Activation of fat burning processes requires not only low insulin levels but also low leptin, according to a study published in 2018. In fact, this confirmed the notion that fat burning begins after not eating for at least 6-8 hours.

At the same time, physical training, by increasing glucose utilization, reduces this interval. In addition, they increase the heart rate (forcing the body to consume more oxygen) – and lead to the production of adrenaline, also associated with the activation of fat burning processes.

What to do?
The mechanism of fat burning is quite simple – adipose tissue receives a signal from the body, causing it to release free fatty acids into the bloodstream, where they are used as a source of energy to support metabolic functions and needs of certain organs.

However, the presence of glucose in the blood (and therefore the production of insulin) limits the release of free fatty acids from the cells. Simply put, if there is food in the stomach, the body will receive energy from that food – and will not burn existing fat at all.

The authors of the above study suggest that in order to activate fat burning processes, the body must switch from glucose metabolism to fat metabolism – citing low plasma levels of leptin and insulin as key features of this condition.

Does insulin block fat burning?
Insulin is a hormone produced by the body to use carbohydrates (glucose or ‘sugar’ in the blood). It usually increases after eating carbohydrate foods, forcing the body to either use calories for metabolic purposes or store them in fat.

Length of the dining window
Modern literature is increasingly using the term “dining window” – meaning the total length of the period in which food is consumed. Statistics 4 suggest that this parameter for most people is 15 hours – and in 50% of cases even exceeds it.

Given the time it takes for the body to digest food (up to 3-5 hours in some cases), many people are constantly on a digestion regimen – and rarely experience a condition in which leptin levels are minimal.

What should be done to burn fat?
Regular physical training is one of the most effective methods to start fat burning processes. Because the oxidation of free fatty acids requires a lot of oxygen, cardio literally helps the body burn fat.

Metabolic data show that 2.9 liters of inhaled oxygen are required for complete oxidation of 1 kg of fat to form 2.8 liters of carbon dioxide and 1.1 liters of water.

The importance of diet
Despite the contribution of training to increasing energy expenditure, where this energy comes from also plays a role. If you eat food just before sports, you will probably burn the calories from this food, not fat reserves.

The theory of the feeding window and its effect on metabolism sets out the 5th hypothesis that the proper functioning of metabolism involves controlling periods of eating – and periodic abstinence from it.

If a person eats food almost around the clock, then his body is extremely rarely in a natural mode of burning calories – it works only to increase reserves.

The process of switching from carbohydrate to fat metabolism involves low insulin levels – as well as low leptin levels. Achieving this condition requires both control of eating periods and physical training.

Consult a specialist to clarify the contraindications.

Leptin mediates the glucose-fatty acid cycle to maintain glucose homeostasis during starvation, source
Physiological process of fat loss, source
Dangers of high leptin and factors that lower levels, source
When to eat: The science behind periodic starvation, source
Time-limited nutrition for the prevention and management of chronic metabolic diseases, source

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