Slim shady

29.9% Body fat
5’8”, 142lbs, size 6

Photo by Roxana Marroquin

At a visit to a personal trainer last month, a 110-pound, 5'6", size-two friend discovered—much to her surprise—that her tiny body was actually composed of 25.6 percent body fat, a slightly high ratio by the American Council of Exercise’s standards. Her BMI (body mass index, or a simple ratio of weight to height) of 18.3 suggests that she’s underweight, so how could this be? Because, as her fat percentage—a more in-depth indicator of health—suggests, she has only a little muscle mass yet a fair share of blubber.

Though the percentage sounds scary (“More than a quarter of my body is lard?!?”), someone in her size 26 jeans shouldn’t freak out: “When it comes to fat, there’s a lot of confusion about what is appropriate and what is too much,” explains clinical nutritionist Lynn Goldstein, founder of You Are What You Eat (youarewhatyoueat.com). “Women need a certain amount of fat to regulate body heat and hormones: at the very minimum 10 to 12 percent, according to the American Council on Exercise.” A female that’s 20 to 39 years old and 25.6 percent body fat fits into the “acceptable” range (21–31 percent)—just shy of the hallowed “fitness” category (21–24 percent)—and is perfectly healthy for the time being.

36% Body fat
4’11”, 108lbs, size 2

Photo by Roxana Marroquin

So if you need fat and you’re small by genetics, then why should a slightly higher-than-ideal percentage matter? Because, as registered dietician Samantha Heller, M.S., explains, “it increases your risk for heart disease, diabetes and cancer as life goes along, especially since people tend to be more physically inactive and eat more as they age.” Adding insult to injury, fat doesn’t just sit there like a stubborn lump. “It’s an active substance,” Heller points out. “When we have too much for our bodies, it produces inflammatory compounds that lead to chronic diseases.” Muscle, on the other hand, ensures physical independence well into our golden years. Therefore someone with a low muscle mass “needs to start moving,” Heller urges, noting that the National Institute of Health recommends 30 minutes of vigorous exercise daily. While our pal’s weight won’t change drastically as she swaps muscle for fat (realistically, she would exchange about five pounds of fat for muscle), the mass gained is less bulky, which will make her look more toned.

30% Body fat
5’4”, 123lbs, size 4

Photo by Roxana Marroquin

Heller, who has worked in cardiac rehab for more than ten years, laments that she sees skinny fat people all too often: “Because they’re lean, they think they can eat whatever they want and don’t need to exercise.” Some people even claim that they’re less hefty when they don’t work out. Heller disagrees: “Muscle weight definitely happens, but only when people are doing some serious weight training.” As for an increased appetite, “it tends to taper off and stabilize as your body gets used to working out,” assures Goldstein, so you should be able to build muscle without raiding your kitchen after every run. “Many of us have lost sight of the fact that what’s important is being healthy, not skinny,” says Heller, who recommends weighing yourself once a week, if at all. “What we do and put in our bodies has an effect on our health, whether we’re thin or not.”