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Is Cardio Alone Enough for Fat Loss?

When the average person thinks of training for fat loss, they think of cardio, also known as cardiovascular training, like jogging, running, walking, etc. Let’s see if cardio really helps with fat burning (body fat loss).

Most people do cardio because they believe it burns fat. The actual amount of fat that is oxidized (the scientific term for burning fat) during a training session is minimal. You’re looking at grams of fat, and the total amount of grams is generally closer to having one digit than three digits.

Let us understand how our body utilized body fat as an energy

When we are talking about body fat, we are actually talking about triglycerides. Fat is stored in our bodies in the form of triglycerides. A triglyceride is made up of 3 fatty acids that are held together by a glycerol backbone (hence the name “tri-glyceride”).

Fats enter the body through this structure via food but must break up into their individual parts to pass through the body’s digestive wall. They then reform into triglycerides as they leave the digestive wall.

This body fat is stored in your body fat stores, also called adipocytes. These cells hold the majority of your body’s fat. Some amounts are also stored in your muscles, and some are found floating in your blood.

How does the body burn fat for energy?

Your body stores fat in the form of triglycerides. But it cannot be used as a source of energy (to burn fat) in that form. Your body needs to break this triglyceride into fatty acids, which can be used as a source of energy by the body, thereby helping you burn body fat.

Fat burning, also known as “fat oxidation,” is thus a three-step process:

  1. Lipolysis: This is a term used when the stored triglycerides in adipocytes (fat cells) are broken down into fatty acids and glycerol.
  2. Transportation: The broken fatty acid then gets transported to the cells.
  3. Oxidation: Once the fatty acids are inside the cells, they should get oxidized by the mitochondria. (Mitochondria is the powerhouse of the cell.) remember?) to be converted into energy.

If the transported fatty acids are not used (Step 2), they again get converted into triglycerides and stored in body fat cells (adipocytes).

Your body normally prefers to use fat as fuel for everyday tasks. The reason for this is that it is a simple way for the body to use it. Also, when it gets converted into energy, it yields double the energy compared to other sources (carbohydrates or proteins). Specifically, it oxidizes the fatty acids floating around in your blood, which can come from your body fat stores or a prior meal.

Aerobic vs anaerobic process

Aerobic: The term “aerobic” means the presence of oxygen. When you perform your day-to-day activities along with some low-intensity activities like walking, jogging, swimming, cycling, etc., you mostly rely on the presence of oxygen so that you can keep doing it for a longer period of time. Cardio, also known as cardiovascular activity, is an aerobic process. Your body will use fat as a primary source of energy since you need less energy consistently over a period of time. These activities can be vaguely termed “low-intensity” activities.

Anaerobic: The term “anaerobic” means the absence of oxygen. These activities include high-intensity exercise done for a shorter period of time. When your exercise is intense enough, it will cross the threshold of aerobic activity (using oxygen) and start performing activities in the absence of oxygen (anaerobic). Once this happens, your body will not use fat as a source of energy. Rather, it will use glucose as a source of energy. That’s because you need a high amount of energy in a very short period of time, and glucose helps you achieve that.

These activities include:

  • High-intensity interval training
  • Weight lifting
  • Sprinting

Fat oxidation (fat burning) is an aerobic process that requires oxygen. When you do cardio, you use the aerobic process, thereby primarily using fat as a source of energy. When you do weight training, you use glucose as a source of energy. This is the reason why people rely on cardio to burn fat. This is the reason people hop on the treadmill for hours and ignore weight training, thinking they are burning fat.

But the above pointers do not tell you the entire story yet.

Yes, it is true that you burn more fat while doing cardio activities. If you compare one hour of a weight training session with one hour of a cardio session, your body will utilize more energy from the fat while performing cardio as compared to weight training. But everything boils down to the concept of the first law of thermodynamics (the law of energy balance).

Law of thermodynamics

In spite of all the controversy surrounding weight loss, this is an area where science has come exceedingly close to having mathematical proof of how exactly weight loss or gain occurs. This proof comes from the laws of physics, specifically the first law of thermodynamics (‘movement of energy”).

The first law of thermodynamics states that the energy of the universe remains the same. It can neither be created nor destroyed but can only be converted from one form into another.

For our purposes, the following energies are most important:

  • Chemical energy is the stored form of energy in food and your body tissues (muscles, body fat, etc.).
  • Kinetic energy is the energy of moving objects. When we ‘burn” body energy to move, chemical energy is transformed into kinetic energy.
  • Thermal energy is the energy we feel as temperature. When we ‘burn’ bodily energy, chemical energy is transformed into thermal energy. This transformation is called heat production.

Energy balance

It logically follows that losing energy from the body equals losing mass (body fat, muscles, etc.). The stored chemical energy in tissue (body fat, muscles) is lost from the body and transferred to the outside world in the form of heat or movement.

This means gaining body energy from the chemical energy in food equals gaining mass (body fat, muscles, etc.).

Now we can formulate the one universal rule that explains fat loss, fat gain, or maintenance:

  1. If you consume more calories from food than you use in your day-to-day activities, your body stores the extra energy from the food, and you generally gain weight. This can be termed a positive energy balance. If you are in a positive energy balance over a period of time, you will start gaining a noticeable amount of weight.
  2. If you expend more calories through activities than you consume through food, your body expends energy from body tissues, and you lose weight or fat. This can be termed a negative energy balance. If you are in a negative energy balance over a period of time, you will lose weight.
  3. If you are in energy balance, your weight generally remains stable.

Notice I emphasized ‘generally”? That’s because energy balance does not linearly correspond with total weight change. This happens over a period of time when you are consistently over or under the energy balance.  Thinking about energy balance is far more useful than thinking about acute activities like ‘fat burning. What I mean by that is that what happens at any particular moment of time (you doing cardio for 1 hour in the gym) is only a snapshot of what happens over a day, weeks, months, and years in terms of all the energy (kinetic, chemical, and heat).

When you do cardio for fat loss, you will burn fat at that moment, but if you do not control the energy that comes from the food and what the rest of the day looks like, you won’t be burning fat and losing weight. This means that doing cardio for fat loss won’t work unless you take control of your diet and track your calories from food.

Here’s an example of fat balance across a day to show that cardio does not make you lean unless it results in negative energy balance. Throughout the day, you will be at different points of time whereby you will be storing and losing fat. The only thing that ultimately matters is the total balance, which is dictated by energy balance.

If the transported fatty acids are not getting used (Step 2) then they again get converted into triglycerides and get stored in body fat cells (adipocytes). I mentioned this point earlier. So when you do cardio for fat loss your triglycerides gets broken down into fatty acids into the blood to get used. However, when you get more energy from the food, the extra energy again gets converted into triglycerides and thereby net effect will depend on how consistent you are creating positive or negative energy balance. You can do all the cardio in the world, but if you don’t sufficiently create net negative energy balance, physics dictate that you won’t lose net body fat mass (body fat).

There are many studies that showed that reduction of body fat simply depends on energy deficit (negative energy balance), and that independently of the method for body fat loss, solely a negative energy balance is responsible for the amount of body fat reduction.

The chemical energy we get from food which is track via calories is used by the body to do activities (kinetic and thermal energy). So the extra intake of energy which comes from food needs to be controlled first. People who create negative energy balance via food has greater chance of losing body fat than people who relies a lot on cardio and never think about what and how much they are eating.

Diet combined with aerobic training (Cardio) is the best combination people can use to lose body fat. Cardio has the benefit in physical fitness in addition to fat loss. It does not matter what kind of cardio activities you use, your first attention should go to the calorie intake from the food to lose body weight.


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Niraj Verma

Niraj Verma

Niraj is a data scientist, freelancer, and a certified nutrition and fitness consultant. He help people achieve their fitness goals, which include but are not limited to fat loss, muscle building, body recomposition, general well-being, and consultation. He believes our body is a work of science, and to achieve a fitness goal, we need to do things that are backed by scientific evidence and at the same time it's practical for you to implement.

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