'I've been a failure at treating obesity for the past 25 years," Prof. Elliot Berry says wryly. Conquering obesity, leading metabolic specialist and former dean of the Hebrew University-Hadassah Braun School of Public Health and Community Medicine in Jerusalem's Ein Kerem says with a sigh, "is simple in theory but so, so difficult in practice." Easy in theory, because as long as you ingest fewer calories than you burn throughout the day, you ought to lose weight. Difficult in practice, because so many people eat much more than they need to and exercise much less than they should. "There is nobody who has been found on this planet who cannot lose weight. By eating less and exercising more, anybody can lose weight," Berry says emphatically. "The problem is maintaining it. Some people are metabolically very inefficient, and some are very efficient. Some are like the cows of Pharaoh's dream [in Genesis] - they can eat up the fat cows and not gain weight. Others can just look at food and gain weight." Aside from exceptional cases, though, obesity is a function of behavior rather than genes, Berry continues. "The great increase in obesity in the past decade means that it has nothing to do with your genes, which couldn't have changed in such a short period, but rather has to do with your environment," he says. "Portion size has grown so much in recent years that the calories available to a person have increased, since the 1950s, by 1,000 a day. So there's too much available out there. And, there's too little exercise. There's too much TV, and not enough walking." Further complicating matters are the metabolic changes that take place as one loses weight. "As you lose weight, you become more metabolically efficient, so you don't need as much food to maintain your weight. If you don't continue to decrease your food intake, you'll gain weight again," Berry explains. That's exactly what happens to a lot of obese people who are only able to temporarily keep weight off. All right - that, and the fact that a lot of people seem to be "addicted" to food. "But exercise is also addictive," Berry notes. "Look, you can't expect people to stay on a [restrictive] diet all their lives. It's just too hard, which is why you have to encourage exercise. You can't fight fast food, but you can give people the education to make the right choices. People have a whole range of food and lifestyle options, and it's our job to be role models for them." The slim, London-born doctor practices what he preaches, wearing a pedometer to ensure he moves around enough each day and running 10 kilometer races throughout the year. Too few people are willing to take a disciplined, lifelong approach to diet and exercise, though, preferring crash diets and pills that Berry says are "for people with more money than sense." Often, people only come around once they suffer a serious health problem. "There's nothing like a heart attack or diabetes to encourage someone to lose weight," Berry says. He'd like to prevent that, of course. He and other health care professionals are trying to fight the "barbecue culture and chronic overeating" that are making obesity so common in Israel, but they feel they can not win that fight alone. The Health Ministry, Berry says, is acutely aware of the problem, but it has no budget for combating it. Meanwhile, he says, the Treasury is inattentive. (The Health Ministry is planning a media campaign to fight obesity as part of its 2020 program to improve the public's health by another 12 years.) "That's why I firmly believe that it has to come from the Prime Minister's Office. When you have more than 60 percent of your population either overweight or obese, you're talking about a national crisis," he says. "When obesity is beginning to show up in populations, like Ethiopian and Yemenite immigrants, who had no tradition of obesity, there's a serious problem. This issue has to become a national priority.