You'll find them in any supermarket; you can even prepare them at home, using whatever fruits and vegetables you have in the fridge. The so-called 'detox' juices and smoothies are now widely available and more people are using them but, as with diets, they can't work miracles and some are more recommendable than others. Let's look at some of the truths and myths about these combinations.

Despite what you might think, these juices and smoothies might not help you lose weight because some of them contain quite a few calories. Depending on the foods used, they can help weight loss to a greater or lesser extent. In any event, it is always best to opt for the home-made varieties, because the ready-prepared ones contain more sugar. "Everything that is packaged contains calories and preservatives, to stop the fruit and vegetables going off," explains María Rojo, a diet and nutrition technician and creator of the blog 'Chatidieta'.

Although the juices and smoothies are healthy, it is always better to eat whole fruit. Most of the proteins are in the skin of the fruit, so they are lost when this is removed or the fruit is blended. That's why it is much more advisable to eat fresh fruit than drink it in the form of juice or a smoothie. "They [the smoothies] are better than eating something from a vending machine, and they do provide you with vitamin C, but not much more than a normal orange juice," says María. That's why the best option for a snack on the move is to take a plastic container of fruit with you if you can, or buy cut fruit instead of a smoothie.

Smoothies are not substitutes for a balanced diet. These juices and smoothies can help someone to lose weight on a specific occasion, but bear in mind that afterwards it will be necessary to complement them with a healthy, balanced diet to avoid the rebound effect.

"If you have these for lunch and dinner for a month you will lose weight because you're not eating much, but then you'll suffer from the rebound effect because your body will want to recover the nutrients it is lacking," says María Rojo. It is also bad for the health, because it can cause muscle cramps and drops in blood sugar levels due to the lack of nutrients.

They are diuretics, but only because of the amount of water they contain. Most people know about the diuretic effect of these juices. However, not all of them know that in reality this is due to the flavour the fruit gives to the water, helping us to drink more of it. "They contain a lot of water to make them last longer. There are some sachets that are flavoured, and people think they help to lose more weight, but in reality the only thing they are doing is drinking more water," says María, who also points out that not all fruits have the same effect on people. "If you're used to eating pineapple, for example, you won't end up going to the bathroom as often," she says.

They can make you feel full up, depending on the vegetables and fruits selected. Each one has different properties, so it is advisable to think about what you want to achieve before making or buying a 'detox' juice. "Because there is such a mixture it can fill you up, especially as you normally take these mid-morning. But they are not a substitute for meals," says María. Some recipes, particularly the ones using vegetables, have a filling effect. If you opt for diuretic fruits, you'll achieve exactly the opposite, because you will go the bathroom more often and generate more hunger. Also, people who want to start using green juices are advised to include 60 per cent vegetables and 40 per cent fruit.

When buying a ready-prepared detox juice, be careful. Not all brands are the same, and the contents of all of them differ according to what you want to achieve. María says that some brands take advantage of people's lack of knowledge to sell their product as a miraculous slimming aid. It is also very important to check all the ingredients carefully, because not all fruits or plants are beneficial for everyone. "They use plants that are innocuous, but some infusions that are inoffensive can't be drunk by pregnant women or people with thyroid problems. I have patients with kidney problems now because they had these smoothies for a long time," she warns.

Some recipes are highly recommended. Bearing in mind the needs of each person and the properties of the fruits and vegetables, some very nutritious juices and smoothies can be made. For example, you can use a combination of cucumber, celery and mint if you want to improve your digestion and benefit from the diuretic effect, or try one with lemon, blackberries and spinach if you want to eliminate toxins and detoxify the body. There are endless possibilities, where the only limit is in the willingness to be inventive.