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In the scientific and nutritional communities, there are reasons to suggest that casein, a milk protein, is linked to cancer development in some animals and humans. Some studies have shown that casein and cancer are linked in rats, yet it is unclear if these findings hold true in the human population. Nutritional research that studies the correlation between casein and cancer has generalized findings to suggest that all animal protein helps cancer flourish. These generalizations have led to many objections and criticisms, as more research on all types of protein should be studied thoroughly before claiming a hypothesis is true.
Casein is the protein found in milk and other dairy products, and is sometimes allergenic to those who suffer from food intolerances such as gluten or lactose. Both casein and cancer have been linked in a few nutrition studies on rats, such as The China Study by Dr. T. Colin Campbell, suggesting a possible link in humans as well. In these studies, researchers fed a group of rats isolated casein and tried to determine its effect on cancer cells. Many researchers claim that increasing casein protein in the rats' diet caused cancer cells to activate and grow.
The rats fed isolated casein powder differ from the other rat subjects fed wheat or soy, as these rats showed no alteration in immune system status and cancer growth. Results like these led many of the researchers to determine that introducing a plant-based diet into animal and human diets can decrease the likelihood of cancer. It is unknown if casein and cancer are linked in the majority of the population; however, it is accepted that there is a slight link between dairy protein and cancer. Research studies on this correlation still contain some flaws, which present a number of scientific objections and criticisms.
Many critics to the studies on the relationship between cancer and casein point out that rats are often fed a diet consisting of powdered isolated casein in casein studies, which doesn't occur naturally in human or animal diets. In fact, many objections stem from the fact that casein is consumed as part of a whole food, along with naturally occurring fatty acids as well as carbohydrates. Also, the generalization that stems from these studies that all humans should avoid animal protein needs to be thoroughly examined, as this claim is only a hypothesis. Pasteurizing, heating and fermentation can also change the structure of casein, leading many critics to believe that other variables should be analyzed in future research.