Demystifying Metabolism (Part 2): Diet

Metabolism has a mystic status – we hear about its power over our ability to gain/lose/maintain weight and we wonder how much of that power is in our hands. Technically (and simply put), metabolism is the sum of all the chemical processes that allows us to maintain life. But we just know that we want it to be higher.

I explained how exercise and muscle impact metabolism in Part 1 of this two-part series. So it’s time now to look at how our diet affects metabolism, without all the popular media hype and a focus on legitimate science.

A Metabolism Refresher

The energy expenditure for bodily functions is called your resting metabolic rate (RMR) (about 70 percent of your energy expenditure) and the energy it takes to move or exercise is the rest – lifestyle metabolism (about 30 percent). (For a full description, check out Part 1.) You can always increase your physical activity to impact your lifestyle metabolism and short-term RMR, so can you impact your RMR through diet?

Food is Simply Fuel for Your Body

There’s no voodoo about metabolism and eating – if you consume more calories than your body can use, it’s stored as fat. Does the content of your diet affect your metabolism? Yes, but only in terms of what type of fuel you’re providing it for energy.

Your body uses carbohydrates to fuel high-intensity anaerobic activities, such as fast running or weight lifting, and fat is used for lower intensity activities such as walking (however, there is no “fat-burning zone“). If you don’t eat enough calories (and protein) and you exercise intensely, your body eventually uses muscle protein for energy. For this reason, people who exercise intensely should avoid a low-carb diet (you won’t have enough energy for intense exercise anyway). A low-carb diet may be useful in the beginning stages of weight loss, but for weight maintenance, exercise – and carbs – are required.

What other parts of your diet might affect your metabolism? You can slow down your metabolism by eating too much or eating too little (but being overweight in and of itself doesn’t cause a slow metabolism – see the next section). So let’s look at several scenarios – eating too much, eating too little, and meal frequency.


Immediately after finishing off a humongous meal, your body starts secreting the hormone insulin and inhibiting the breakdown of a few other hormones. For the next few hours this hormone tells your body to “store, store, store!” [1] The result is that much of what you just feasted on will be stored as fat because the insulin has triggered a change in what’s used for fuel. Excess fat the body can’t use becomes body fat and excess protein is also stored as fat.


While feasting causes excess energy to be stored as fat, fasting actually “unleashes potentially damaging toxins to circulate throughout the body,” according to the National Academy of Sports Medicine [2]. Fasting breaks down adipose tissue and releases toxins found there into the bloodstream – not good. The body will also slow down metabolic processes to conserve energy – also not good. While people fast for many different reasons, if you are healthy and doing it to “cleanse” your body, think again. Fasting should be avoided for many health reasons, not just fat storage, and there is no evidence that “cleanses” or detoxes do anything your body can’t do naturally.

Very Low-Calorie Diets

For those trying to lose weight, it’s been proven that very low calorie diets trigger a “famine” response in the body, causing it to burn fewer calories for the same amount of work – that is, a slower metabolism. Most experts recommend against going below 1,200 kcals per day [3]. You may lose fat in the beginning of such a diet, but at some point your body will begin trying to hold on to it. And if you’re very fit and don’t eat enough, your metabolism may be slower than someone who does.

Frequency of Eating

Some suggest eating every two to three hours (high-fiber foods, lean proteins, and complex carbs) to raise metabolism, and this may work for some people. But there is no consensus on this issue and studies are conflicting.

Can Being Overweight Cause a Slow Metabolism?

You think you can’t lose weight because you have a slow metabolism – it’s a very common belief. But there’s no evidence to back this up, and many studies have shown no correlation between obesity and a low RMR. As Scott Pullen, a master instructor for the National Academy of Sports Science, says:

The preponderance of data available… suggests that obese individuals not only have a BMR  similar to non-obese individuals, it may even be greater.”[4]

The bottom line is that to lose weight, you must create a calorie deficit. When you lose weight, your metabolism actually decreases – your body has less mass to maintain and thus fewer calories to burn. So when people can’t lose weight but say they’re on a low-calorie diet, the main culprit is usually underreporting of calories.


[1] Insel, Paul,  Don Ross,  Kimberley McMahon,  Melissa Bernstein. Nutrition: Custom Edition, 4th Edition. Jones & Bartlett Publishers, 04/2010, page 297

[2] Insel, Paul,  Don Ross,  Kimberley McMahon,  Melissa Bernstein. Nutrition: Custom Edition, 4th Edition. Jones & Bartlett Publishers, 04/2010, page 297

13 thoughts on “Demystifying Metabolism (Part 2): Diet

  1. When I was trying to lose 100 pounds, it was definitely about eating to lose weight (i.e. eating processed foods that were low calorie and only caring about the calorie count). Once I lost the weight I changed my thinking of food. Food is fuel and as such I try my best to eat REAL food and not processed. My body definitely feels better for it.

    Figuring out how many calories to eat to maintain weight is something I’m still working. My range is between1500-2200 depending on my activities. Eating frequent small meals/snacks has helped me a LOT!


    • It’s a serious challenge for ANYONE to figure out their exact calorie intake. Kudos to you Lisa for relearning your approach to food. I also think of food as fuel, maybe too much so because I have trouble getting ENOUGH calories!


  2. I think fueling ourselves us definitely a learning process & I went thru many! 😉 The many small meals work for me but I know they don’t work for others.. some have trouble keeping calories in the right range cause they eat more then they think they are eating. People think they eat less than they do & they think they burn more calories exercising than they really do… can’t out train a bad diet! 😉

    I know it is controversial about fat cells still being there if you were fat when you were young & they expand if you eat too much. I tend to believe this as I was fat & I do gain easily… but the thing about this is that I really work hard to be the way I want to be. If there is a “set point” & I know people go back & forth on this too – me feeling there may be a sort of wet point.. I fight to work past it. I know my body really tries to stay at a bit higher weight than I am at but I am willing to do what it takes to break that barrier. That is it right there – it is a choice on what a person wants to look like, how much they want it & what they are willing to do to get there. I do my thing but I never expect people to be as crazy as I am! 😉

    Always a great post Suzanne!


    • No, I don’t know if it’s controversial. There was a study done showing that people who’ve gained and lost weight in the past do in fact have more trouble keeping weight off in the future. You are an inspiration – you show it CAN be done!!


    • Fasting does seem to be the current “solution” for staying lean, “detoxing,” etc. I agree that it makes no sense. But then, people will do all kinds of things to get around basic clean eating 😉


  3. Interesting facts, I haven’t known that it is not good to eat low-carb meals while intensive training and exercising. Now I already know that it burns only the muscles nut the fat. That’s why I prefer the cardio-training and spinning.


  4. Suzanne, I agree that the bottom-line of it all is having a calorie deficit, one hopefully achieved by making healthy food choices. When I’m trying to take off a couple of pounds and don’t record calories carefully, the pounds come off because I’m always “under-reporting” them in my mind!


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