I proudly stand at BMI 25.1 and bodyfat 2 standard deviations below average for women my age. Never used any supplements other than protein powder/bars. Just years and years of lifting, where I have gone between very high weights (I compete in powerlifting), high volume (training for a bodybuilding competition now), and both (Crossfit). If you work hard for a very long time and have good gehneticks, it's certainly possible. I'm the type who packs on muscle if I even lay my eyes on a barbell.