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Calculating weight loss from exercising

If a person who normally requires an average of 12,000 kJ (3000 kcal) of food energy per day consumes 13,000 kJ per day, he will steadily gain weight. How much bicycling per day is required to work off this extra 1000 kJ?

Solution

[link] states that 400 W are used when cycling at a moderate speed. The time required to work off 1000 kJ at this rate is then

Time = energy energy time = 1000 kJ 400 W = 2500 s = 42 min. size 12{"Time"= { {"energy"} over { left ( { {"energy"} over {"time"} } right )} } = { {"1000"" kJ"} over {"400 W"} } ="2500"" s"="42 min" "." } {}

Discussion

If this person uses more energy than he or she consumes, the person’s body will obtain the needed energy by metabolizing body fat. If the person uses 13,000 kJ but consumes only 12,000 kJ, then the amount of fat loss will be

Fat loss = ( 1000 kJ ) 1.0 g fat 39 kJ = 26 g, size 12{"Fat loss"= \( "1000"" kJ" \) left ( { {1 "." "0 g fat"} over {"39 kJ"} } right )="26"" g,"} {}

assuming the energy content of fat to be 39 kJ/g.

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A person is measuring the amount of oxygen in blood and metabolic rate using a pulse oxymeter. The pulse oxymeter is strapped to the person’s wrist, and the index finger is inside the clip.
A pulse oxymeter is an apparatus that measures the amount of oxygen in blood. Oxymeters can be used to determine a person’s metabolic rate, which is the rate at which food energy is converted to another form. Such measurements can indicate the level of athletic conditioning as well as certain medical problems. (credit: UusiAjaja, Wikimedia Commons)
Energy and oxygen consumption rates for an average 76-kg male (power)
Activity Energy consumption in watts Oxygen consumption in liters O 2 /min
Sleeping 83 0.24
Sitting at rest 120 0.34
Standing relaxed 125 0.36
Sitting in class 210 0.60
Walking (5 km/h) 280 0.80
Cycling (13–18 km/h) 400 1.14
Shivering 425 1.21
Playing tennis 440 1.26
Swimming breaststroke 475 1.36
Ice skating (14.5 km/h) 545 1.56
Climbing stairs (116/min) 685 1.96
Cycling (21 km/h) 700 2.00
Running cross-country 740 2.12
Playing basketball 800 2.28
Cycling, professional racer 1855 5.30
Sprinting 2415 6.90

All bodily functions, from thinking to lifting weights, require energy. (See [link] .) The many small muscle actions accompanying all quiet activity, from sleeping to head scratching, ultimately become thermal energy, as do less visible muscle actions by the heart, lungs, and digestive tract. Shivering, in fact, is an involuntary response to low body temperature that pits muscles against one another to produce thermal energy in the body (and do no work). The kidneys and liver consume a surprising amount of energy, but the biggest surprise of all it that a full 25% of all energy consumed by the body is used to maintain electrical potentials in all living cells. (Nerve cells use this electrical potential in nerve impulses.) This bioelectrical energy ultimately becomes mostly thermal energy, but some is utilized to power chemical processes such as in the kidneys and liver, and in fat production.

An f M R I scan of a human head with energy consumption in the vision center shown by a bright spot. This brightness indicates the energy consumption.
This fMRI scan shows an increased level of energy consumption in the vision center of the brain. Here, the patient was being asked to recognize faces. (credit: NIH via Wikimedia Commons)

Section summary

  • The human body converts energy stored in food into work, thermal energy, and/or chemical energy that is stored in fatty tissue.
  • The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate, and the corresponding rate when at rest is called the basal metabolic rate (BMR)
  • The energy included in the basal metabolic rate is divided among various systems in the body, with the largest fraction going to the liver and spleen, and the brain coming next.
  • About 75% of food calories are used to sustain basic body functions included in the basal metabolic rate.
  • The energy consumption of people during various activities can be determined by measuring their oxygen use, because the digestive process is basically one of oxidizing food.

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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