The science of weight loss is complex, but can be understood better by these 10 facts:

1.   When do we use fat and when do we use carbs.

 Through regular activities that do not increase the heart rate significantly, fat is the main source of fuel for the body.  If we go for a leisurely walk for example, we use mainly fat. If we increase the intensity and begin to speed walk or walk up hill, meaning increasing the heart rate, we will then begin to switch fuel sources from fat to carbs.

This doesn’t mean we stop using fat when we increase the intensity of our training session or activity. It simply means the source shifts to more carbs than fat. For example, if we burn 100 calories walking, we burn 60% fat, or 60 calories, and 40% carbs, or 40 calories. But if you burn 1000 calories lifting weights and the fuel source shifts, we are now burning 40% fat or 400 calories from fat and 60% carbs or 600 calories from carbs, so higher intensity wins. More calories burned, more fat burned.

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 2.     High intensity interval training (HIIT) has a better fat burning effect.

When lifting heavy with longer rest periods, we don’t have as much of a fat burning effect after the workout. On the other hand, when we use HIIT as our method of training, we burn over 20% more calories from fat, compared to 5% more calories when heavy lifting for up to 24 hours after the session.

3.     RMR (Resting Metabolic Rate)

Roughly 70% of our daily calorie expenditure is spent while we are at rest. About 20% is from exercise and around 10% is from the thermic effect of food. If we build more muscle mass our resting metabolic rate automatically increases, meaning we burn more calories at rest.

 4.     Reducing calories for extended periods of time leads to plateau.

What I see with many people looking to lose weight, is that they tend to keep their calories at that same calorie deficit after they have lost weight.  We must recalibrate the amount of calories we take in after we have lost weight. To lose fat without exercise, we need to take in around 8-10 calories per lbs of weight.

On days that we do workout, we need to take in about 10-13 calories. After about 2 weeks of calorie deficit, our body stops producing a hormone called leptin. Low levels of leptin inhibits the fat loss process. So, strategic cheat meals or days that go over your normal calorie intake, can help boost leptin that will then kick start the fat burning process again.

 5.     Sugar is the devil .

Anytime we put carbs or sugar in our body, we trigger an insulin response. An insulin response is when insulin is released into the blood stream to absorb that sugar we have ingested. When we put too much sugar in our system for the insulin to break down, it ends up storing as fat.

 6.     Glucagon

When our insulin levels are elevated, glucagon, our fat burning hormone is suppressed. This is why intermittent fasting (IF) is one successful way to fat loss. IF ensures long periods of no insulin being released into the bloodstream, meaning longer periods of glucagon thriving, meaning more fat burning.

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7.     Cortisol

Cortisol is a double edged sword. On one hand, when we are placed in stressful situations it triggers the release of adrenaline, which puts us in that fight or flight mode that sets us up for lifting, etc.

On the other hand, when we are continually under stress, the body is constantly releasing cortisol, which triggers the release of sugar from the liver, which in turn releases insulin from the pancreas, which causes more fat storage.  Not managing stress properly will eventually lead to fat storage.

 8.     Carnitine

Carnitine is a nutrient essential for fat burning. It is responsible for transporting fat cells for use as energy. The more carnitine we have, decreases the levels of glycogen used in high intensity workouts, meaning time to exhaustion is extended. This essentially means we can train for longer and burn more calories and fat.

9.     Caffeine helps burn fat.

Caffeine encourages fat cells to be released for fuel. That does not mean a 20oz  coffee before a workout. Keep it single digit ounces.

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10.  Spot reduction.

There is no such thing as spot reduction in science of weight loss. You can do 500 sit ups a day for the rest of your life and you will not see a six pack, so stop buying 7 minute abs products. You need to burn more calories than you take in. Doing full body exercises will help you burn more calories/fat. That fat will not be lost where you specifically want it. You will lose body fat all over. Once you lose that layer of fat from your abdomen, which will typically be last to go, will you then get your six pack.

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Pollyanna Hale Health and Lifestyle coaches
Lost 13 Kg in Total
Mel, 32y Location: London, United Kingdom Working with Pollyanna changed everything. I lost 13kg, got toned and have more energy than ever! Get same results!


Chriz Zaremba Fitness Consultant
Lost 45 Kg in Total
Chris, 50y Location: London, United Kingdom Lost 45kg after the age of 50 and now competes and wins physique competitions and runs marathons Check our weight loss plans