Nope. Dieting does not damage your metabolism. Thanks for coming out, that's all I have for you today. Just kidding! But seriously, dieting damages your metabolism about as much as painting your house ruins the walls. Dieting changes your metabolism but it does not damage it. In today's post (at the request of a reader!) we get to the bottom of what happens to your metabolism when you lose weight. 

In your worldly travels you have likely met someone who has lost weight. In rare circumstances you may have met someone who lost weight and kept it off. More likely, said person lost weight and regained it. They may have even gained more weight back than they ever lost. Yikes! I trust you, and I believe you are not naive enough to think your coworker/friend/relative did this on purpose. No one in their right mind goes through the struggle of losing weight only to choose to gain it all back. So why does it happen? If you are fortunate enough to have a body builder in your life (someone who actively competes in aesthetics), they have likely spun you a few yarns about their friend in the industry who "ruined" his or her metabolism by dieting for a show. If you are truly blessed, you will know someone who gets their jollies in by explaining, in detail, their weight loss/regain/loss "journey" with you. In all seriousness, a lot of people struggle with weight loss and I wish we talked more thoughtfully about it more often. Social media "transformation" photos are great, your coworker's impressive pre-prepared meals are great, but weight loss and more importantly, weight maintenance is really hard. Photos of meals in tupperware on Instagram don't tell the full story. 

LET'S START BY TALKING ABOUT METABOLISM

Your metabolic rate is the total amount of energy produced and used by your body per unit of time (Seeley, Stephens, & Tate, 2008). The amount of energy you put into your body should equal the amount of energy your body expends, otherwise you will gain or lose weight. Your body produces energy (ATP) at about the same rate it is used (Seeley, Stephens, & Tate, 2008). Since most ATP production involves the use of oxygen, metabolic rate is measured by estimating the amount of oxygen your body uses per minute. Your metabolic rate has three components: basal metabolic rate (BMR), the thermic effect of food, and muscular activity.

Your BMR is the amount of energy needed to keep your body functioning at rest (Seeley, Stephens, & Tate, 2008). BMR accounts for 60% of total daily energy expenditure (TDEE). Basically, your BMR is the number of calories your body needs to survive...even if all you did was lay in bed all day. Sounds like a win right? Yes, you actually need calories just to sit on the couch. There are a few factors that can affect your BMR. Muscle tissue is more metabolically active than adipose tissue (even at rest) so the more lean mass you have, the higher your BMR. Plug: do your resistance training! Increasing your muscle mass (not tone ladies) will increase the number of calories your body needs at rest! Do it, do it, do it. Men have proportionally more muscle mass and less adipose tissue than women and thus get to enjoy a higher BMR. Ever wonder why your boyfriend can eat a whole pizza and still have a six-pack? Meanwhile you glance at a pepperoni and your pants feel instantly tighter. Yep, another one of the joys of being female. Younger people will also have a higher BMR than older people because of increased cell activity during growth (Seeley, Stephens, & Tate, 2008). Those enjoying the symptoms of a raging fever will also experience an increase in their BMR. In fact, for each degree Fahrenheit increase in body temperature your BMR increases by 7%! (Seeley, Stephens, & Tate, 2008). Would not recommend this approach to boosting your BMR though... Finally, BMR can increase by 20% during pregnancy because of the increase in metabolic activity of the growing fetus (Seeley, Stephens, & Tate, 2008)! So if fever and pregnancy aren't available options for you, you're stuck with resistance training to boost your BMR. Don't make me say it...obviously I don't advise getting pregnant or feverish just to eat more...

The thermic effect of food is the energy it takes to ingest and digest food and accounts for around 10% of your body's TDEE (Seeley, Stephens, & Tate, 2008). Not all of the energy your body can derive from a meal is stored for later use. Some of the energy is used as the meal is digested and absorbed leaving slightly less energy than you took in for storage. Different nutrients have different thermic effects. Your body uses 0 to 3% of the energy from fat during digestion and absorption, 5 to 10% for carbohydrate, 20 to 30% for protein (Acheson, 1993), and 10 to 30% for alcohol (Westerterp, Wilson, & Rolland, 1999) . When you digest fat, more of it makes its way into energy storage because less of it is used in the breakdown and conversion process than for carbohydrate, protein, or alcohol. That doesn't make dietary fat the enemy, it just makes protein sound like a real pain in the a$$ for your body to digest and absorb. Be that as it may, high protein diets are important to maintain fat-free mass while dieting (Westerterp-Plantenga, 2003). The bottom line is that you have no control over the thermic effect of food. Meals that are rich in lean protein and carbohydrate have a higher thermogenic effect than fatty meals but the significance of thermogenesis in terms of energy expenditure pales in comparison to your body's main energy expenditure: muscular activity.

Your body's muscular activity accounts for 30% of your body's TDEE. The energy you lose through physical activity is the only component of TDEE that you can control. Sure, you can change your diet to eat more lean meats and less fat but you can't change the thermic effect of the food you consume nor do you have any say on what your body's BMR is (indirectly you can lose/gain weight to change it but you don't get to pick the number of calories it takes to power your body at rest!). The amount of energy expended through physical activity is dependent on the amount of muscular work performed and the duration of the activity (Seeley, Stephens, & Tate, 2008). Although listening to your spouse talk about work can make you feel tired....there is little muscular activity at play and thus limited energy expenditure...i.e. little to no change in BMR. Plug #2: lifting weights requires a significant amount of muscular activity and thus increases your metabolic rate! Hey now! You should lift weights! Now that you are up to speed, let's find out how and why your metabolism changes when you lose weight.

HAVE YOU EVER HEARD OF "SET POINT THEORY"?

The basic idea is that your body has a preferred weight and level of fatness. Your body, not unlike myself, hates change. From an evolutionary standpoint, this was a good thing. During long periods of energy deprivation (aka when the hunters sucked at hunting and the gatherers had to gather enough twigs and berries to feed the entire village....) it would have been handy to store energy as fat really efficiently. Nowadays, when you can't swing a cat without hitting a Tim Horton's, the advantage of being able to store fat in seemingly limitless quantities is not so hot.

When you lose weight, your body does everything it can to get it back.

Believe it or not, weight loss is not as simple as calories in < calories out. There are a host of genetic, environmental, and physiological factors at play. Whether you are a lean or obese individual, the amount of energy your body stores as fat is regulated by your central nervous system (Rosenbaum & Leibel, 2010). Your ability to maintain a lower body weight is governed by powerful neuroendocrine, metabolic, and autonomic system responses (Rosenbaum & Leibel, 2010). The actual phenomenon some people refer to as "metabolic damage" is likely what is known as adaptive thermogenesis.

Basically, when you lose weight your body responds by reducing your energy expenditure. A 10% reduction in body weight in lean or obese individuals results in a 20-25% decrease in daily energy expenditure (Leibel, Rosenbaum, & Hirsch, 1995; Weigle, Sande, Iverius, Monsen, & Brunzell, 1988). This means that someone who diets down to 150lbs will require less energy to stay at 150lbs than someone who has always weighed 150lbs. Your body actively opposes weight loss. Long-term studies of subjects who were able to sustain their new body weight still had a reduced energy expenditure up to 7 years later (Leibel, & Hirsch, 1984; Van Gemert et al., 2000; Weyer et al., 2000). In fact, the more weight you lose, the greater the reduction in your metabolic rate (Camps, Verhoef, & Westerterp, 2013).

Let's pause for a moment.