It turns out that even a slight energy imbalance will, over time, have consequences. Eating only 50 calories a day more than you burn will over time translate into about one pound a year, or 30 pounds over three decades.
- Is it fair to blame low fat-dietary advice (such as that contained in the USDA food pyramid) for the obesity crisis?
No. While the percentage of fat in the American diet dropped from 40 percent in 1990 to around 34 percent today, the absolute amount of fat actually increased. The true explanation for the obesity epidemic is much simpler: Americans are eating more total calories. In the 1990s, the bulk of those additional calories came from carbs -- mostly refined starches and sugary drinks.
- How do diets work?
All diets work by restricting calories. Since simply telling people to eat smaller portions doesn't sell books, most commercial diet plans are built around a nutritional trick that makes it easier to restrict dietary intake. Food is made up of four basic components (so-called macronutrients): water, fats, protein and carbohydrates. Therefore, it follows that telling people to avoid fats or cut out carbs will automatically eliminate a lot of food choices. Low-carb diets, for example, are effective, because they remove an enormous number of potential foods such as bread, pasta, rice, potatoes, etc., from a person's diet.
- Which diets work best?
While all diet programs claim success, it turns out to be rather difficult to compare them scientifically. In order to control for all the differences between people that might affect the outcome -- from genes to exercise -- scientists must randomly assign subjects to competing diet programs. The different diet cohorts must then be followed for a reasonably long period, say a year, during which time they are checked closely for compliance.
Two studies published in the May 22, 2003 issue of the New England Journal of Medicine attempted such a scientific comparison of low-fat diets and low-carb diets. The studies, which were plagued by high drop out rates -- around 40 percent -- both reported a small advantage at six months for low carb diets. By 12 months this advantage had disappeared. [See A Low-Carbohydrate as Compared With a Low-Fat Diet in Severe Obesity by F.F. Samaha and others, and A Randomized Trial of a Low-Carbohydrate Diet for Obesity by G.D. Foster and others for more details.]
- How are low-fat and low-carb diets supposed to work?
The body runs on two primary fuels: fats and carbohydrates. Fats are stored for later use and carbs are turned into glucose where they can be burned immediately.
The theory behind low-fat diets is simple. Ounce for ounce, fats have twice as many calories as carbs and so (according to low-fat diet gurus) eating more carbs -- including pasta, bread, potatoes -- will "fill you up" with fewer calories. This makes perfect sense: On the face of it, fat should make you fat.
The theory behind low-carb diets is more complicated. While carbs are lower in calories, they are not as filling as fats. Moreover, say low carb diet gurus, carbs -- especially highly processed starches like white bread, white rice and pasta -- are rapidly converted to blood sugar, or glucose. This glucose triggers a surge in insulin -- a hormone that moves glucose into muscle cells -- that causes blood glucose levels to come crashing down. The gut and brain sends out new hunger signals, we eat more, the process repeats itself. Over time we put on weight. This metabolic roller coaster is avoided, experts agree, by diets high in fats and proteins and also by diets rich in slowly digested unrefined carbohydrates like brown bread and brown rice.
An ultra low-carb diet like the Atkins diet has its own metabolic consequences. When there is little or no glucose in the blood stream, fats are burned, producing substances called ketones. In the induction phase of an Atkins diet, so much fat is burned that the resulting ketones can actually be smelled in dieters' breath. [Note: Dietary ketosis, as it's called, is not to be confused with ketoacidosis, which is a life-threatening condition most often associated with uncontrolled insulin-deficient type 1 diabetes.]
- Do any diets work over the long term?
Very few people are able to keep weight off for more than a few months. While this issue is still being researched, most experts argue that 80 to 95 percent of dieters will regain any lost weight within a year. The most likely reason for this is that people eventually tire of a diet low in fat or carbs and return to their old eating habits.
- But there are some people who do succeed? How do they do it?
The National Weight Loss Registry is tracking some 3,000 people who have lost 30 pounds or more and kept it off for six years. One thing these individuals have in common is that they are very active. They take well over 10,000 steps a day, compared with the 4,000 steps most of us take. The lesson seems to be that keeping weight off is impossible without lots of exercise.
However, over the past 30 years we have become much less active: we drive everywhere, take elevators instead of stairs and use numerous labor saving gadgets in our homes. According to the latest data from Eurodata TV Worldwide, Americans watch an average of 4 hours and 25 minutes of television a day, second only to Japan.
- What are the health effects of different diets?
The relationship between diet and health has been very complicated to pin down. Long-term controlled experiments are more or less impractical. While scientists can observe with great accuracy everything a person eats over a few days, most diseases develop over years or decades. On the other hand, it is impossible to monitor precisely what a person eats over a period of 20 or 30 years.
One approach is to use large prospective studies. In the late '70s, Harvard began tracking the diets of hundreds of thousands of health professionals. The study participants regularly fill out food questionnaires and complicated formulas are used to convert the data into percentages of saturated fat, cholesterol and so on. The participants are also followed to see what diseases they contract. Then, after controlling for all possible confounders (such as smoking, for example), the scientists look for associations between dietary intake and disease.
The head of Harvard's Herculean effort in nutritional epidemiology is Walter Willett. Willett and colleagues found that not all fats are bad. Saturated fats and trans fats (those used in margarines and many baked goods) are associated with heart disease and stroke, but unsaturated vegetable and fish oils are positively good for us. Willett also found associations between excess consumption of refined carbohydrates and diseases like type 2 diabetes. Details of some of the findings can be found at The Nutrition Source and in Willett's book, Eat Drink and be Healthy.
- Can a healthy diet help us avoid disease?
Absolutely. The Harvard studies indicate that the correct dietary choices -- that's moderate consumption of lots of vegetables and fruits, good (unsaturated) fats and good (unrefined) carbs -- in conjunction with regular exercise (and of course avoiding smoking) would prevent about 82 percent of heart attacks, about 70 percent of strokes, over 90 percent of type 2 diabetes, and over 70 percent of colon cancer. By comparison, the best statin drugs can reduce heart attacks by about 20 or 30 percent.
- Are Americans really overweight?
No question. According to the Surgeon General and the Centers for Disease Control (CDC), we are facing a major public health crisis. Most experts estimate that about two-thirds of Americans are overweight and about half of those -- i.e. one third of the population -- are so heavy they meet the clinical definition of obesity.
Recently, the CDC predicted that obesity would soon overtake smoking as the number one cause of preventable death. Currently obesity is linked to some 400,000 premature deaths -- 16 percent of all deaths -- from conditions like heart disease, cancer and diabetes.
- But what do the terms "overweight" and "obese" really mean?
Most experts estimate body fat with a measure called the Body Mass Index or BMI. To calculate your BMI, take your weight (in pounds) and multiply it by 703, then divide the result by the square of your height (in inches). Or, if you prefer, you can look up your BMI on this chart from the National Heart, Lung and Blood Institute. A BMI between 18.5 and 25 is considered normal, one between 25 and 30 is defined as overweight, and one over 30 gets the label obese. People with BMIs over 40 are sometimes referred to as "morbidly obese."
- Is the BMI a fair measure?
The BMI gives a pretty accurate measure of body fat, but the scale has limits. It's really only applicable to "ordinary" people 20 to 65 years of age. The BMI is not an appropriate measure for children, for the elderly, for pregnant and nursing women, or for heavily muscled athletes. For the rest of us, however, it does give a snapshot of how fat we are.
Most people with a BMI of 26 don't think of themselves as overweight, but consider this: The risk of heart diseases, diabetes, and high blood pressure actually begins to rise at BMIs of around 22, rather than 26. When experts wrote the USDA's 2000 Dietary Guidelines for Americans, they briefly considered using a BMI of 22 as a cut-off for "normal" weight, but realizing this would label the vast majority of Americans as overweight they settled on a threshold BMI of 25 instead.