Despite decades of research, we still don't fully understand why some people gain weight more easily than others, and also have greater trouble losing excess weight. It is known that body fat levels are increasing in America, along with the rest of the developed world (and increasingly in the developing world too). For example, in 1990 fewer than 10% of Australian adults were obese, while in 2001 the corresponding figure was more than 16%. In 2003 the Worldwatch Institute reported that, possibly for the first time in human history, the number of overweight individuals worldwide rivals the number of underweight. Clearly, something is wrong with our diet and/or lifestyle. However, it is known that avoiding excessive energy (i.e. calorie) intake is critical to weight control. That is, you will inevitably gain weight if your energy intake exceeds your energy output. But this doesn't mean crash dieting or attempting to lose huge quantities of weight in a short time. Gradual weight loss, at a rate of 0.5-1.0 kg per month, is most likely to lead to sustainable weight loss, because the body hardly notices such a low rate of change. By way of contrast, rapid weight reduction stimulates hunger (the body interprets sudden weight loss as impending starvation and does all it can to encourage higher food intake). A slow rate of weight loss also means that you can eat enough food to ensure adequate intake of essential nutrients such as vitamins, minerals and dietary fiber. Also, while reducing your food intake will help in weight control, it may not be sufficient in itself. You must still ensure that your level of physical activity takes your energy expenditure above your energy intake. It is difficult to over-emphasize the importance of physical activity, not only in weight control but for general health and fitness as well.The simplest and most appropriate physical activity for most people is walking. Just adding 30 minutes of brisk walking to your daily activity pattern may be enough to tip the energy balance in favor of reducing weight. If you lose 0.5-1.0 kg of excess body fat per month, your weight will have come down by about 9 kg in a year. For someone whose initial weight was 90 kg, this represents a 10% weight reduction, enough to make a substantial difference to your health and metabolic fitness (i.e. blood pressure, cholesterol, triglycerides and so on). Once your weight has stabilized at the new (lower) level, it is essential that you continue with your new lifestyle of slightly lower food intake and increased physical activity, or the weight will simply go back on. In other words, there is no point going on a diet; rather, the changes you made to lose weight must now be a permanent part of your life. This will not be easy. Although the solution seems straightforward (eat less and increase physical activity), as already mentioned in this FAQ, losing weight is difficult enough in the first place; keeping it off is even harder. If we fully understood the actual cause (or causes) of the worldwide increase in body fat levels, we would be in a better position to develop strategies to halt the epidemic (and perhaps even begin reversing it). Clearly, both diet and lifestyle (particularly physical activity or rather a lack of activity) are involved in the obesity epidemic, but the exact role of diet is still very controversial.