For years, I lived with a boyfriend who ate the exact same things I did.
If anything, he ate better than I did. He didn’t have my sweet tooth. I couldn’t resist a few cookies a day, while he snacked on pumpkin seeds and tomatoes. Yet he put on weight and I didn’t.
How does that work?
How can two people, eating much the same, with similar exercise habits, both working in sedentary jobs, end up with vastly different weight issues?
If anything, I was the one who should have been putting on weight. I was half his size. I was female! Everyone knows women carry more fat than men. 
So I did some investigating.
And what I found is that how much you eat and how much you move is just the tip of the iceberg.
There are many reasons you might not be losing weight. Here are 5 of the most surprising.
1. Central heating
How often do you get cold? Really cold?
How often do you get hot? Really hot?
If the answer for both is, “Not often,” then the ambient temperature may be hindering your efforts to lose weight.
Before the days of central heating and air conditioning, our human bodies had to work hard at feeling comfortable. Those hats and gloves were more than fashion accessories; they fought off the cold. Our bodies worked overtime to sweat us cool and shiver us warm.
All that work burned calories. Without lifting a finger.
Shivering boosts your metabolism, burning about 100 calories in 15 minutes. The Atlantic claims that “just enduring winter weather counts as exercise.” 
So turn down the thermostat to 60 degrees, or get outside more often. Burn calories instead of burning through your power bill!
2. Staying indoors all morning
When are you most likely to get outside:
In the morning or in the evening?
If you tend to get outside in the evening, then you might want to change your routine.
A study found that being an early bird was associated with a lower body mass index (BMI)—the earlier the better.
Subjects who enjoyed early morning sun exposure had a lower BMI than subjects who got outside a bit later in the morning. Subjects who didn’t get outside until after work weighed in heavier. And let’s not even talk about those who never got outside!
Natural sunlight beats caffeine in waking you up naturally and regulating your circadian rhythm, which helps you stay active all day and wind down at night. The optimal amount of sun exposure is 20 to 30 minutes each morning. Unfortunately, sitting at a sunny window doesn’t count.
It turns out those early morning joggers were onto something!
If you can’t fit in a workout before going to work, consider parking a short distance from work and walking the rest of the way. Have breakfast outside on your porch, or use your morning coffee break as an opportunity to get outside the office.
3. Your meds
Taking birth control, antidepressants, corticosteroids, beta-blockers, or diabetes medication?
Then the reason you can’t lose weight may be as close as your bathroom cabinet.
Certain medications make you gain weight. In fact, Dr. Ken Fujioka of the Scripps Clinic Nutrition and Metabolism Research Center believes that 1 in 4 of his patients gain weight because of the medication they’re on or a medical condition. 
If you’re on medication and have found yourself gaining weight unexpectedly, talk to your doctor. He or she may be able to prescribe a similar drug without the side effects.
4. You’re not eating enough
You heard me right. Eating too little and overtraining can cause your weight to plateau.
Registered dietitian nutritionist Laura Schoenfeld explains, “I’ve never had a weight loss client who was actually overeating.” 
When you eat too little to sustain the level of activity you’re engaging in, your body goes into crisis mode. It slows down your metabolism so that you use fewer calories. You can even end up putting on weight, despite eating hardly enough to survive.
Eating too little can also cause you to feel chronically cold, lose hair, stop having periods, and feel emotionally volatile.
So make sure you’re eating enough fat, protein and carbohydrates to reassure your body you’re not starving. Leave the crash diets to the celebrities. Contrary to popular opinion, eating less is not always a virtue.
5. Your friends
Think of your 5 closest friends. How many of them are overweight?
Your answer tells you a lot about your own chances of keeping the weight off.
A 2007 study found that the people closest to you have a big impact on your weight. If one of your closest friends becomes obese, your risk of following suit jumps 170%.
Violet Owens of YouBeauty.com describes it graphically:
[O]besity is like a communicable disease, spreading from person to person, especially friends and family.” 
A 2011 study by Arizona State University explored why. It turns out that we’re remarkable mimics. We tend to do what our friends do, without thinking about it. 
For example, imagine that you’re having a meal with your best friends. All of them have seconds. Will you be the only one to put down your fork?
Now imagine that, instead of remaining at the table for a second helping, one of your friends pushes her plate aside and announces she’s going for a walk. Who wants to come with her? Will you go?
None of you are thinking about weight at all. You’re not making decisions based on your weight loss goals. You’re just enjoying each other’s company, and part of being together is following one another’s lead.
That doesn’t mean you have to ditch your friends to lose weight.
It just means that you need to step into a new role. Be the one who suggests going for a hike instead of going for a drink. Your friends will thank you!
Other Weight Loss Barriers
These 5 barriers to weight loss are just scratching the surface.
A number of other factors could be preventing you from losing weight, including chronic stress, insulin sensitivity, leaky gut, insomnia, hypothyroidism, your weight set point, perimenopause, and so forth.
As for why my guy gained weight while I didn’t, I have my suspicions.
I was an early bird while he was a night owl. I worked in a freezing home office while he enjoyed corporate central heating. I relaxed by doing housework while he relaxed on the sofa.
Lifestyle matters more you realize.
So get healthy first. Focus on sleeping well, eating healthful foods, getting outside every day, tackling any medical conditions, and learning to handle stress with grace.
As you feel better, you’ll make better choices—and hopefully starvation won’t be one of them.