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Feed efficiency is the measurement of ones efficiency in converting food into ones desired output.
Simply put, increased feed efficiency will allow you to gain more off the same amount of food.
In a bodybuilders case the outcome is is obviously performance and the building of LMM. When using Tren you use what you eat more efficiently.
The drug was originally used for cattle so that cattle farmers could yield a larger amount of beef with no extra cost for the cattles feed.
Same amount of feed.....bigger cows.