Fitness Carter: When dieting and exercise hurt you

It's hard not to get excited when you finally make the decision to get in shape. And who wouldn't be tempted to speed things up with an extra workout here, eliminating certain foods there? Understandable, sure, but definitely not wise: Overdoing exercise, especially after a hiatus, can cause serious injuries, and drastically cutting calories can make you irritable, forgetful, and may even age you.

Here's why too much of a good thing can be a bad thing—and how to strike a healthy balance to still get the results you want.

Mistake: You drastically cut fat, carbs, or calories

Why it's bad: Your skin, hair, and nails will suffer—and so will your mood.

When you cut calories, you deprive yourself of certain nutrients that promote healthy cell division, cell regeneration, and overall skin tone and texture, explains Dr. David E. Bank, director of the Center for Dermatology, Cosmetic and Laser Surgery in Mount Kisco, NY. "The skin also requires essential fatty acids—which the body can't produce on its own—to maintain hydration. A diet that's too low in fat could cause dry skin, hair loss, and brittle nails."

Other key youth-boosting nutrients include vitamins A, C, and E. Being deficient in A, for example, can cause acne, dry hair and skin, hyperkeratosis (thickening and roughness of skin), and broken fingernails. A lack of vitamin C can affect collagen synthesis (the "glue" that binds our ligaments, bones, blood vessels, and skin), and low levels of vitamin E can cause chronic skin conditions such as eczema and psoriasis to flare up. (Check out the perfect diet for glowing skin ^[1] .)

Fat is also an essential building block for brain cell membranes, explains Susan M. Kleiner,author of The Good Mood Diet. Different types of fats play different roles in the brain, but DHA, which we get primarily from fish oil in our diets, is linked to cognitive function, memory, and mood. If your diet contains less than 25 percent total fat, everyday coping skills may diminish, and you may feel increased anxiety, frustration, and stress.

Mistake: You go the extra extra mile

Why it's bad: You could end up with chronic foot pain.

If you've ever felt a stabbing pain in the arch of your foot after working out, you might be suffering from plantar fasciitis. This painful foot injury occurs if you put excessive strain on the underside of the foot, explains Ryan Halvorson, a personal trainer from the IDEA Health & Fitness Association. It can strike runners and walkers alike when the muscles of the foot are overused due to repetitive motion. The arch support becomes strained, small tears develop, and the tissue stiffens as a protective response. Plantar fasciitis is especially common in women who often wear high heels. Halvorson recommends wearing lower-heeled shoes and—when you just can't skip the high heels—purchasing good orthotic inserts with sturdy arch support. (It also helps to wear the right sneakers for your workout; see our top picks here!^[2] )

Mistake: You're a slave to one type of workout

Why it's bad: Your knees may suffer.

Any body part is at risk when you overtrain, but knees are especially injury-prone, says Halvorson. Knees can cave in toward the center of the body, especially when running, and put pressure on the knee joints. Using orthotics in shoes can help prevent your arches from collapsing and putting additional load on the knees. (Ease knee pain with these yoga moves.^[3] )

"It's very important to vary workouts and cross-train," says Halvorson. If all you do is run (and run and run), over time, soft and hard tissue structures start to break down faster from all the repetitive strain.

Mistake: You pick up where you left off after a break

Why it's bad: Your back may rebel.

Even if you killed it in Zumba three months ago, don't assume you can jump back in if you've taken a significant hiatus. Why? Chances are that during your break, your core strength was reduced, and a weaker core—paired with sitting down for extended periods of time—is the typical cause of back pain, says Halvorson. Your deep abdominal tissue ends up not being strong enough to support the spine, and the hips and butt muscles aren't strong enough to hold the pelvis in a neutral position.

Halvorson suggests easing back into things by strengthening and developing your core muscles and glutes.

Mistake: You don't properly fuel your workout

Why it's bad: You'll tax your immune system.

When it comes to your health, think of your body as a computer: Many systems run simultaneously, and your brain prioritizes them according to importance. "The immune system is secondary to heart functioning, so if you're not eating enough, your natural immunity is at a lower ebb," says Kleiner.

Plus, a Georgia State University study found that when you don't eat enough calories, your metabolism actually slows down and you burn fewer calories. Take the guesswork out of proper fueling with these 20 Perfect Workout Snacks.^[4]

Mistake: You don't let your body rest

Why it's bad: You'll lower your sleep quality.

If you can barely keep your eyes open all day but toss and turn at night, it could be a sign of overexhaustion, says Halvorson. Other clues that you're working out too much include extreme muscle soreness that persists for several days, unintended weight loss, an increased resting heart rate, interruptions in your menstrual cycle, or decreased appetite.

"Plan your rest as well as you plan exercise," says Polly de Mille, a registered clinical exercise physiologist at the Hospital for Special Surgery in Manhattan. Rest is a critical component of exercise, because it's when rebuilding takes place, she says. "If there's no balance between breakdown and recovery, then the muscle is in a state of chronic inflammation—and what may start as a simple case of soreness can turn into an actual overuse injury."

Mistake: You're trying to lose 5 pounds—again

Why it's bad: You'll overeat when stress hits.

Yo-yo dieting can cause binge eating, especially on high-fat foods, according to recent research published in the Journal of Neuroscience. Study author Tracy L. Bale, associate professor of neuroscience at the University of Pennsylvania, used mice to examine how stress systems were affected after dieting. The results? Repetitive dieting was shown to cause long-term changes in DNA that can make you more sensitive to stress.

Bale says her findings support the need to find healthy ways to reduce stress—like doing yoga and meditating—and the importance of stopping the deprivation-binge cycle.

Mistake: You go on an extreme short-term diet

Why it's bad: You'll gain weight when you stop (and be really miserable in the meantime).

If you ever tried a fad diet (e.g., the two-week cabbage soup diet or these 25 unbelievably bad pieces of dieting advice), you probably know that the weight always comes back. A 2007 study in American Psychologist reviewed the results of several long-term studies on subjects who followed various low-calorie plans and found that the potential benefits are too small—and the potential harms too large—for dieting to be recommended as a safe and effective treatment for obesity. Several other studies show that most dieters often gain back more weight than they've lost.