For most of human history food was something you found or grew and then gladly ate to stay alive. There was little thought to its quality or nutritional value, let alone calorie counting.
That began to change in the 19th century as scientists discovered thermodynamics (the study of heat and energy). They then applied that knowledge to improving the efficiency of steam engines. They used those same concepts of heat and energy to study how different foods improved the health and value of livestock and increased their capacity for work. Eventually, one man turned to studying nutrition and work in people. He thereby caused future generations to scrutinize food labels, to obsess over weight and exercise and to count calories.
Portrait of Wilbur Atwater. National Agricultural Library.
The Weight Watcher
Wilbur Olin Atwater was born in New York in 1844 and grew up in Connecticut and Vermont. He earned a degree in agricultural chemistry from Wesleyan University in Middletown, Conn., in 1865. He then received a doctorate in the same field from Yale University four years later.
Atwater chose a good time to get interested in the science underlying agriculture. The 1862 Homestead Act gave some 400,000 American families land on which to start new farms and raise livestock. That same year, the Land-Grant College Act (or Morrill Act) gave land grants to the states to establish colleges focusing on A&M—agriculture and the mechanic arts. Agriculture was big business and it drove the economies of most states. Atwater’s made his initial contribution to that economy by studying the mineral content of various fertilizers. Then he went to Europe.
Connecticut Experiment Station
In Germany, he spent two years studying physiological chemistry and visiting European experimental agriculture stations. He returned to Wesleyan University in 1873 to become the school’s first professor of chemistry. Inspired by European agricultural stations and their research on crops and fertilizers, Atwater persuaded Wesleyan and the Connecticut Legislature to establish an agricultural station in the basement of Judd Hall at Wesleyan. By 1877, a permanent Connecticut Experiment Station was established at Yale University.
Years later, Atwater and colleagues persuaded Congress to establish experiment stations at the land-grant colleges previously created by the Morrill Act. The 1887 Hatch Act resulted from those efforts. Atwater became the director of the second agricultural experiment station at Storrs Agricultural College in Connecticut. In 1888, the Office of Experiment Stations was created to monitor and appraise the research of the state-level stations. Atwater served as its first national director. Eventually, the work of the stations was turned into Farmers’ Bulletins so farmers and ranchers could keep up with the latest findings about feed, fertilizers and crop management.
Exiting the respiration calorimeter. National Agricultural Library.
Measuring Heat
Returning to Germany again, Atwater observed his European colleagues using thermodynamic principles to study respiration and metabolism in livestock. He thought it would be instructive to do similar studies on people. When he returned to Wesleyan, Atwater built a “respiration calorimeter” to study human metabolism.
Atwater’s calorimeter, an enclosed four-by-eight-foot chamber, could measure a test subject’s body heat during exercise, oxygen intake, carbon dioxide output, and the amount of energy provided by various foods. Atwater then used his students as most of his test subjects; some of whom may have wanted extra cash, a better grade or both. Over several years, Atwater and his guinea pig students quantified the dynamics of metabolism and determined the balance between food intake and energy output. He called that measure of food energy the Calorie.
Calorie Counting
Calorie comes from the Latin, calor, for heat. The French scientist Nicolas Clément used it as a unit of heat in the early 19th century. The Calorie (large “C”) actually is 1,000 calories (small “c”) or one kilocalorie. This is the amount of energy needed to raise the temperature of one kilogram of water one degree Celsius. Atwater determined the caloric content of foods using a “bomb calorimeter,” which burned whole foods or individual samples of fat, carbohydrates and protein and measured the resulting heat. Atwater’s system, for example, found protein contained 4,000 calories per gram, as did carbohydrates. Fat contained 9,000 calories per gram.
Exercise bicycle. National Agricultural Library.
This research was more than ivory tower information for scientists and scientific journal editors. His 1896 report to the Department of Agriculture, “The Chemical Composition of American Food Materials,” listed the calories and nutritional content of 500 different foods. With that data, Atwater then made practical recommendations for economical diets that still would provide sufficient nutritional value. He emphasized the importance of an inexpensive and balanced diet that included more proteins, beans and vegetables in place of carbohydrates and indigestible fiber. Atwater also concluded Americans consumed too much fat and sugar and did not exercise enough—conclusions still true today.
4,000 Calories a Day?
“A woman with light exercise, for example, required 2,300 calories per day, while a man with a corresponding activity level needed 2,830 calories,” according to a Century magazine article Atwater wrote in 1888.
Winning over a skeptical public about calorie intake and exercise was not made easier by the fact that the beefy Dr. Atwater looked as if he enjoyed 4,000 calories a day. Worse perhaps, his research showed that people generated heat (energy) from alcoholic beverages (7,000 calories per gram). In other words, alcohol had nutritional value. This did not win him any friends in the Methodist Church or the temperance movement, and he belonged to both. Nonetheless, Atwater and his colleagues had started a “scientific eating” movement.
World War I
During World War I, the U.S. Food Administration used publications and good old-fashioned propaganda to make calorie counting a familiar concept to the larger public. The government then rationed certain foods for shipping to allies and encouraged the public to seek out alternative foods of similar nutritional content. The Food Administration, for example, would encourage citizens to substitute a piece of beef (500 calories) for extra helpings of beans. The government also deemed rationing and not overindulging at the dinner table both healthy and patriotic. Many restaurants even started posting calorie counts on their menus so customers could keep track of their mealtime calories during the war.
WWI Food Sign. Library of Congress.
After the war, rationing ended, but calorie counting did not. Fashion, foods, lifestyles and economies changed, and people tried to change with the times. As a result, dieting became common. Diet and Health, the first diet book published in the U.S., sold 2 million copies and stayed in print from 1918 to 1939.
Atwater died in 1907. He started a food revolution with his analytical research and turned the old French word calorie into a mantra and an obsession that lives on in the 21st century. Though today’s calorie counters may have to switch to counting joules—a more common international unit of energy that equals 0.239 calories (small “c”).
Measuring respiration. National Agricultural Library.
For more information:
Check out the Mayo Clinic Calorie Calculator to see your recommended daily calorie intake.
Visit the USDA’s Food Data Central for calorie and nutrient information about various foods.
See Scientific American’s July 2015 special issue on “The Science of Food.”
The author of this story, Edward McSweegan, is a Rhode Island microbiologist who writes about infectious diseases and history. This story was updated in 2023.
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