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 programmes 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.

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 processedstarches 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 unrefinedcarbohydrates 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.

But there are some people who do succeed? How do they do it?

There are various stats out there tracking some 3,000 gym goers. These 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 labour saving gadgets in our homes.

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.

Can a healthy diet help us avoid disease?

Absolutely yes. So many 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.

Why do people get fat?

People put on weight when they take in more calories than they burn. If you think of food as fuel, the energy content of the fuel is measured in calories. A slice of bread, for example, has about 100 food calories. If you were to add up all the calories you consumed in a day -- breakfast, lunch, dinner and snacks -- that's your energy input. Your body uses this energy for everything from breathing to moving around. Everyone is different, but over the course of a day, an "average" man expends something like 1800 calories and an "average" woman around 1500 calorie. 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 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.

How can I get enough nutrients from my diet without consuming too many calories?

At BPPC we encourage you to choose a variety of nutrient-dense foods and beverages to help achieve recommended nutrient intakes. Foods like vegetables, fruits, whole grains, low-fat dairy products, and lean protein foods can help you get the nutrients you need without excess calories. Avoid excess calories by limiting consumption of foods high in added sugars and solid fats, and alcoholic beverages; these provide calories but are poor sources of essential nutrients. And, because calorie intake must be balanced with physical activity to control weight, stay active.

How can I burn off my stored body fat?

We all need some body fat, but if stored fat is excessive it may increase risk of diet-related diseases such as heart disease, diabetes, and some cancers. This is particularly true if excess fat is in the abdominal area. According to the CDC, a body mass index, or BMI, of 25 or higher is an indication that your weight may be unhealthy. Also, a waist circumference of over 40 inches in men and over 35 inches in women indicates excessive abdominal fat if BMI is 25 or higher. The best strategy for losing excess weight and stored body fat involves calorie reduction, increased physical activity, and a behavior change plan.

How many calories do I need to burn to lose a pound of weight?

You need to burn off 3,500 calories more than you take in to lose 1 pound. This translates into a reduction of 500 calories per day to lose 1 pound in a week, or 1000 calories per day to lose 2 pounds in a week. (1-2 pounds per week is generally considered to be a safe rate of weight loss.) This can be achieved by eating fewer calories or using up more through physical activity. A combination of both is best.

I would like to gain weight and still want a balanced diet. How can I do this in a healthy manner?

Losing, gaining or staying at the same weight all depend on how many calories you eat and how many calories your body uses over time. If you eat more calories than you use, you will gain weight; conversely, if you eat fewer calories than you use, you will lose weight. Because many people are overweight, there are many resources geared toward losing weight. Some of these resources explain the principles of weight balance and can provide guidance for you to gain weight in a healthy manner; you will just need to focus on portion sizes for weight gain, rather than weight loss.

Which fats should you cut back on to lose weight?

Saturated fats. Less than 10% of your calories should come from saturated fats, say government dietary guidelines. Not eating one type of food doesn't translate to cutting overall calories. Besides, fat can help you feel full after eating, which may curb your desire for seconds or dessert. Your body needs some dietary fat to function. Replace butter and processed foods with more healthy polyunsaturated and monounsaturated fats, like olive oil, cold water fish, tofu, avocado, and small amounts of nuts. Although lowering saturated fat isn't magical for weight loss, it is beneficial for overall health.