How Eating (or not eating) Really Effects Your Metabolism

By December 15, 2016 No Comments

The truth is, the thermic effect of food (the energy it takes to digest it) only accounts for about 10% of your caloric burn in a given day.

70% of your caloric burn is actually due to your resting metabolic rate; the rate at which your essential function and movement burns energy.

The remaining 20% of your daily caloric expenditure comes from other things, like exercise and other activity over and above primary functional movement.

I looked at the research and I’m going to try to make this as simple as possible for you to understand, so I’m not going to quote specific studies here.

But I am going to explain the findings and conclusions of a number of well-respected studies and show you how to make them work for you.


The Effects of Different Diets on Metabolic Rate and Fat Burning

The basic findings of the research I found was that of the three different types of diet/exercise lifestyles, low-calorie, diet and exercise, or high-calorie, there was no difference between the groups as far as metabolic rate.

In other words, whether you eat a low or high-calorie diet does not raise or lower your basic metabolic rate.
Let me explain why this is.

Your resting metabolic rate is based on your amount of lean mass or muscle.

Muscle uses calories, fat and bone do not. So the only way to lower or raise your metabolic rate is by adding or losing lean muscle mass.

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The #1 Way to Raise Your Metabolic Rate

What that means to you is that if you want to raise your base metabolic rate, you need to add more muscle to your body.

Let me tell you threes reasons why this is good news:

It means you can eat like a real human being and get the calories and balanced nutrition you need to be healthy and to be happy.
Adding muscle mass isn’t as difficult as some people make it out to be. It’s especially quick for women, and especially for women who are just starting out on a strength-training program.

It frees you from the hamster-wheel of cardio. Yes, cardio is good. Yes, it can burn some good calories. But if you’re doing an hour on the treadmill every day hoping to burn more fat, your time is better spent doing 30-minutes of strength-training instead.

[Related article: Worst exercise for women over 35 and what to do instead]


What this Research Means in Regard to Fasting

I’m a huge advocate of intermittent fasting, for several reasons that I’ll explain in a moment.

Now, let me first differentiate fasting from not eating.  Quite often when I talk to my lady clients they talk about skipping meals or forgetting to eat or just not eating throughout the day and bingeing on something late at night. Or just reducing down the calories to a decifit. This is not fasting.  Fasting is mindfully letting your body rest and reset by not consuming solid food for a number of hours.
But one of the biggest arguments against intermittent fasting has always been that it causes your metabolic rate to slow down.

Well, the research studies I’ve just mentioned have shown that NOT to be the case.
A very, very low calorie diet, such as with a fast, had no impact on basic metabolic rate. However, fasting does have a great impact on your ability to lose stored fat and it’s not because of the calorie deficit.

There are two states in the body; there’s a fed state and a fasted stated.

When you eat food you’re considered to be in the fed state for several hours, as your body is digesting the food.

In the fasted state you have no food to digest. The body is essentially just working on what it has, and it’s going through a bunch of processes.

In one respected study, they looked at fasting for up to 36 hours. What they found was that in the first 24 hours, there was a 70% decrease in blood insulin levels.
That is extremely important to your fat loss goals. Insulin is a storage hormone; it’s an anabolic hormone. It stores sugar and fat in your fat cells, the sugar and fats that your body doesn’t need for immediate energy. When insulin is high, there’s a lot of fat storing going on. When it’s low, your body is prompted to let go of the fat that’s already been stored.

The study showed that after the first 24 hours, there was a diminish in returns, so the more you fast, the less benefit to your blood insulin levels, so I don’t recommend fasting any longer than 36 hours. The other thing is, when we’re in a fasted state, our body is looking to move to a different fuel source, which is essentially using more fat as a fuel.

In the fasted state, your body is looking to conserve blood sugar, looking to conserve muscle glycogen, and it’s going to turn to fat and break that down for energy.

When insulin goes down, there’s another hormone that the pancreas produces and secretes called.

Glucagon actually helps to maintain adequate blood sugar levels in the opposite fashion that insulin does. While insulin takes sugar out of the blood, glucagon will break down fat or blood sugar stores and increase sugar in the blood.

During a fasted state, when insulin levels are lower, glucagon is increased because now the pancreas is signaling a need for more sugar in the blood.

So glucagon goes into the fat and muscle cells and break down stored fat so that it can be converted back into a usable form of energy.

No, You Won’t Lose Muscle Because of Fasting

At the same time, the adrenal glands will kick out cortisol and epinephrine.

Those two hormones will go to your fat cells and activate hormone-sensitive lipase, which is a specific enzyme in the fat cells used to break down fat and allow it to be metabolized for fuel. Additionally, fasting is one of the best known ways to increase human growth hormone, who’s role is to maintain and grow muscle (amongst other things).

[Related article: How to boost your metabolism to overcome thyroid and adrenal fatigue to become a fat burning machine]


Yes, it sounds odd, but not eating (for 18-24 hours) actually preserves your muscles by the surge in growth hormone that results from being in a fasted state.

This is why the scare-tactic that fasting will cause you to lose lean muscle is a fallacy. Your body is designed to turn to stored fat for fuel in the absence of glycogen. It’s only after being in a fasted state for several weeks that your body will then begin to catabolize or rob muscle of proteins to use as fuel.

By fasting for only 24-48 hours (and I think 24 hours is plenty), you’re dramatically reducing your insulin levels and activating fat burning as fuel. You’re not in danger of losing lean muscle.

By the way, one other way that intermittent fasting helps you with your fat loss goals is that it gives you a healthier sense of the importance of food and the accuracy of your craving for it.

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You learn to disconnect a bit from food as a source of pleasure, which can really help you the rest of the week, as you try to battle cravings and make good choices.

So this is why my 28 Day programme contains intermittent fasting and why I follow a 7 day cycle that includes a fast day every week from anything between 12-24 hours.

To find out more about this programme or if you have a question about fasting, comment below or click on the link for more information and join the 100s of ladies embracing this cycle to maximise fatloss and building lean muscle.

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Lisa Barwise

Author Lisa Barwise

Hi, I'm Lisa. I consider myself a Wellness Alchemist, the catalyst in the transformation of Strong Women around the world. Strong of mind, body and character.

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