Firstly, a relevant video by Flanders and Swann.
The title of this blog post is from the "Physics Man" sketch on The Now Show. Work (also heat) is another word for energy and there are two different units for it.
The calorie (cal) is the amount of energy required to heat 1g of water by 1°C. This is a tiny amount of energy. The dietary Calorie (Cal) = 1,000cal = 1kcal.
The Joule (J) is the SI unit of energy. 1J = 1kg*m^2/s^2.
1Joule/sec = 1Watt (W).
1kcal = 4.186kJ.
At rest, an average human body uses ~1kcal/min = ~4,186J/min = ~69.8J/sec = ~69.8W.
The brain uses ~5g of glucose/hour = 18.75kcal/hour (1g of carb = 3.75kcals, usually rounded-up to 4) = 78487.5J/hour = ~21.8W.
The heart uses ~10W. The liver, kidneys, gut and lungs run continuously so they use energy all of the time.
Skeletal muscle uses a variable amount of energy using a variable proportion of fuels, depending on what you're doing with it. A chap called Steve sent me a spreadsheet of results in 2004 when he underwent a metabolic test on a stationary bike while breathing through a respiratory gas analyser, which calculated kcals oxidised and fuel utilisation by measuring Respiratory Exchange Ratio (RER).
At 1kcal/min (resting), he oxidised ~95% from fat (~0.11g/min), ~5% from carb (~0.01g/min).
At 2kcal/min (12% max), he oxidised 100% from fat (0.22g/min), 0% from carb (0.00g/min).
At 3kcal/min (18% max), he oxidised 100% from fat (0.33g/min), 0% from carb (0.00g/min).
At 4kcal/min (24% max), he oxidised 99% from fat (0.44g/min), 1% from carb (0.01g/min).
At 5kcal/min (29% max), he oxidised 48% from fat (0.27g/min), 52% from carb (0.69g/min).
At 6kcal/min (35% max), he oxidised 62% from fat (0.41g/min), 38% from carb (0.61g/min).
At 7kcal/min (41% max), he oxidised 58% from fat (0.45g/min), 42% from carb (0.78g/min).
At 8kcal/min (47% max), he oxidised 46% from fat (0.41g/min), 54% from carb (1.15g/min).
At 9kcal/min (53% max), he oxidised 42% from fat (0.53g/min), 58% from carb (1.39g/min).
At 10kcal/min (59% max), he oxidised 44% from fat (0.49g/min), 56% from carb (1.49g/min).
At 11kcal/min (65% max), he oxidised 38% from fat (0.46g/min), 62% from carb (1.82g/min).
At 12kcal/min (71% max), he oxidised 41% from fat (0.55g/min), 59% from carb (1.89g/min).
At 13kcal/min (76% max), he oxidised 37% from fat (0.53g/min), 63% from carb (2.18g/min).
At 14kcal/min (82% max), he oxidised 30% from fat (0.47g/min), 70% from carb (2.61g/min).
At 15kcal/min (88% max), he oxidised 14% from fat (0.23g/min), 86% from carb (3.44g/min).
At 16kcal/min (94% max), he oxidised 0% from fat (0.00g/min), 100% from carb (4.27g/min).
At 17kcal/min (100% max), he oxidised 0% from fat (0.00g/min), 100% from carb (4.53g/min).
There are some interesting points about Steve's data:
1. Over a wide range of exercise intensities, the number of grams of fat Steve oxidised/min was fairly constant.
2. Up to 24% of maximum exercise intensity, Steve derived almost 100% of his energy from the oxidation of fat. Steve was on a LC diet, which shifts fuel usage away from carb and towards fat. This is known as "fat-adaptation".
3. Despite fat-adaptation, above about 45% of maximum exercise intensity, Steve derived more energy from the oxidation of carb than the oxidation of fat.
4. Despite fat-adaptation, above about 80% of maximum exercise intensity, Steve derived almost all of his energy from the oxidation of carb rather than the oxidation of fat.
Note that 17kcals/min = 1186.6W, or 1.19kW! Steve was aerobically fit. A less aerobically fit person derives a higher % of energy from the oxidation of carb than an aerobically fit person. This level of exercise intensity can be maintained for a few seconds only, as carb is oxidised both aerobically and anaerobically, which exhausts PhosphoCreatine stores in muscles and also causes an accumulation of lactate in muscles.
Muscle mass is very metabolically-active compared to fat mass, as one pound of fat mass oxidises only about 2kcal a day. See also Dissecting the Energy Needs of the Body – Research Review
See also It's all in a day's work (as measured in Joules) Part 2.
Here's another Physics Man.