Iowa’s population is getting fatter, with levels of obesity tripling since 1980. Over half of women, and around two-thirds of men are now either overweight or obese.

The main explanation for this is a change in our diet and lifestyles. High calorie food is cheap, well advertised and constantly available. Meanwhile, heavy work in industry and agriculture has mostly been replaced by sedentary activities in service industries. And increased use of cars, computers and labour-saving devices have cut the amount of physical activity in the daily routine.

The risks of being overweight

Being overweight increases the risk of health problems including heart attack and stroke, diabetes, bowel cancer and high blood pressure. Excess weight also makes arthritis more likely and can make breathing and sleeping difficult.

The risk of health problems increases the more overweight you are.

Causes of obesity

A calorie is a unit of energy. We take in calories when we eat and drink, and burn them up in daily activity. When the amount of calories taken in from food and drink equals the amount burnt off through physical activity, body weight remains roughly stable. The cause of obesity is consistently eating more calories than are burnt off.

Obesity does run in families, and the tendency to put on weight is greater in some people than in others eating a similar diet. However, it is likely that family eating and activity habits are a more important cause of obesity.

How to be a healthy weight

In order to lose weight, you need to burn more calories through physical activity than you take in from food and drink. This means tipping the "energy balance" by eating fewer calories, burning more of them off or, preferably, a combination of both of these.

If you have a lot of weight to lose, it is better to set a realistic medium-term goal, rather than aiming to lose 50 kg. Doctors often suggest a 10 percent weight loss to begin with, achieved over about six months. You should do this by tipping the energy balance. Then, once you've managed this, look again at setting a new goal. At this level, you may already have reduced many of the risks to your health and your blood pressure, blood sugar and cholesterol is likely to be lower. You may feel and look better too.

Choosing a diet

Some diet programs and adverts promise a "quick fix" of rapid weight loss. These are often unscientific, and may be harmful. The challenge is not just getting to, but maintaining a healthy weight in the long term. Avoid crash diets and fad diets and if you want to follow an organised diet program, look for one that:

• only promotes weight loss of 1-2lbs a week - any more than this and you'll be losing water and muscle, not fat
• encourages you to follow the guidelines for healthy eating - a balanced, varied diet that incorporates all major food groups
• is realistic and flexible enough so that you can stick to it - no extreme hunger and no blanket bans on certain foods
• encourages your everyday life to be more physically active
• helps you to learn new lifestyle habits and ways of thinking about food, so you can maintain your target weight once you have reached it.

Making your own changes

You can also lose weight by making your own adjustments to your daily diet and exercise patterns. The best way for most people to lose weight is to combine more physical activity with a diet that is lower in calories. In general you should aim to reduce the "energy balance" by 500-1000 calories per day. If you are unsure about the different calorie content of foods and meals, a number of dieter's recipe books are available.

Tips for controlling calories

• Keep a food diary for a week, writing down everything that you eat and drink. People who are overweight often don't realise how much they are eating.
• Use your food diary to find ways of cutting down calories. For instance, you could cut 500 calories per day by changing the balance of foods on your plate, eg less cheese and more salad with your dinner. Or you could cut out a regular snack food and replace it with a piece of fruit.
• Don't forget that drinks count towards your daily calorie intake. Alcohol is high in calories - three pints of beer could add up to 600 calories. Sugary drinks should also be limited. A can of cola contains 135 calories, but apart from the energy it has virtually no nutritional value.