Negative Calorie Food Myths
Negative calorie foods, said to require more energy to digest than they provide, are a popular food fad.
Today we're going to point the skeptical eye at one of the more popular food pseudosciences, the idea that some foods take more energy to eat and digest than they provide in calories, thus allowing you to eat all day long and lose weight in doing so. It's the ultimate in magically easy solutions: if you feel hungry, choose a negative calorie food. You can eat until you're stuffed and get skinnier in doing so, and you can do it all day every day. Today we're going to study the data and see if this is indeed the miracle we're all hoping it is, or if it's just another gimmicky food fad.
Just listen to a few of these amazing, miraculous sounding claims:
It sounds too good to be true: We can eat all day long, and our bodies will get less than no calories. It's the guilt-free miracle path to skinniness that we all want. And it even sounds sciencey!
Fortunately, this is a testable claim upon which we can actually shine the skeptical eye of science. We can look at the actual calorie content of the foods in question, we can actually measure how many calories your body burns in digesting them, and we can determine your body's true caloric requirements. Let's look at some of these.
The Basal Metabolic Rate (BMR) is the amount of energy your body burns in a day when at rest; in essence, your base number of calories. Do nothing, lie in bed, and this is the number of calories that you would have to eat to neither gain nor lose weight. Various calculators for this exist that give slightly varying results, but individual variances among different people exceed the differences between the several calculation methods. So, for the purposes of this discussion, we're going to use the Revised Harris-Benedict Equation, and for our official test person, we're going to take a hypothetical healthy man, 35 years old, who is 168cm tall (about 5'6") and 68kg (about 150 lbs). The formula is a little bit different for men and women to account for the normal physiological differences between the genders. In order to spare Skeptoid listeners the need to do elementary school arithmetic in their head, I'll simply state that the Revised Harris-Benedict Equation, like other similar equations, is a simple formula that multiplies your height, weight, and age each by a separate constant, and then totals them all up with another constant.
By this equation, our test man's BMR is 1,607 calories. That's if he lies in bed and has not eaten recently, thus not busy digesting a big meal. If he is sedentary, meaning he eats a normal amount, and sits at his computer desk most of the day, and walks around as much as needed to get around the house, we multiply the BMR by 1.2, giving our test man a daily budget of 1,928 calories. He's probably someone whose pounds include a lot of fat. But if he's moderately active, meaning he goes to the gym a few times a week or plays some sports, we multiple the BMR by 1.55, giving him a daily budget of 2,491 calories. If he's very active, meaning he exercises every day and definitely stays fit, we multiple his BMR by 1.725, giving him a daily budget of 2,772 calories. This is a guy who's lean and whose pounds are made up of more muscle than fat, so it requires more calories to maintain his weight. Note that between 1,607 and 2,772 is a huge range. And remember, these numbers are just our starting point; if our test man wants to track his calories, he only starts with this number, before adding the calories he consumes and deducting the calories he burns through exercise. The main takeaway here is that people are all very different — even those of about the same age, height, and weight.
The next thing to understand is what's called the thermic effect of food. This is the rise in metabolism needed to digest it. We all have a resting metabolic rate, when we're not doing anything, and that's what gives our test man the BMR of 1,607 calories. But when we exercise or our body takes on some other duty, such as digesting food, that rate rises. Thermic effects are very different depending on the food, and just as different depending on the individual person. Fat, for example, is really easy to digest, so it has a low thermic effect and usually contains a lot of calories; thus fatty foods are (in general) a poor choice for weight loss. At the other end of the scale are foods that are high in protein. Proteins have to be broken down into amino acids in order to be digested, and doing so takes energy. Similarly, foods containing complex carbohydrates and fiber require extra work for your body to reduce them into the needed building blocks. The body has to call upon additional biochemical resources to digest such foods, so high-protein foods and high-fiber foods often have a high thermic effect.
The availability of the energy required to perform these processing tasks varies a lot by individual. Two of the strongest factors are probably obesity and insulin resistance, and I say probably because most of this is theoretical. It's backed up by a fair amount of observation, but very little empirical data exists. Few or no thorough studies have been done testing the idea of negative calorie foods. But some studies have shown that the thermic effect may be lower in obese people who eat the same meal as a thin person, meaning their bodies don't work as hard to digest it; but this is not a proven link.
Insulin resistance is probably the strongest factor. Also known as metabolic syndrome, this is when your body's insulin becomes less effective at reducing blood sugar. It's almost always associated with being overweight, and it brings an increased risk of diabetes and heart disease. Together with aging, levels of physical activity, and other factors, obesity and insulin resistance levels will result in different levels of thermic effect when digesting the exact same meal, by two test men whose metrics are otherwise identical.
The bottom line is that it's wrong to say any food is a negative calorie food, because that depends on so many factors.
However, the possibility that some foods, under ideal circumstances, can be negative calorie for some people is not discounted. The fact is that a negative calorie food does fall within the range of possibility, in the leanest people, with the lowest insulin resistance, and with the exact right food. Celery is often described as the poster boy of negative calorie foods, and it's true that this is probably the best candidate. Celery is almost all water, so it has very few calories to begin with; and what it does have is mainly fiber. The unraveling and digesting of fiber does have a high thermic effect. For a few very lean, very healthy people, eating celery may actually reduce their daily calorie count. But if and when it does, we're probably talking only a single digit number of calories. Other foods often cited as negative calorie, such as some citrus fruits, are much less likely than celery to ever actually be negative calorie in the real world.
Our bodies use 5 to 10 percent of our energy digesting food. This relatively large 5% range is due to all the same variables that we've already discussed. So our test man may use as few as 96 or as many as 277 calories digesting his normal daily diet amount of food. The range seems big, but even the high end is not very much. Digestion just doesn't take that much energy.
Why so little? Because kilocalories of energy go a really long way. Our test man has to run pretty hard for 15 minutes to burn 100 calories, and that's a lot of huffing and puffing. Allowing celery to digest is a lot less strenuous. As a weight loss strategy, going after negative calorie foods gives a very low return on investment; and the most frustrating part is that they work best for the leanest, healthiest people: those who need them the least. For people of normal or above-normal weight, no food is realistically negative calorie; and celery by itself is an extremely poor, nutrient-starved diet.
Another problem with foods with high thermic effect is that most of them are not very satiating. You can eat celery for an hour and still feel hungry. To trigger the biochemical reactions that tell your brain that it's happy and well fed, you have to eat some of those foods that are disappointingly high in calories and low in thermic effect. This is one reason why cravings are usually for those yummy foods high in fat.
There is no harm at all in seeking a negative calorie food, so long as your expectations are correct that it's rarely actually going to produce the desired effect, and so long as you're also eating a normal balanced diet. As a snack they're great, but unlikely to satiate. The best diet advice of all remains unshaken: Eat a healthy, balanced diet; don't overeat; and get plenty of exercise. There's not a fad diet in the world that can compete with that.
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