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BMI and BMR posted: Mon 2017-06-19 17:30:03 tags: fitness
When I quit smoking in 2007, I started putting on weight to the tune of about 10 lbs per year, peaking around 210#. When I met Gamer Girl, I joined her in learning more about weight management and fitness.

The two paramount concepts I learned were BMI and BMR. Body Mass Index is a table-based lookup correlating weight and height. If you're not a bodybuilder, it is an approximate indicator of how much of your body mass is unnecessary fat. (If you are a bodybuilder, your exceptional lean muscle mass could falsely suggest you're overweight, but that's a pretty rare problem.) This NIH web page has a fine BMI calculator. In my case, it tells me a) at 5'9.5" and 190#, I'm overweight (BMI 27.7), and b) the top of my "healthy" weight range is 171# (24.9 BMI), so that's my target weight. Pound for pound, it's healthier to be overweight than underweight, so I'd be ecstatic to hit my pre-smoking-cessation weight of 160ish, but I'd rather tone up what I have than weigh any less than that.

Basal Metabolic Rate is a bit more complicated. It's the number of calories per day required to sustain a given body weight at an inactive activity level. This BMR calculator takes into account height, weight, age and gender to predict base daily calorie needs. Any additional activity increases calorie needs. With my stats, my BMR is 1800 calories per day. My sedentary activity level probably bumps my total daily calorie needs up to about 2000.

Weight loss is ultimately a result of calorie deficit. The conventional calculation is that a pound of body fat represents about 3500 calories, and 2 lbs per week is the standard advice for "safe and sustainable" weight loss. Doing the math, we see that means a 7000 calorie per week deficit, or a nice round 1000 calories deficit per day. So if my daily needs are 2000 calories and I want to lose 2 lbs per week, then I need to limit my intake to 1000 calories per day. If I increase my activity level then I can (and should) bump up my minimum calorie intake accordingly.

Severe calorie restriction (starvation) is known to trigger dangerous stress hormone levels that cannibalize muscle and organ mass in order to protect fat storage. The 2 lbs per week rule is deemed "safe and sustainable" because it allows a calorie intake well above that starvation-hormone-trigger level. Barring serious gland disorder, a moderate calorie restriction diet will not trigger the starvation hormone sequence.