Its OK, all my drinks are sugar free.

Imagine that I take you to a chemistry lab. I put some water in a glass beaker and then I go around the shelves opening jars of chemicals adding a bit of this, a bit of that and a bit of the other to the beaker. I stir the water until all the powders dissolve and then I pass it to you and say, “Here, drink this.”
I am guessing you would refuse because you don’t really know what I put in there.

I agree that it doesn’t look very appetising, so I add something else to make it a brown colour and then dissolve carbon dioxide in it to make it fizzy. I invite you to drink it again. It looks like something you might drink but I am still thinking you will say no. It is odd that you would refuse because if I poured it out of the beaker and packaged it in a brightly coloured ring-pull can and called it Diet Cola, not only would you drink it but you would pay me for it.
Those chemicals I put in the beaker would typically be carbonated water, caramel colour, aspartame, phosphoric acid, potassium citrate, natural flavours, citric acid and caffeine. What do they do?

Aspartame: There are many conflicting studies on the safety of aspartame. Animal studies show more of an increase in cancer risk than human studies have. But there is definitely enough of a connection to avoid or limit your exposure to this artificial sweetener. Also, artificial sweeteners trick your body in to thinking a load of sugar is coming and you get a surge in insulin, which reduces blood sugar and makes you hungry.
Phosphoric acid: This is acidic enough to erode tooth enamel and some studies have shown it increases your chance of kidney stones.
Potassium citrate: It can cause irritation of the stomach lining, nausea and diarrhoea.
Natural Flavours: Hundreds of chemicals can be used to mimic the taste of natural flavours. Natural flavours can be anything derived from foods found in nature, but doesn’t make them a health food.
Caramel Colour: In 2011, the International Agency for Research on Cancer concluded that caramel colouring is possibly carcinogenic to humans. Caramel colouring is in a host of products, like baked goods and soy sauces, so the total we consume from many different products makes it more of a concern.

Apart from this stuff being chemical soup, it is unlikely to help you lose weight. It may well be ‘zero calorie’ but weight is controlled by hormones not calories. Insulin is triggered by too much sugar in the blood and insulin prevents access to the fat stores. The artificial sweeteners in these ‘diet’ drinks can be 100 times sweeter than sugar. When the brain registers this level of sweetness, it can release insulin in a learned response, just like a Pavlovian dog. The insulin stops you burning fat and makes you feel hungry when its level starts to fall. So it is possible that having a diet drink makes you eat more. As if that wasn’t enough, these non-food chemicals disrupt the bacteria in the gut and studies have shown this can lead to weight gain. Always remember, they don’t make this stuff for your health; they make it for their wealth