Table 5.

Energy expenditure of systems and organs under sedentary conditions (approximately 10,000 kJ/day)^a [69,70,136-139].

System/organ	Energy expenditure per day (kJ/day)
Muscle at restb	2,500
Central nervous system (brain and spinal cord)	2,500
Immune system in a quiescent statec	1.600
Liverd (including immune cell activity)	1,600
Heartb	1,200
Gastrointestinal tract (including gut immune system, without liver, kidney, spleen)d	620
Kidneys	600
Spleen (erythrocytes plus leukocytes; 90% anaerobic)	480
Lungsd (including lung immune system)	400
Skind (including skin immune system)	100

^a10,000 kJ = 2,388 kcal. ^bActivated muscle has a much higher metabolic rate: for example, a Tour de France bicyclist needs approximately 30,000 kJ/day, which is 20,000 kJ more than under sedentary conditions. The 20,000 kJ are used predominantly by the muscles and also the heart. At the upper limit of gastrointestinal resorption, the total body daily uptake (absorptive capacity in the gut) is 20,000 kJ/day. ^cModerate activation of the immune system increases daily energy needs to approximately 2,100 kJ/day, and strong activation increases the daily need to 3,000 kJ/day. ^dEnergy need is difficult to estimate independent of the immune system in some organs.