I know it’s rude to bring this up during the holidays, but it’s not that dress or those pants that make our collective rumps look big.  We all know we’ve gotten fatter, and it’s happened remarkably fast.

Although many reasons have been offered, the epidemic of obesity remains a bit of a puzzle.  As Cutler and colleagues note in their excellent analysis of why we’ve gotten so chunky, our collective weight gain is consistent with an average daily increase of 150 calories.  Such a dinky number could be the result of any number of causes, which is part of the reason we’re still a bit in the dark about what’s up with our fat.

Cutler et al rule out a number of factors as likely causes of the obesity surge:

• The cost of calories has dropped. True enough; food inflation has lagged behind non-food inflation.  However, obesity is negatively associated with income (especially for women), and has increased significantly among the poorer groups despite a relatively flat income.
• Fast food. Studies in this area go both ways; for example, a 2003 Yale study concluded that kids who visit fast food restaurants eat about 187 more calories daily.  This sounds bad, but we have to remember that it’s extremely likely that families that eat more are overrepresented among fast food clientele.  In other words, it’s possible that fat people cause fast food. Other clever studies have failed to find an association between access to fast food restaurants and obesity, and the most recent thorough review of the literature concluded that the question remains open.  Cutler et al use food diaries to show that increases in consumption of fast food are pretty much offset by decreases in calories of food prepared elsewhere.  Tellingly, those diary studies show whopping increases in snacking, however.
• Inactivity. Kids these days, right?  Cutler and team looked at activity diaries and poo poo this one as well.  Although overall energy expenditure (calories burned) dropped between 1965 and 1975, it’s been pretty much flat since then.  We do watch more TV (and surf the web), but we also sleep less and engage in more sports and walking.  (By the way, this doesn’t let you off the hook for exercise — it just means that a sedentary lifestyle may not be the main cause of obesity in the US.)

So if it’s not access to cheaper, faster food by sedentary, lazy people, what’s making us fat?  Here are some possibilities:

• Meal preparation is easier than ever. This is Cutler’s main argument; as the cost associated with food preparation drops, the number of meals (i.e., snacks) increases and the type of meal shifts.  (I wrote about this earlier this year.)  Microwave ovens make it easier to eat popcorn (or worse), so we do.  Frozen tater tots are a snap as a side dish with that meat loaf but not worth the effort if you have to make them from scratch.  If you think this is hooey, consider this: nature’s ready-made snacks are fruits and veggies.  Better food preparation technologies have made it just as easy to open a bag of chips as to peel a banana.
• Fat is the new black. Okay, that’s an overstatement.  But as the prevalence overweight and obesity increases, the chance your weight makes you the outlier in your social group drops.  In other words, the social cost of being heavy has decreased over time.  Christakis and Fowler’s network analysis of the Framingham data suggest that having an obese friend is a bigger risk factor for obesity than having an obese wife or sibling.  This is exactly the kind of effect that could explain changes in rates of overweight and obesity that look like an epidemic: you “catch” it from your friends.
• An unhealthy lifestyle is cheaper than ever. The widespread use of medications to treat hypertension, high cholesterol, diabetes, and heartburn have fundamentally changed how we manage these conditions.  And the ability of these medications to prevent heart attacks, stroke, kidney failure, and other serious problems is a godsend.  But these medications also make it possible to achieve better health outcomes without relying on better diet and more exercise.  It’s quite possible that we’ve unintentionally made unhealthy lifestyles less of a problem, and that weight gain is a side effect.

So what can we do?  The basic in-out equation still holds: if you want to lose weight, you need to decrease calories in and/or increase calories out (i.e., activity).  So watch what you eat and exercise more (check with your doctor about what’s right for you).

Beyond that, consider the following:

• Cook more meals from scratch. I know this takes time and effort… that’s the point.  Increase the cost (i.e., time and effort) of the meals you eat, and you will eat fewer meals.
• Load up on nature’s snacks. You’re wired to minimize upfront costs.  Don’t fight it; make sure you have fresh fruits and ready-to-eat veggies on hand.  Make them easier to get to than prepared snacks such as chips and cookies.  (And consider stapling those bags of chips and cookies shut - anything you can do to create a hesitation factor is to your advantage.)
• Get rid of your microwave oven. I know, you only use it to heat water for your organic chai tea and to defrost your frozen edamame.  But do you remember how awesome you looked in the late 70s?  Back then, only 8% of American households had microwaves; in 1999 that number had risen to 83%.  Obesity is far less common in Italy than in the US and the UK; less than 14% of Italian households have microwave ovens, but 66% of British households do.
• Hang out with people who look the way you want to look… especially if it’s awkward. Do what you can to increase the social costs of being overweight.  Be careful not to overdo this; your happiness and sense of belonging are worth a lot.  But consider getting a bit outside your comfort zone.  (By the way, all of my social interactions are awkward, so I shouldn’t have a problem here.)
• Use your medications as one of several tools, not a shortcut. ALWAYS take your medications as prescribed by your physician: better to be a bit chunky and avoid the heart attack than to be thin and dead.  But take seriously your role in managing any health conditions you might have using all the tools at your disposal: better diet, more exercise, less booze, better sleep habits.  Ask your doctor about whether and how healthier behaviors can work to your advantage.  I bet you find you’ll drop some weight in the process.
• Do whatever it takes to make exercise as fun as possible. Buy a Wii Fit.  Get rid of the TVs in your house except for the one in front of your treadmill.  Watch mindless drivel but only when you’re working out.  Find a guilty (but calorie-free) pleasure and figure out how to embed it into your workout.  Not a treat you give yourself afterward; something you can include as part of the workout.
• Lower your exercise goals. Actually walking one mile four times a week is much better than planning on walking three miles every day.  Lower the bar: you’ll find you’re more likely to get going, and once you’re going you can always do a little more.
• Admit that this will take time. You’ll feel all the costs of your discipline upfront and the benefits will come a lot more slowly.  There’s no way around this.  We’re wired for an environment of scarce calories and an uncertain future, which means we tend to eat as though there’s no tomorrow.  You’ll try, you’ll succeed, but you’ll also fail.  Keep at it, and I swear you’ll lose some weight.

Happy holidays.  And here’s to a wonderful 2010 for you and your loved ones!