While networks settle in for their long winter’s nap of reruns, two cable outlets launch new series tonight — both with the strange confessional aspects that “documentary” reality shows can bring.

“I Used to Be Fat” (9 p.m., MTV) follows teens from their final weeks of high school until their opening weeks of college. They want to make their summer count, particularly on the scale. Every new episode will focus on a different teen, from a 306-pound student from a close-knit Italian family whose mother equates food with love to an obese girl who opted to be home-schooled rather than face the taunting of her peers.

As we’ve come to expect from MTV series like “Real Life,” not all of the teens will succeed in losing the weight but “I Used To Be Fat” will drain maximum drama from their attempts.

• Weight-loss struggles pale besides the trials of the subjects on “My Secret Addiction” (8 p.m., TLC), a new hourly series. Don’t go looking for something as humdrum as “Hoarders.” These folks include a woman who can’t sleep unless she has her blow-dryer with her in bed. On full blast! It’s murder on her skin (not to mention her utility bills), and she runs the risk of burning down the house. Stranger still, this hairdryer obsession has been with her since she was 8 years old.

Other subjects include people with a predilection for eating detergent. Another snacks on toilet paper. And one man has taken body building to an unhealthy extreme.

“My Secret Addiction” joins TLC’s peculiar niche of shows about extreme situations, subcultures and behavior. The season opener of “Toddlers & Tiaras” (9 p.m., TLC) follows.

• “Great Performances” (8 p.m., PBS, check local listings) puts the accent on talent from South and Central America with “Celebracion: Gustavo Dudamel and the L.A. Phil with Juan Diego Florez.” The first half of the concert is devoted to Italian opera, notably Rossini. Then Florez sings works from composers closer to his native Peru and performs a popular cancion his father used to sing at home.

Tonight’s other highlights

• A vacation in Greece proves eventful on “Undercovers” (7 p.m., NBC).

• Director Clint Eastwood’s 2006 war drama “Letters From Iwo Jima” (7 p.m., IFC) presents the battle from the Japanese perspective and offers a companion piece to his film “Flags of Our Fathers” from the same year.

• On two episodes of “Human Target” (Fox), Winston retrieved (7 p.m.), one for the wives (8 p.m.).

• On two episodes of “Law & Order: Special Victims Unit” (NBC), Marcia Gay Harden guest stars (8 p.m.), rape on campus (9 p.m.).

• An earthquake rattles some more than others on “Modern Family” (8 p.m., ABC). Nathan Lane guest stars. In a second helping (9 p.m.), Claire’s perfectionist vision of Halloween unravels.

• A gambling debt misunderstanding ends in murder on “The Defenders” (9 p.m., CBS).