Did you know? Part 2.

Last night, I got to use this factoid at a dinner. Upon hearing that diesel vehicles have a fuel efficiency of 1 mile per chicken, people immediately wanted to know what the fuel efficiency is in terms of human fat. Last December, I read that a company in Norway struck a deal with a hospital in Miami to supply it with 3,000 gallons a week of liposuction fat for conversion into biodiesel. In the article, it says that 3,000 gallons of liposuction fat is enough to make 2,600 gallons of biodiesel. The average diesel engine gets about 30 miles to the gallon of #2 diesel. But biodiesel (in pure form) gets 10% lower fuel efficiency relative to traditional diesel fuel (source). So, an average diesel engine will get say 27 miles to the gallon of pure biodiesel. 3,000 gallons of liposuction fat = 2,600 gallons of biodiesel = 70,200 miles driven. So, 23.4 miles per gallon of liposuction fat.

But that's per gallon, not per pound. Body fat has a density of 0.918 grams per cubic centimeter (source). There are 453.59 grams in a pound and there are 3,785.41 cubic centimeters in a gallon. Thus, a gallon of liposuction fat would weigh 7.66 pounds. And 23.4 miles per gallon would translate into 3.05 miles per pound.

As an additional sanity-check here, diesel fuel typically has an energy density of 40.9 MJ/L (source). There are 3.785 liters in a gallon so 40.9 MJ/L equals 154.8 MJ/gal. If a diesel engine gets 30 miles to the gallon, it takes 154.8 MJ to go 30 miles. A pound of human fat contains 3,500 kcal (source). A kcal contains 4.184 kJ of energy so 3,500 kcal would equal 14,644 kJ or 14.644 MJ. If 154.8 MJ goes 30 miles, 14.644 MJ would only get you 9.46% of that distance - or 2.84 miles.

In terms of the difference between 2.84 miles per pound of liposuction fat vs 3.05 miles per pound, one thing to keep in mind is that some of the energy content of biodiesel comes from the alcohol component of its production (typically methanol). So the 2.84 figure is probably more accurate than 3.05.

Regardless, that’s too much detail for casual conversation. The short answer is that the fuel efficiency of a diesel engine is about 1 mile per chicken or 3 miles per pound of liposuction fat. Use that factoid at your next cocktail party and let me know how it goes. =)

If you really want to impress your guests, you could also mention that according to the American Society of Plastic Surgeons (ASPS), 325,000 of liposuction operations were carried out last year to remove fat from various parts of the body - particularly the thighs, abdomen and buttocks – and that the average woman sheds an average of 20 pounds of abdominal fat by liposuction (source). So, 325,000 operations x 20 pounds per operation = 6.5 million pounds of liposuction fat. At 3 miles per pound, 6.5 million pounds of liposuction fat would get you 19.5 million miles. And, at 30 miles per gallon, that would offset 650,000 gallons of diesel fuel. Unfortunately, the US consumes about 64 billion gallons of diesel a year - 100,000x the amount you could get from liposuction fat. But every little bit helps and Americans get fatter by the year so perhaps this will make more of a dent over time. =)