British nutritionist Dr Stanley Ulijaszek says fast food is mostly to blame because it's so easily available.
Although it's pleasant to eat, this kind of food is very high in fat and sugar.
A massive one in five of the world's population is classed as overweight.
Experts see this as an 'obesity epidemic' sweeping the globe.
And you can't just swap the takeaways for salads; watching television and playing computer games are also being blamed for a rise in the number of overweight children.