As of 2015, research has not addressed milk thistle's direct effect on weight loss, according to Mayo Clinic. WebMD reports that evidence suggests the herb decreases insulin resistance, but a PLoS One journal article reports contrary findings. As the Journal of Clinical Investigation reports, insulin resistance can contribute to obesity.
Insulin resistance is characteristic of Type 2 diabetes, according to WebMD. As a 2014 PLoS One article describes, insulin resistance decreases glucose uptake, which means that blood sugar levels remain higher. The PLoS One article details a study on the effect of silymarin, the active ingredient of milk thistle, which is scientifically known as Silybum marianum, on insulin resistance in rats. The study concluded that silymarin actually induced insulin resistance in some of the rats in the study while increasing it in others and that caution is necessary in the use of silymarin in patients with Type 2 diabetes.
Mayo Clinic states that good scientific evidence exists for milk thistle's positive effect on blood sugar control in people with diabetes. However, it also cautions that better research is needed.
Good evidence also exists for milk thistle's helpfulness for diabetes-related kidney disease and for liver conditions such as cirrhosis and chronic viral or alcohol-induced liver disease, according to Mayo Clinic. However, further research is necessary to substantiate these uses.