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United States Department of Agriculture

Agricultural Research Service

Fighting fat involves ‘caloric deficit’ tactic
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Tom Johnson

The results from the Karolinska Institute study have important implications regarding interventions to reduce or prevent obesity. Fat mass depends on two factors: the number of adipocytes and the amount of fat each adipocyte stores. Comparisons of adipocyte numbers in obese and lean people showed that the number of adipocytes increases more rapidly between birth and adulthood in obese people and that the total number of adipocytes in adults, while constant, is also higher in obese people. There also was no difference in the rate of adipocyte turnover between obese and lean people.

Most of us have seen photographs or newsreels showing the spectacular flashes of light, the powerfully destructive shockwaves and mushroom clouds that appeared over the Nevada desert and Pacific atolls when nuclear weapons were tested above ground from 1951 until 1963. Then, the tests were banned with the signing of the Limited Test Ban Treaty.

Although the radioactive fallout produced by the atmospheric tests increased the risk for developing some types of cancer, it also has provided scientists interested in obesity with an opportunity to study the life cycle of cells called adipocytes that harbor fat in our bodies.

It is commonly thought by scientists in the field of obesity research that once an adipocyte is born, a person has that cell for a very long time — if not for the rest of his or her life.

However, scientists at the Karolinska Institute in Sweden have used carbon-14, a radioactive form of carbon that entered into the atmosphere as a result of above ground nuclear testing, to challenge that thought.

Before atmospheric testing began, the amount of carbon-14 in the atmosphere was stable. When above-ground nuclear testing began, the amount of carbon-14 in the atmosphere increased and did not drop until it began diffusing out of the atmosphere after the testing was stopped in 1963.

Plants incorporate atmospheric carbon-14 in the form of carbon dioxide. Humans then incorporate carbon-14 into their cells by eating plants and animals that eat plants.

The amount of carbon-14 in a human reflects the amount in the atmosphere at any given time. In particular, the carbon-14 ends up in the DNA of new chromosomes formed when a cell divides. Thus, the amount of carbon-14 in the DNA of a cell can be used to determine the birth date of the cell because it represents the amount of carbon-14 in the atmosphere on the day that the new cell was formed by cell division.

By examining the carbon-14 in adipocytes from people in the U.S. born before atmospheric nuclear testing began and after testing stopped, the scientists at the Karolinska Institute were able to determine when the cells were born and how long they lived.

They found that DNA in adipocytes from people born before nuclear testing had levels of carbon-14 that were higher than the atmospheric levels on their birthdays. This indicates that the adipocytes in these older individuals were formed after they were born and after atmospheric nuclear tests were stopped.

Examination of adipocytes from people born after the bomb tests ended showed that they contained carbon-14 levels corresponding to contemporary atmospheric levels. This indicates that some of their adipocytes had disappeared. These findings allowed the scientists to conclude that new adipocytes are constantly born to replace dead ones and that in about eight years, humans replace 50 percent of their existing adipocytes. However, even though adipocytes turn over during one’s lifetime, the research also showed that the number of adipocytes in humans steadily increases from birth to early adulthood and then remains constant for the remainder of a person’s life.

This all suggests that obese people acquire adipocytes faster during childhood and adolescence and maintain more fat cells during adulthood than lean people. Because adipocytes can store large amounts of fat, and there is no way known at the present time to reduce their numbers, reduction in the amount of fat stored within the cells is the only way to lose weight if you are obese or overweight. If you are lean, the way to prevent becoming overweight is to prevent accumulation of fat in adipocytes.

A pound of body fat is equal to about 3,500 calories. Thus, if someone wanted to lose a pound of fat in a week, they would need to create a caloric deficit of 500 calories a day (500 calories times seven days equals 3500 calories). It is also possible to lose about 10 pounds in a year by creating a caloric deficit of 100 calories a day.

Until the time arrives where drugs are developed that can cure obesity and induce weight loss by affecting the number of adipocytes and their turnover, the best way to reduce the amount of fat stored in the cells is to create caloric deficits by reducing caloric intake and burning calories by exercising.

Increasing the amount of fruits and vegetables in the diet should be part of any weight-loss program because they are low in calories and high in fiber that fills you up. Exercise should be something that you really enjoy doing because you need to do it for 20 to 30 minutes a few times a week.

Once you decide what combination of diet and exercise works best for you, you have to maintain the combination on a regular basis because the number of fat cells you have is not changing even though you lose weight.

Remember that the results from the study at the Karolinska Institute are show that those adipocytes are still there waiting to store excess calories, as fat, if people deviate from their diet/exercise program.


Last Modified: 6/25/2009