• Hypotheses explain the rapid rise in obesity over recent decades.

• Energy balance: Weight gain due to consuming more calories.

• Carbohydrate-insulin hypothesis: Excess carbohydrate intake stimulates the insulin response.

• Effect of Protein: Eating insufficient protein causes constant hunger.

• New hypothesis: sugar, especially fructose, is to blame for obesity.

• Fructose suppresses mitochondrial function, causing hunger and thirst.

• In the long term, regular exposure to fructose can damage mitochondria.

• Corn syrup, honey and cane sugar are common sources of fructose.

• Fruits are still healthy, but they contain less fructose than juices or candies.