1200 calories per day diet

1. You aren't eating enough.
You may need to bump up your calories to stoke metabolism. When you dip below about 1, 200 calories per day, not only are you not eating enough to get all your nutrients, but your body slows your metabolism in order to hold on to precious calories, says Christine Gerbstadt, MD, RD, author of The Doctor's Detox Diet.

Also, if you skip meals to lose weight, your body could lose its ability to feel full. Blame evolution, which has designed our bodies to resist famine and not the buffet table. For example, if you skip breakfast, the body assumes food is scarce. You need a morning meal to let your body know it's okay to burn calories. "Within 1 hour of waking, you should consume a 350- to 500-calorie breakfast, with 10 to 15 grams of protein and fiber to stoke the metabolic fire, " says Gerbstadt. (Make your morning meal count with these 12 healthy ideas.)

2. You reward exercise with food.
Burning 300 calories during a workout is cause for celebration...but rewarding yourself after exercise with a high-calorie treat doesn't add up to weight loss. You're likely to overestimate how much the workout burned off and underestimate how much you ate. "Even if you're just working out for well-being, you still have to keep calories in check, " says Heidi Skolnik, author of Nutrient Timing for Peak Performance.

3. You slurp diet drinks.

Photo by Sam Bloomberg Rissman/Getty Images

Research suggests that diet drinks may backfire: The taste of something sweet without the calories can cause your body to hold on to calories as fat. In one study, diet-soda drinkers had a 178% greater increase in waist circumference over 10 years, compared with non-diet-soda drinkers. "Artificial sweeteners can actually raise your insulin levels and lower your blood sugar, which may stimulate hunger and move existing calories into storage in your fat cells, " says Sharon P. Fowler, MPH, one of the study's coauthors. Plus, fake sweeteners may not quell a craving like real sugar can, because sugar triggers a longer dopamine release. So even after downing two Diet Cokes, you may still want the candy bar. (If that's not enough to make you skip the soda, these weird side Effects of diet soda surely will.)

4. Your friends are heavy.
Your chances of being overweight or obese increase half a percent with every friend in your network who is obese, finds a recent study from Harvard. That more than adds up: Your chances of obesity double for every four obese friends you have, say researchers. Even if that friend lives thousands of miles away, your chances of gaining weight still go up, according to a New England Journal of Medicine study. That may be because your perception of being overweight changes—living larger seems acceptable since the heavy person is a friend. (Interestingly, having an obese neighbor that you don't know does not raise your risk.) Experts also think that a person's lifestyle and behaviors can subconsciously rub off on those in the individual's inner circle.

But you don't have to ditch overweight friends to lose weight. In fact, if you embark together on an exercise plan, you can increase your fun and calorie burn: Research from Oxford finds that exercising with friends as a team can actually make the agony of exertion less intense. The same hormones that are released during social bonding, endorphins, also help quell pain. And once a friend starts to lose weight, you have a greater chance of losing as well (the mechanisms work both ways).