How Antibiotics Affect Your Weight

It is well-known that livestock producers feed cattle antibiotics to promote weight gain and some research has suggested that the same can occur in humans with prolonged use of the drugs. The process is complex and several reasons for the correlation have been described.

Disruption to Digestive Tract

The first known study that links antibiotic use with weight gain happened in the 1950s. In research published in the Journal of Nutrition, healthy Navy recruits who were given broad-spectrum antibiotics such as penicillin to prevent streptococcus were found to gain an average of 4.5 pounds over the course of seven weeks. Long term use of broad-spectrum antibiotics, which refers to a medication that fights a wide variety of bacteria, may disrupt normal digestive functioning and cause an overgrowth of certain types of digestive system pathogens. 

Yeast is a fungus that is prone to overgrowth during a long-term antibiotic treatment. Candida is one of the 400 organisms that live in the mouth, digestive system and on the skin, and is not harmed by the use of an antibiotic medication. Candidiasis occurs when bacteria is wiped out during antibiotic use and, in the absence of competition, yeast colonies overgrow. The yeast feed on sugar from the diet and have been linked to hormonal imbalances that cause sugar cravings.

Some research also indicates that yeast overgrowth can decrease thyroid function and lead to lowered metabolic functioning. A slower metabolism means that fewer calories are burned for fuel and are placed in fat storage, leading to weight gain.

Elimination of H. Pylori

Antibiotics may affect bacteria known as helicobacter pylori, or H. Pylori. This bacteria is normally present in controlled amounts in the stomach and is best known for its role in the incidence of ulcers and gastric cancer.  However, scientists also believe that H. Pylori mediates a hormone called ghrelin that has several effects on the body, including the stimulation of the brain centers that control hunger and in the suppression of fat utilization in adipose tissue.  Ghrelin also stimulates gastric emptying, which can lead to overeating due to lack of satiety. 

Disruption of Good Bacteria

Antibiotics are important for killing the “bad” bacteria that cause illness, but the drugs are unable to target one bug while ignoring another. Therefore, many antibiotics kill some of the 100 trillion naturally-occurring beneficial bacteria, often called flora, that reside in the intestinal system. About 85 percent of these bacteria serve important functions such as boosting immunity and aiding digestion.

Studies have been conducted on the role of a particular beneficial bacteria called lactobacillus. This probiotic has been associated with reducing glucose absorption in the gastrointestinal tract, preventing it from being digested by the body and stored as fat. Probiotic supplements and food products fortified with “good” bacteria are now being marketed as weight control products.

The Bottom Line

One round of antibiotics is not likely to cause significant weight gain or obesity. However, the overuse of antibiotics has many negative health consequences, including the creation of “superbugs,” or resistant bacteria that cause illness.


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  • Lisa Gibbs

    This time last year I weighed 110 lbs. I looked and felt great in all my clothes. I started having bladder infections that kept coming back and have been on constant antibiotic therapy. I now weigh 158 lbs. I feel sluggish and worst of all I am fat for the first time in my life. I have tried and tried so hard to lose weight but I just keep gaining. My doc did a complete blood work up and everything came back normal. The weight gain is rolls of fat on my belly, thighs and hips. I feel so fat, sluggish and downright ugly. no matter how much I diet and exercise the weight will not go away. I don’t want to be fat for the rest of my life. Also I was always small busted and now they are huge. I now hate my body and just don,t now how to get rid of this ugly fat.