Energy sources at rest

Keto Masterclass

Cyclical Ketogenic Diets Review

Get Instant Access

At rest, under postabsorptive conditions, fatty acids constitute the primary energy source, accounting for approximately 60% of energy requirements, leaving about 20% for carbohydrates and proteins, respectively. Postabsorptive conditions are said to be present when no nutrients are entering the blood from the intestinal tract. The energy liberated per gram of nutrient combusted is 17 kJ for carbohydrates and proteins and 39 kJ for fat. Therefore, the demand for fat combustion at rest can be covered by the adipose tissue liberating 5 g of fatty acids per hour, of which 1.5 g is taken up by the liver and 2 g by skeletal muscle. The carbohydrates are provided by the liver, which releases 7.5 g/h of glucose, of which 4.5 g is derived from glycogenolysis and 3 g from gluconeogenesis. This covers mainly the 6 g/h of glucose which is used

Table 1.2.2 Typical changes in energy source (g/h) when an individual exercises at successively higher intensity (postabsorptive state) [24,31].

Rest

Exercise 100W

Exercise 200W

Exercise 250W

Glucose,from liver store

4.5

16

25

40

Glucose,from liver glucose neoformation

3

4

5

5

Glucose,from muscle store

-

40-45

100

150

Fat, from adipose tissue or muscle

5

18

26

22

Values are given in g/h and refer to the postabsorptive state. Exercise metabolism during the absorptive state will vary with meal composition and the time following the meal. Generally, however, more energy will be derived from the blood (glucose, fatty acids, triglycerides) and less from the substrate stores in skeletal muscle and adipose tissue.

by the central nervous system and the red blood cells. Liver glycogenolysis is made possible by the liver store of glycogen (about 75 g in the fed state), whereas substrates for gluconeogenesis are lactate, glycerol and amino acids taken up by the liver from the blood. The amino acids used for gluconeogenesis derive mainly from net proteolysis in skeletal muscle, which releases around 4.5 g amino acids/h. The latter also constitute an important energy substrate for the intestines and the liver [24].

Absorptive conditions prevail for several hours following each meal, with most nutrients being taken up by the body during the first 2-3 h. If it is assumed that the main absorption phase takes place during the first 2.5 hours following each meal, with three meals a day, absorptive conditions will prevail for 7.5 h/24 h, whereas the body during the remaining 17.5 h per day is closer to a postabsorptive state. In the absorptive state, the metabolic situation is different in that now carbohydrate oxidation dominates, normally covering three-quarters of total substrate oxidation, while the substrate stores of the body are refilled rather than used. If 10500 kJ/24 h is covered by three meals of 3500 kJ each, containing 50% carbohydrates, 35% fat and 15% protein, 100 g glucose, 30 g fatty acids and 30 g amino acids will be made available to the body during the absorptive period after each meal. Due to the shift from predominantly fat to predominantly carbohydrate oxidation during this period, around 50 g of the 100 g of carbohydrates taken up by the body will be oxidized, whereas the remaining 50 g will be used to refill the liver glycogen store (25 g) and the glycogen store in skeletal muscles (25-30 g during non-glycogen-depleted conditions) [24]. The 30 g of fatty acids from each meal enters the blood in the form of a special lipoprotein, the chylomicron. The chylomi-cron triglycerides (TGs) are degraded by the enzyme lipoprotein lipase (LPL), which is localized in capillary endothelial cells in most organs of the body, but with especially high levels in adipose tissue, myocardium and skeletal muscles. The fate of the enzyme-liberated fatty acids is mainly storage in adipose tissue, but also uptake in liver and skeletal muscle [27]. In muscle tissue the fatty acids are oxidized and/or used for the restoration of muscle TG stores reduced by exercise [28]. In the liver, the chylomicron remnants are taken up and fatty acids resynthesized to triglycerides, which are incorporated into very low density lipopro-

tein, exported and finally also stored in adipose tissue. The 30 g amino acids from each meal are added to the 400-500 g of amino acids being formed per 24 h mainly from the degradation of protein stores of skeletal muscle and liver. Of these amino acids, 300-400 g are used for synthesis and approximately 80 g are used for oxidation in the intestines and the liver [24].

Was this article helpful?

0 0
Good Carb Diet

Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

Get My Free Ebook


Post a comment