When it comes to having a healthy diet sometimes people opt for what they think is the healthier option rather than what's actually good for you.

The most common change in someone's diet is to swap out a product for the 'low-fat' or 'healthy' version, but nutritionists say that this is the worst thing you can do. London nutritionist Rhiannon Lambert gave some advice to The Daily Mail about foods which people should avoid. First up is the sugar substitute agave, which a lot of blogger are currently promoting. Rhiannon says, "Agave is a cactus native to Latin America and the syrup is made from the pulp of the cactus leaf. The processing involved in its production destroys all of the health promoting properties of the agave plant. She adds, "Agave syrup has a low Glycemic Index (GI) score - a measure of how quickly the sugar in a food enters your bloodstream - because of its low glucose content. But this is irrelevant as agave syrups are dangerously high in fructose content, often as high as 90 per cent.

"In this way, agave syrup is similar in composition to high fructose corn syrup, which is a processed sugar common in the US and is thought to be largely responsible for much of the country's obesity epidemic."

Another type of food which people eat thinking they're being healthy are low-fat yoghurts, but it turns out they have a very high sugar content. Rhiannon says, "low-fat, sugar-sweetened yogurt contains too much sugar to qualify as a nutritious choice. Many types of low-fat and non-fat yogurt are as high in sugar as desserts. Stick to Greek yoghurt and add fruit for flavor." Also on the dairy side of things, she says to drink full fat milk instead of low fat as "it is perfectly healthy as the higher the fat content, the slower the release of intrinsic lactose sugars." There's bad news for those who love to snack on protein bars, they're a big no-no for Rhiannon. She says, "these bars are labeled as a healthy, low-sugar, low-carb treat or dessert, but they contain sucralose, an artificial sweetener made from sugar in a multi-step chemical process where three hydrogen-oxygen groups are replaced with chlorine atoms.

"Research suggests sucralose raises blood sugar levels, making us hungry when levels crash - and that contributes to weight gain."