Your liver has to do 80% of the work when it comes to metabolising alcohol. A chunk of this will end up as fat. But that is not the only problem as common mixers such as tonic water, coke, sprite and fruit juices contain very high amounts of sugar. White sugar contains an equal mix of glucose and fructose; but high fructose corn syrup (HFSC) is often used and the fructose component can be even higher.

• Glucose is the simplest sugar that all our cells can absorb for energy. Whilst some glucose is processed in the liver, much of this is used for glycogen production and little finds its way into fat reserves. Glucose scores a 100 on the glycemic index and will spike insulin as soon as it is drank in the form of a mixer.
• Fructose is pretty toxic for the body and can only be metabolised in the liver. The consequences of ingesting large quantities of fructose are pretty dire for the body.  Read more about the food and drink industries scandalous use of fructose here.