of weight that you can lift. Those are simple ways of doing it, and physicians can evaluate people that way. Also, people in the physical fitness area can also evaluate people that way.

But you ask me, "Can physicians use this?" What I think is helpful for physicians to use is to look at change in BMI. Let's say you're twenty, and your BMI is about 23. So now you've just become forty, and your BMI is about 33. That isn't all muscle. It's excess fat, and that's not good for your health. So if you measure BMI change, and a physician does it, that can help. And you look at other things. Is your blood pressure high, is your glucose high, and what's happening specifically. Weight loss can improve these conditions.