Continued from Part 1: What is Yerba Mate. Aside from these various health benefits, another benefit that has helped Yerba Mate gain popularity and recognition is its use in the treatment of obesity, and its effect on weight loss. There are numerous studies that have found Yerba Mate to have beneficial effects for weight loss, and because of this, Yerba mate is a common ingredient that you often find in diet pills and fat burners. There are numerous studies showing the weight loss benefits of Yerba Mate in lab mice, and some studies conducted using human subjects. Below is a Yerba Mate weight loss study done on lab mice.
In a systematic review done on a number of herbal weight loss supplements, numerous double-blind, randomized controlled trials were reviewed to assess all clinical evidence of adverse events of herbal food supplements for body weight reduction. The herbal products assessed include Ephedra sinica, Garcinia cambogia, Paullinia cupana, guar gum, Plantago psyllium, Ilex paraguariensis (Yerba Mate) and Pausinystalia yohimbe. The results of this systematic review showed that there are adverse side effects and events reported for a number of these herbal weight loss supplements, and suggested that the reported risks are sufficient to shift the risk-benefit balance against the use of most of the reviewed herbal weight-loss supplements. However, they made note that exceptions to this are Garcinia cambogia and yerba mate, which merit further investigation.  What we can gather from this review is that despite the adverse side side effects associated with many of their reviewed herbal weight loss supplements, Yerba Mate was one of the exceptions; however, more studies are needed.
Drinking Yerba Mate Tea & Weight Loss Studies
In another similar type of study (this was not a systematic review, but a clinical trial), the effects of 12 commercially available plant preparations were investigated on non-obese men and women. The results of their study found no significant increase in energy expenditure with the preparations, and no change in respiratory quotient was shown, except after treatment with Yerba maté (Ilex paraguariensis), where a drop was observed, which indicates a rise in the proportion of fat oxidized. 
There are also several studies done on herbal extract mixture preparations (which contained Yerba Mate extracts) on weight loss where several different types of herbal supplements are mixed in a formula to induce weight loss. In a randomised double blind, placebo controlled clinical trial done at the Poznan University of Medical Science in Poland, a botanical extract weight loss formula was evaluated. Two types of tablets were used: 1) extracts of Asparagus, Green tea, Black tea, Guarana, Yerba Mate and Kidney beans; and 2) extracts of Kidney bean pods, Garcinia cambogia, and Chromium yeast. 105 health and overweight subjects took part in this clinical trial. The two types of tablets were consumed twice daily with 2 main meals. Their results showed a significant change of the Body Composition Improvement Index, with increased body fat loss. 
Dangers of Yerba Mate Side Effects
Yerba Mate is a stimulant and has various positive and negative physiological effects. Yerba Mate tea can help increase the focus and alertness of the user, and it may also make the user feel various negative side effects such as anxiety and jitteriness. Also, numerous studies have found that consistent drinking of Yerba Mate tea is associated with various forms of cancer. Drinking Yerba mate tea has been associated with cancers of the esophagus, oropharynx, larynx, lung, kidney, and bladder. One study conducted at the National Cancer Institute in the US to determine whether drinking yerba mate could lead to exposure to known carcinogens (a substance that causes cancer). The dry leaves of 8 different commercial brands of Yerba Mate tea was made with hot or cold water. Their results found that very high concentrations of carcinogenic polycyclic aromatic hydrocarbons were found in yerba mate leaves and in hot and cold infusions. 
Studies performed in Uruguay have found a high risk of bladder cancer associated with drinking Yerba Mate tea. Drinking Yerba mate tea was positively associated with bladder cancer, and the risk increases with increasing duration and amount of yerba maté drinking.  Yerba Maté consumption has also been associated with an increased rate of oral, oropharyngeal, esophageal, and laryngeal cancers. Clinical evidence suggests that drinking Yerba Mate tea is carcinogenic, and plays a role in the development of cancers; however the exact mechanism of carcinogenesis of Yerba Maté is still unknown. 
Guayaki yerba Mate
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Non-alcoholic beverages and risk of bladder cancer in Uruguay.
De Stefani E, Boffetta P, Deneo-Pellegrini H, Correa P, Ronco AL, Brennan P, Ferro G, Acosta G, Mendilaharsu M.
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