Why you might pack on pounds If college is your first time away from home, it's probably your first shot at choosing your own meals. And while mom may have said no to pizza for breakfast, lunch, and dinner, she's not there to stop you from overeating at the all-you-can-eat cafeteria. Combine that with late night food deliveries and snacking during study sessions, and it's hard to believe that the freshman 15 isn't more like a freshman 50.
Stay thin "Keep a daily food diary so you become accountable for what you're eating," says Elisa Zied, RD, author of Nutrition at Your Fingertips. "You're probably logging many hours at a desk, so stash nuts and fruit nearby so you're reaching for healthy snacks rather than junk."
(Related: Discover how to lose up to 8 pounds in 3 days with The Belly Melt Diet)
Why you might pack on pounds The common cold has been linked to Ad36, a virus that could make you gain weight, according to a 2011 study from the Pennington Biomedical Research Center in Louisiana.
Stay thin Since researchers aren't exactly sure why the Ad36 virus is out to get your waistline, and a vaccine is still in the works, all that scientists suggest right now is to wash your hands frequently or eat these foods that fight sickness.
(Related: 7 Causes of Weight Gain You Can't Control)
Why you might pack on pounds Stress raises levels of cortisol, a hormone that can make you feel hungry, according to a recent study published in the American Journal of Epidemiology. Plus, if you make a habit of eating when you feel frazzled, your body may begin to link stress with needing food.
Stay thin Choose an activity to replace stress eating, says Jason Block, MD, lead study author and assistant professor at Harvard Medical School. It should be something that can relax you at least momentarily. For ideas, check out the best workouts to banish stress.
Why you might pack on pounds You're going out to dinner with your new boyfriend or girlfriend instead of whipping up meals at home. You're also not as concerned about the way your body looks because you've already found love.
Stay thin If you're a woman and want to eat the same foods as your metabolism-busting man, aim to eat about a third less of your meal than he does, says Lisa Young, PhD, RD, author of The Portion Teller Plan.
Why you might pack on pounds When you're depressed, it's easy to turn to Drs. Ben and Jerry to make yourself feel better, says Young.
Stay thin For the first few days, it's okay to spend time on the sofa, but curl up with a bowl of low-calorie frozen yogurt instead of ice cream. And then get out of the house so you're not home alone with the fridge. Bonus: If you venture out, you may meet someone new. (Or you could try to meet a healthy match online.)
Why you might pack on pounds Eggnog, mashed potatoes, stuffing, Christmas cookies, and potato latkes are just a few of the high-calorie foods that are probably on your "to eat" list every November and December, says Rania Batayneh, MPH, owner of Essential Nutrition For You, a nutrition consulting firm. The average person gains a pound during the holidays and never loses it, according to a study published in the New England Journal of Medicine. (Search: Healthy holiday recipes)
Stay thin Sip sugar-free hot cocoa spiked with Peppermint Schnapps over eggnog, and for dessert, pick pumpkin instead of pecan pie to cut calories in half. If mindless munching at holiday parties is your downfall, carry a drink in one hand and a clutch in the other so you won't be able to pick at the spread.
Why you might pack on pounds You did the wedding dress workout plan, your big day has come and gone, and now you can relax. "You're comfortable, and you might think to yourself, 'Well, I am married now. I don't need to impress him anymore,'" says Nicole Glassman, holistic nutritionist and owner of Mindful Health in New York City. Also, if your schedules differ, you may eat a large snack before dinner--or eat dinner twice--in order to dine with your spouse. And who wants to get out of bed to hit the gym?
Stay thin Working out with your partner gives the two of you time to bond and keeps you both motivated, says Glassman. When you're eating with your special someone, pay attention to your own body's needs and recognize that your partner's food requirements may be vastly different than yours.
Why you might pack on pounds When you sit all day, your body tends to swell and become fatigued, says Glassman. And when you're tired, you're more likely to turn to the vending machine conveniently located a few feet from your desk. Your pick-me-up may provide a quick boost of energy, but you'll crash shortly after and might become too tired to go to the gym after work, says Glassman.
Stay thin Take a 20-minute walk during lunch to boost circulation and ward offer afternoon junk food cravings. Staying hydrated will also combat fatigue and help your body move fluids out of your system. Aim to drink eight glasses of water daily. Finally, keep healthy snacks, like almonds and apples, in your desk drawer so you don't turn to sugar. (Video: Outfit Your Desk for Health)
Why you might pack on pounds You need to shed the baby weight, but you've temporarily lost your workout mojo. Post-pregnancy hormonal changes can make you weepy and unmotivated, says Rallie McAllister, MD, co-author of The Mommy MD Guide to Pregnancy and Birth. Weight loss is put on the back burner--sometimes for years. In fact, moms gain an average of 20 pounds in just 10 years, according to a study in the American Journal of Preventive Medicine.
Stay thin Take a break from mom duties to enjoy your favorite form of exercise and you'll remember that you had a life--and a body--before baby.
Why you might pack on pounds When your 20s are over, your muscle mass--and metabolic rate--begin to decrease. Muscle tissue torches calories even when you're sitting, but the less of it you have, the fewer calories you'll burn, explains Keri Glassman, RD, owner of Nutritious Life, a nutrition practice in New York City.
Stay thin Compensate for muscle loss by understanding that you're not going to be able to eat like you did in your 20s, Glassman suggests. Also try adding an extra workout each week or cutting out a dessert to make up for the extra calories your body won't be burning. You can also work this exercise routine into your mix--it was designed for people in their 30s.
Why you might pack on pounds When you're trying new foods, ditching exercise for sleeping in, and not sticking to a routine, a getaway creates the perfect storm for weight gain, says Batayneh.
Stay thin Picking a rental property over a hotel means you'll have an opportunity to cook meals at home, which can cut down on your daily calorie intake. When you're dining out, choose one item to indulge in, whether it's a margarita or freshly baked bread. Finally, pack healthy snacks such as a Kind Nuts & Spices bars (each has less than 5 g of sugar along with 5 to 7 g of protein and fiber) in your bag so you don't go overboard when you need a little nosh. Here's more advice on how to stay fit on vacation.
Why you might pack on pounds When you're chilled to the bone, eating can be one of the fastest ways to warm up. Digesting food increases your metabolic rate and generates heat, McAllister says.
Stay thin Just because you're hungry or cold doesn't mean you need a 450-calorie latte. Warm up with a bowl of chicken or vegetable broth-based soup or a cup of tea.
Why you might pack on pounds "I get so much sleep," said no mom ever. Being tired is a way of life for many new mothers, but it can do serious damage to your waistline. Research has linked a lack of sleep to weight gain, as sleep deprivation drives down levels of the appetite-suppressing hormone leptin while increasing levels of ghrelin, a hormone that stimulates hunger, McAllister says.
Stay thin If eight hours a night isn't going to happen, make sure you get the best quality of sleep possible. Use darkening shades, buy a white noise fan, and keep the temperature in your bedroom at 66 degrees, which has been shown to optimize sleep.
Related: 5 Ways to Sleep Well and Lose Weight
Why you might pack on pounds It doesn't get dark as early, so you're probably eating dinner later or snacking into the wee hours of the night. You could also be throwing back beers at a baseball game or sipping sangria on a deck, and those calories add up quickly, Zied says.
Stay thin You don't need to give up ice cream or barbecues, just don't indulge of in all of summer's tempting treats in one day. But it's all about calories in and out, so don't do both on one day. And we hate to be a buzz-kill, but if you find that you're always gaining weight during the summer, you may want to do a calorie count to figure out when you have room in your diet to splurge on a hot dog -- and when you need to stick with a salad.
Related: Simple Swaps for Summer Weight Loss
Why you might pack on pounds The hormonal changes that happen during menopause make women more likely to gain weight in their abdominal areas, Keri Glassman says. In fact, women burn about 200 fewer calories per day when they're post-menopausal, according to a study published in the International Journal of Obesity.
Stay thin While the average person may burn 200 fewer calories daily, your experience may be different, notes Keri Glassman. Slowly reduce your calorie intake or increase your exercise frequency until you stop gaining.