Of the two loin subprimals, the sirloin sits farther back toward the rear leg, where the muscles get more exercise. The meat from this section can be tougher than cuts from the front part of the loin, called the short loin. The sirloin is separated from the short loin at the front tip of the hip bone by a straight cut through the seventh lumbar vertebra.
The sirloin is almost always broken down into two boneless wholesale cuts: the top sirloin butt and bottom sirloin butt. This is done by cutting along the natural seam between the gluteus medius, the primary muscle of the top sirloin, and the knuckle, a group of three muscles (the rectus femoris, vastus lateralis, and vastus medialis) sometimes called the sirloin tip.
Sirloin cuts from the top butt contain about 175 calories per 3.5-ounce portion and 5 grams of fat.1ï»¿ They also deliver 30.8 grams of protein, plus the B vitamins responsible for red blood cell health and minerals such as zinc, phosphorous, and selenium.
Roasts from the bottom portion of the sirloin deliver about 182 calories, 26.5 grams of protein, and 8.3 grams of fat, plus the same beneficial vitamins and minerals.
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