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What Changes can I Expect During
Weight Loss in my Energy Needs?

You hear a lot about how dieting lowers your metabolism (resting metabolic rate [RMR]), as if that's bad and wrong. Here's the real deal.

It is NORMAL and expected for your RMR to get smaller as your body gets smaller.

First, metabolic needs are predominantly based on body mass (weight). Second, body composition--the relative amounts of lean-body mass (muscle and organs) versus fat mass add a little bit more accuracy to prediction of metabolic needs.

It is also NORMAL (but not necessarily expected) to have a small suppression in RMR secondary to energy restriction.

It’s not necessarily expected because we don’t see it in everyone. Rather it seems to be related to the magnitude of energy restriction.[i] In other words the bigger the difference is between your body’s current maintenance-calorie requirements, and the weight loss calorie level that you choose, the higher the likelihood of a minor suppression in RMR.

For most people—since they're eating 1,200 or more calories per day—that drop won’t be more than 5%. For the minority of people that eat fewer calories (which should only occur under medical supervision) the reduction may be 5- to 15%.[ii]

In neither case will weight loss STOP, or stall, beyond what’s explainable by those changes. Under no circumstances will EATING MORE be necessary. Not everyone will experience this reduction in metabolism, but for those that do it dissipates a week or so after they resume a calorie intake level that matches their current body weight (and activity level).

Fluctuations in body water, which may be related to salt sensitivity or variations in carbohydrate intake, can lead to shifts of a few or even several pounds on the scale. What's important is to trust that your body works just fine (it does), and not to let the scale distract you! (See my article on weight loss plateaus for more details.)

Also see:
Choosing a Calorie Level for Weight Loss?
How Much Physical Activity is Needed for Successful Weight Management?
How do I stabilize at my goal weight?

[i] Prentice A, Goldberg GR, Jebb SA, et al. Physiological Responses to Slimming. Proc Nutr Soc. 1991;50:441-458.

[ii] Poehlman ET, Melby CL, Goran MI. The Impact of Exercise and Diet Restriction on Daily Energy Expenditure. Sports Med. 1991;11:78-101.