Training on reduction - fasting or after a meal?

According to popular opinion, training performed in the morning on an empty stomach is one of the most effective ways to reduce body fat. This view is based on the assumption that in the absence of food and energy intake, the body is forced to draw energy directly from fat stores. So let's find out if fasted training really accelerates fat burning.

• What are the benefits of regular exercise?
• How often should you exercise?
• How does fasted training affect metabolism?
• Post-meal training and its effects on the body
• Fasting or post-meal training during fat reduction?
• Potential risks associated with fasted training

What are the benefits of regular exercise?

It is well known that regular exercise helps achieve and maintain desired body composition, as well as significantly reducing the risk of a wide variety of chronic diseases. Systematic exercise helps improve many parameters of carbohydrate metabolism and reduces the risk of developing type 2 diabetes by improving glucose tolerance and tissue sensitivity to insulin. In addition, regular physical training reduces resting heart rate, blood pressure and the concentration of atherosclerotic lipid markers such as total cholesterol, lipoprotein fraction LDL (the so-called "bad" cholesterol), non-HDL cholesterol and triglycerides. Systematic exercise, on the other hand, increases physiological cardiac hypertrophy and the concentration of HDL fraction lipoproteins (the so-called "good" cholesterol) and improves blood flow through the body's tissues. In addition, studies have shown that regular physical training is an effective method of improving mental state, as it reduces the level of stress experienced and reduces tension, anxiety, symptoms of depression and anxiety disorders.

How often should you exercise?

According to current World Health Organization (WHO) recommendations, one should do 150 to 300 minutes of moderate-intensity physical activity or 75 to 150 minutes of high-intensityphysical activity per week to keep the body in good health. Current WHO guidelines also recommend regular muscle- and bone-strengthening (strength) exercises involving all muscle groups at least twice a week for all age groups. Thus, physical training of about an hour should be performed at least three times a week to simultaneously avoid excessive fatigue and increased risk of injury from too much and too intense exercise. However, the WHO guidelines do not specify the preferred time of training (morning, afternoon or evening) or the nutritional status (fasting or after a meal).

How does fasted training affect metabolism?

The endocrine system and autonomic nervous system, which are closely related to metabolic responses to exercise, show diurnal variability. In addition, physiological responses to exercise can vary depending on the time of physical training. It has been shown that training in the morning on an empty stomach (8-12 hours after eating the last meal) increases fat oxidation, which is due to transiently reduced levels of glycogen, especially liver glycogen. Increased fat oxidation, however, does not mean that there is increased fat burning, as this can come from intramuscular triglycerides, or visceral fat. Recent scientific reports indicate that training in the morning on an empty stomach appears to be the most effective strategy for reducing energy intake in the short term, although it also results in increased hunger and lower energy expenditure. In addition, fasting training significantly lowers blood insulin levels and increases tissue insulin sensitivity in overweight and obese individuals.

Post-meal training and its effects on the body

We already know that fasted and post-meal training can affect metabolism and exercise performance differently. Eating a meal before a workout increases the body's performance capacity, especially when performing prolonged aerobic exercise. Training after a meal, on the other hand, is characterized by lower fatty acid oxidation during the exercise itself, compared to fasted training in the morning. This is mainly due to elevated insulin concentrations after eating a meal, which can inhibit the breakdown of intramuscular triglycerides and reduce the availability of free fatty acids for oxidation. However, this does not mean that training after a meal impedes effective fat reduction, only that fatty acid oxidation is reduced at higher insulin levels.

Fasting or post-meal training during fat reduction?

Many people who want to lose weight opt for fasted training (e.g., running, orbiter training, fast walking) with the hope of better results. Studies show that both fasted and post-meal training are effective in reducing body weight, body fat percentage, and waist and hip circumference when combined with a well-composed reduction diet. No major differences were found between fasted and post-meal training in terms of body composition improvements. The changes in body composition associated with regular aerobic exercise combined with a well-rounded weight-loss diet are similar regardless of whether a person trains fasted or after a meal. Therefore, all people who want to reduce excess body fat can choose both fasted and post-meal training, depending on their individual preferences.

Potential risks associated with fasted training

The literature reports that regular fasted exercise training can lead to lowered immunity, loss of muscle mass, worsened performance, and hypoglycemia (i.e., blood glucose levels that are too low) and greater risk of injury in some people. In addition, many people can mentally and physically ill-suffer fasted training (especially at higher intensities), although there are some people who even prefer to train at such a time of day. Short-term fasted training in the morning (up to 30 minutes), however, does not appear to significantly increase the likelihood of adverse effects such as fatigue, weakness, nausea and increased hunger and thirst in healthy individuals.

Sources:

• Schoenfeld BJ, Aragon AA, Wilborn CD, et al: Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014 Nov 18;11(1):54.

• Vieira AF, Costa RR, Macedo RC, et al: Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. Br J Nutr. 2016 Oct;116(7):1153-1164.

• Aird TP, Davies RW, Carson BP: Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018 May;28(5):1476-1493.

• Liu X, He M, Gan X, et al: The Effects of Six Weeks of Fasted Aerobic Exercise on Body Shape and Blood Biochemical Index in Overweight and Obese Young Adult Males. J Exerc Sci Fit. 2023 Jan;21(1):95-103.

• Frampton J, Edinburgh RM, Ogden HB, et al: The acute effect of fasted exercise on energy intake, energy expenditure, subjective hunger and gastrointestinal hormone release compared to fed exercise in healthy individuals: a systematic review and network meta-analysis. Int J Obes (London). 2022 Feb;46(2):255-268.

• Iwayama K, Seol J, Tokuyama K.: Exercise Timing Matters for Glycogen Metabolism and Accumulated Fat Oxidation over 24 h. Nutrients. 2023 Feb 23;15(5):1109.