Wednesday, March 28, 2007

The Science of Weight Loss

At the most basic level, losing weight simply involves consuming fewer calories than are burned by your body. This is called a calorie deficit. When people diet to achieve this calorie deficit, however, they often become so hungry that the diet is unsustainable and inevitably fails, causing any weight lost to be regained. Fortunately, utilizing some of the science of digestion and nutrition, we can begin to understand how to maintain a calorie-deficient diet while still being satiated (feeling full), and thus maximizing the chances of sustained weight loss.

The number of calories you burn in a day of doing absolutely nothing is called your basal metabolic rate (wikipedia) and can be estimated based on your age, weight, height and gender (here). If you then add in all the calories you burn walking, talking, brushing your teeth, chewing and everything else, you’ll get your calories expended. Here is an online calorie calculator for common activities. Subtract from that the number of calories you intake to get your calorie deficit. For every 3500 Calories (kcal) you are deficient, you’ll lose 1 lbs of fat, which means that if you have a daily calorie deficit of 500 Calories, you’ll lose one pound of fat per week. Interestingly, the act of keeping track of the calories you burn can actually help you be successful at maintaining a reduced calorie all on its own. Here is some information about what types of habits successful dieters have.

Studies on the effect of exercise on weight loss are, surprisingly, somewhat ambiguous. Two major factors are suspected which combine to limit the effectiveness of exercise on weight loss in certain situations. First, people tend to eat back a average of 30% of the calories they burn exercising, so if you aren’t keeping track of the calories you intake, you can unwittingly counteract the benefits of exercise by eating more. Second, some have suggested that people who are forced to exercise compensate by decrease their activity level while not exercising, thus reducing the effect of the exercise on the total calories burned. (I couldn’t find all the articles I read for this, but here’s a good review article on the topic)

So how does one maintain a calorie deficit without feeling hungry? One way is to eat foods that have a large physical volume relative to the number of calories or energy density. This concept is called Volumetrics (book), developed by Dr. Barbara J. Rolls at Penn Sate. The idea can be crudely summarized as: Eat half a burger and a huge salad rather than a small salad and two burgers. You’ll feel just as full, you’ll be content because you got you eat the burger you were craving and you’ll have eaten far few calories than you would have otherwise. Eating smaller portions is also a central tenant of Volumetrics. A good starting point is to learn the energy densities of the different types of food (sugar, carbohydrates, protein, fat) Wikipedia has a list.

Another way to achieve satiety on a reduced calorie diet is to eat a higher than normal proportion of protein. Protein has been shown to cause a greater level of satiety (fullness) than other forms of food, so you can feel more satiated on fewer calories. Also, it takes more energy for the body to digest protein than other forms of food. This is called the Thermic Effect (wikipedia) and some studies have suggested that it can be as high at 25% for protein. In other words, when you eat 100 calories of protein you will burn up to 25 of those calories just to digest the food, leaving only 75 to go toward your total caloric intake. High protein diets can cause some health problems associated with the metabolism of the abnormally large about of protein but is generally considered safe for otherwise healthy people, if done in moderation. This is a good review article on high protein diets, including safety issues. Unfortunately, only those of you with access to university journal subscriptions will be able to access the full article.

So, what does this all mean? In short, to diet effectively, start by keeping track of your caloric intake and expenditure with a goal between 500 and 1000 Calorie deficit per day. Try to eat eat foods that have low energy density (Volumetically) and replace some of your carbs/fat with some additional protein.

Obviously, this is easier said than done :-)

10 comments:

dave said...

If I had written this post, it would have been titled, "Stop stuffing your face, fatty". I suppose it is slightly more complicated than that.

Donnie said...

Did those studies on exercise correlate their results with the level of exercise? I would imagine that mild exercise would allow you to nudge up your metabolism and decrease your stress levels (reducing some of the motivation to eat), without making you famished at the same time.

But I'm a programmer, not a doctor.

Alan R. said...

That's a good question. The short answer is no, the studies I've seen are not nearly that indricate. A general problem in health/medince related studeis is that they are often limited to very low sample sizes, like 20-30 participants over short time periods, so it's very hard to get meaningful results. One often has to resort to looking at results of many small studies and trying to draw conclusions based observable trends, even on the most basic questions.

dave said...

I'd also point out that even if you ate back 100% of the calories you burned during exercise, you'd be better off. I don't have the link handy, but if you exercise then more of your weight loss is fat. Otherwise, you lose muscle, which lowers your basal matabolism, which means that when you quit the diet you go to a higher equilibrium weight - this is how people end up yo-yo dieting.

Alan Rosenwinkel said...

According to recent research, the reason people eat back only 30% of the calories they burn while exercising is that exercising causes a rise in hormones associated with satiety. The article is titled "'Exercise after eating' diet tip", but I don't see how the research supports that. I would think eating soon after exercising, while your feeling more full, would help you eat less and thus promote weight loss.

Alan Rosenwinkel said...

here is a nice review article in Time magazine on the biochemical basis of weight-loss (and weight-gain).

Alan Rosenwinkel said...

The benefits of interval training are discussed in this NYT article. Apparently you can get pretty dramatic benefits from even just one day a week of interval training -- sprint ... walk ... sprint ... walk.

Alan Rosenwinkel said...

Eating a Low Glycemic-Index diet is also good for health and weight-loss, apparently because it helps maintain a consistent bloodsugar level over time.

jaideep said...

i don't know if they have such a thing, but it would be great if the nutrotional info that they have on a food product is linked to its barcode. that way you could "scan" everything you eat with some sort of small portable device (or a cell phone). this would give you a sense of what you eat and in what proportions. i've always been curious about that, but i'm way too lazy to keep track of that kind of stuff.

Alan Rosenwinkel said...

That would be awesome, but until then I use a really cool nutritional scale. You enter an ID number corresponding to the food you're weighing and it tells you the nutritional information -- fat, protein, calories, carbs, fiber. It does take effort, but it's really helpful for those of us who are watching our weight.

 
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