Professor Steven Blair, from the Arnold School of Public Health at the University of South Carolina, said greater emphasis had to be placed on physical activity to prevent the obesity problem in Scotland from reaching US proportions. He suggested exercise could be prescribed to patients by their doctors.
Professor Blair was speaking ahead of a major conference in Edinburgh today, which will also hear from Scotland's top doctor, Chief Medical Officer Sir Harry Burns.
The event, organised by the Royal College of Physicians of Edinburgh and the Royal College of Surgeons of Edinburgh, will focus on sports and exercise medicine, ahead of the Commonwealth Games in Glasgow in July.
Mr Blair claimed physical inactivity had become "the biggest public health challenge of the 21st century" and said people must consider their levels of activity as well as just their diet.
He said much of the published scientific literature on the causes of obesity was flawed, arguing it placed "an over-emphasis on dietary intake, at the expense of measuring the positive effect of physical activity".
Mr Blair added: "Obesity rates continue to rise and in inter-national obesity terms, Scotland is not far behind the US. If we wish to prevent levels in Scotland reaching that of the US, greater emphasis has to be placed on exercise."
He said: "This could involve creative solutions such as considering the provision of exercise advice, or indeed the prescribing of exercise to patients by doctors and other health professionals.
"An entire industry has built up around diet, but reducing our dietary intake alone will not solve our problems with obesity. Physical inactivity has become the biggest public health challenge of the 21st century and we have to become more active if we are to stop collectively sleepwalking into obesity.
"In simple terms, we are talking about changing the mind-set from thinking 'I must go on a diet' to 'I must become more active'."
The conference will also hear from former Scotland football manager Craig Brown, ex-player Pat Nevin and Dr Michael Turner, who recently retired as the chief medical adviser to the British Horseracing Authority.
Professor Stewart Hillis, emeritus professor of cardio-vascular and exercise medicine at Glasgow University, and Lindsay Thomson, the lead physio-therapist at the sportscotland Institute of Sport will also take part.
Professor Hillis, who worked with the Scotland football team and SFA for many years, said: "The wider positive impact of exercise on health has often been under-estimated, particularly in reducing obesity.
"By bringing together leading experts in the field of sports medicine in the year of the Commonwealth Games, we hope to shine a light on developments in sports medicine, stimulate wider awareness about the many benefits of physical activity and creative thinking about this can be applied to improve health."