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How to Lose Body Fat (According to Science)

There seems to be a lot of confusion around the topic of fat loss and it comes as no surprise. There’s tens of thousands of people and businesses out there trying to sell you the ‘best’ fat loss diet, weight loss pills, diet hormones, etc. The list is endless, so I fully appreciate how confusing it might be for some people.

But let’s go back to basics… The cold hard truth is to lose weight you must start by obeying the first law of thermodynamics – energy can neither be created nor destroyed, it can only be transferred or changed from one form to another.


The food you eat contains energy, which can be used for two things. It will either be used to power your body – heat, movement, organ function etc – or stored in your body as fat, muscle, glycogen and any other tissues you have. Assuming you digest and absorb the food properly of course!

This means if you’re using more energy than you’ve consumed – at any given point in the day – your body needs to top you up by tapping into your reserves of stored carbohydrate (glycogen in muscle and liver) and fat (stored as adipose tissue around your body).

This means over the course of the day you have a fluctuation of fed states after meals, where your body is in a temporary net storage mode, and unfed states, where your body tops you up until the next fed period by tapping into energy stores.

If the energy of all your fed periods is less than the energy used during unfed periods, then you lose weight.

This also means the amount of meals you eat doesn’t have an impact in this regard because a bigger meal sends you into storage for longer, but you also have longer periods without being in storage. Whereas smaller more frequent meals don’t push you as much into storage, but do so more frequently… the net effect on body weight is the same!

Calories matter – they are the key determinant of weight loss.


All this is not to say food choice, or other factors such as sleep, don’t matter. Weight loss isn’t really the desired effect after all. You want to lose fat and retain muscle. This is where protein comes into the mix.

Protein not only helps protect muscle mass – it’s the most filling type of food, assists with dietary adherence and has the highest ‘thermic effect of food’.

To digress slightly, one of the most confusing aspects of the calories in vs calories out (CICO) view of weight loss is it’s a lot more dynamic than people think, which means people think the laws of thermodynamics are violated. This isn’t the case, CICO is just more variable than we often think and TEF is an example of that.

The calories we eat can directly (and profoundly) influence the calories out!

Protein has the highest TEF of all the macronutrients. This means around 1/3rd of calories you consume in protein (calories in) are ‘lost’ in digestion and absorption (calories out). Thus, eating 1000 kcal of protein has a net storage potential of around 700 kcal. CICO is still true, it is just more dynamic than people give it credit for.


This is where the ‘real’ debate often lies when it comes to weight loss. Many people believe carbs make you fat and this is often because intake of carbs causes a release of insulin. Insulin is a storage hormone that causes the increased storage of carbs as glycogen, but also prevents fat tissue being broken down and used for energy.

So, the theory goes, if you keep carbs and insulin levels low then you can break down and use more fat. And as dietary fat doesn’t increase insulin, there’s potential to lose more weight.
The big problem with this idea is it completely ignores overall energy balance – you can’t just ignore calories from fat!

Let’s think about your storage breakdown in the fed and unfed state. If insulin was solely to blame, then you could eat as much fat as you wanted and never go into storage mode. But where would all the energy you’re getting from that fat go?

It is still energy, so it would be stored in the fed state, and there are pathways in your body that do this efficiently and effectively without insulin being present! No matter what you eat, in the fed state you are storing carbs, fats or whatever you’ve eaten and in the unfed state you are tapping into energy reserves such as body fat. It always comes down to energy balance.

To crush one final myth on this subject, under normal circumstances carbs are not stored as fat. Yes, they contribute to fat storage if part of a daily calorie surplus due to actions of insulin. But most of the extra calories you store are likely to come from excess dietary fat… but why?

This is because in a high carb diet the body then ‘burns’ more carbs for fuel and preferentially stores fat as it is easier and more efficient to store than to convert carbohydrates to fats.

This is the same when you have a high fat diet – your body switches to burn more fat because that’s the fuel available, but if you consume more fat than the body is using for energy you still gain body fat.

The body is very clever at not wasting the energy you consume. It would be silly from an evolutionary perspective not to store excess calories incase of a future food shortage.


Although food=fuel we also have to consider how easy it is to overeat on certain foods, how filling foods are and whether they’re packed full of fibre, vitamins and minerals to keep us healthy. This means ‘by the numbers calorie counting’ will work for weight loss, but you also need to consider food choices to help keep you feeling full, prevent binging behaviours and stay healthy in the process.

As a solid set of ‘rules’, this means sticking to lean protein sources (make this the focus of your meals), using whole grain, unrefined carbohydrate sources high in fibre and throw in some fruits and vegetables with small amounts of plant based fat sources (avocado, olive oil etc).

Processed fatty and sugary foods aren’t going to stop you losing body fat if you are in a calorie deficit consistently each day, but they are potentially going to cause overeating and make sticking to a fat loss diet difficult.

Keep these foods to the occasional small treat and on special occasions where food choice might be restricted to ‘junk’ try and factor this into your overall daily (or weekly) calorie allowances!



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