Why Do I Gain Weight so Easily?

Losing weight is a difficult and long process for most people, but if your clothes feel a little tight when you even look at a cheeseburger combo, it can feel especially discouraging. Gaining weight easily might mean you have a slower-than-average metabolism, you need to readjust your perception of calories eaten vs. calories burned or you may have an underlying health condition. If you're struggling to keep your weight under control, consult a doctor to rule out a medical condition.

Take a closer look at your diet when you're gaining weight unexpectedly -- you might accidentally be taking in more than you think, which is contributing to your easy weight gain. People typically underestimate their calorie intake, including when they're eating out, reports a study published in BMJ in 2013. The study researchers fed adults, adolescents and children relatively high-calorie meals -- 836 calories for the adults, and 733 and 756 calories for the kids and adolescents, respectively -- and asked them to estimate their calorie intake. All three age groups underestimated their calorie intake, with adults underestimating their intake by 175 calories. And people tended to underestimate the calories in a healthier-sounding option -- like a sandwich -- compared to food in a burger restaurant.

Make sure you're accurately counting your calorie intake by measuring all the foods you eat, at least for a few days. While constant measurement and portion control take a significant amount of time, the effort can help prevent "portion creep" -- unconsciously increasing your portion sizes to trigger weight gain. If you're unknowingly eating even 175 extra calories per day, you'd gain about 1.5 pounds each month. To avoid accidentally eating too many calories at a restaurant, visit the company's website to check calorie content ahead of time.

Staying active is a great way to burn more calories, but overestimating your calorie burn during exercise can cause you to overeat during the rest of the day, so it may seem like the weight creeps on for no reason. Trusting your instincts on how many calories you burn -- and how many calories you need after a workout -- might make you overeat, according to a study published in the Journal of Sports Medicine and Physical Fitness in 2010. The study authors asked healthy women and men to participate in exercise sessions that burned 200 to 300 calories, then asked them how many calories they thought they burned and how much they'd need to eat to replenish those calories. They found that the subjects overestimated their calorie burn by 300 to 400 percent, and thought they needed two to three times more than the appropriate amount of food to recover after their workout.

Don't rely on the calorie burn listed on your exercise machine -- it might not be accurate for you -- and instead invest in a heart rate monitor to get more personalized insight into your calorie burn. And avoid guessing how much food you'll need in your post-workout snack -- try a small, high-carbohydrate snack like a glass of chocolate milk or chocolate soy milk, and count those calories as part of your daily intake.

If you've used an online calculator to estimate how many calories you need but you're gaining weight anyway, you might simply have a slower-than-average metabolism. While calculators provide a general idea of your calorie needs, based on your body size, age and activity, your actual calorie needs could vary as much as 10 percent from the average calorie burn, explains a review article published in a 2004 issue of Current Opinion in Clinical Nutrition and Metabolic Care.

For example, a 45-year-old woman who is 5 feet 3 inches tall, weighs 160 pounds and has a sedentary lifestyle needs an average of 1,884 calories to maintain her weight. That same woman with a higher-than-average metabolism, however, might need 2,072 calorie daily -- and conversely, if that woman has a slower-than-average metabolism, her daily calorie needs might be more along the lines of 1,696 calories daily.

If you're gaining weight even when you're sure you're eating the right number of calories and getting an accurate calorie burn, try reducing your calorie intake by 5 to 10 percent until your weight starts to level out.

If you've tried everything and you're still gaining weight, see your doctor. Certain hormonal conditions -- like hypothyroidism -- can affect your metabolism and make you gain weight, seemingly for no reason. Certain medications can also affect your metabolism, appetite or both, increasing your risk of weight gain. A doctor can diagnose any underlying issues, as well as refer you to a nutrition professional for help controlling your weight.