Infectobesity: Obesity Due to Viral Infection

So, it could be a virus after all! Many of us wonder why we put on so much weight despite regular workouts and controlled diet. Scientists are of the opinion that obesity maybe the result of ‘combination of issues including virus infection’ and they call it infectobesity. They concluded that apart from obvious explanations such as over-eating, lack of exercise, accumulation of endocrine-disrupting chemicals, and other reasons, (don’t forget the genetics factor!) obesity is of infectious origin. Researchers have even published their findings in Proceedings of the Royal Society B. A number of research studies were undertaken to prove and resolve this theory:

• ‘Animals experimentally infected with SMAM-1, an avian adeno-virus, or two human adenoviruses, adenovirus type 36 (Ad-36) and Ad-37, developed adiposity. Notably, SMAM-1 and Ad-36 were associated with obesity in humans. Although more research is needed to further define the mechanisms and the role of pathogens in the etiology of obesity, they should be included in the long list of potential etiological factors for obesity.’

• ‘The concept of virus-induced obesity is not new. Eight viruses have been shown to cause obesity in animals and there is evidence for virus-induced obesity in humans. Recent evidence on animal and human adenoviruses suggests that these adenoviruses may infect adipocytes to alter enzymes and transcription factors resulting in accumulation of triglycerides and differentiation of preadipocytes into mature adipocytes. The E4orf1 gene of Ad-36 has been shown to be responsible for the adipogenic effect. It appears that a portion of the worldwide epidemic of obesity since 1980 could be due to infections with human adenoviruses.’

‘In recent years viral infections have been recognized as possible cause of obesity, alongside the traditionally recognized causes (genetic inheritance, and behaviour/environmental causes such as diet exercise, cultural practices and stress). Although four viruses have been reported to induce obesity (infectoobesity) in animal models (chickens, mice, sheep, goat, dogs, rats and hamsters), until recently the viral etiology of human obesity has not received sufficient attention, possibly because the four viruses are not able to infect humans. In a series of papers over the last ten years, however, the group of Prof. Dhurandhar (Pennington Biomedical Research Center, LA, USA) demonstrated that a human adenovirus, adenovirus-36 (Ad-36), is capable of inducing adiposity in experimentally infected chickens, mice and non-human primates (marmosets). Ad-36 is known to increase the replication, differentiation, lipid accumulation and insulin sensitivity in fat cells and reduces those cells' leptin secretion and expression. It also affects human primary preadipocytes. In rats increased adiposity was observed due to Ad-36 infection. Recent studies have shown that, in the USA, antibodies to Ad-36 were more prevalent in obese subjects (30%) than in non-obese subjects (11%). We postulate that Ad-36 may be a contributing factor to the worldwide rising problem of obesity. If you are not much into medical jargon, here’s what they are trying to say. Once you have an infection, you develop antibodies to that infection. Antibodies are proteins produced by the immune system to detect and destroy the virus and bacteria that invade the body. Now, these scientists have found that the adenovirus SMAM-1 that infects birds enters the ‘pre-fat’ cells and through certain biochemical changes accumulates large amount of triglycerides thus increasing the number of fat cells especially in the abdomen of the chicken. Adenovirus is a virus that infects the upper respiratory system causing symptoms similar to a cold or flu. Remember bird flu?

SMAM-1 has been found to infect human beings as well. The adenovirus Ad-36 acts in the same way causing adiposity (accumulation of fat) in a number of animals including human being. The virus stimulates the pre-fat cells to differentiate into mature fat cells thereby increasing the number of fat cells and their lipid content. Researches to find the relationship between Ad-36 and obesity in people showed that 30 percent of the obese people screened had antibodies to Ad-36 whereas only 11 percent of lean people had these antibodies.

Although researchers don’t have the complete picture about the possible causes of obesity and its connection with these adenoviruses, they suggest that the viral gene E4orf1 is responsible for the conversion of pre-fat cells into fat cells.

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Interestingly, they also found that these obesity viruses improved cholesterol levels even when they increased fat cells in the body. Insulin sensitivity also increased in these obese people.

In conclusion it can be said that research on these adenoviruses and their effect on obesity can lead to better understanding of obesity and its causes, and of course, it could aid in developing treatments for obesity and, yes, high cholesterol.

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References:

1. Dhurandhar NV. Contribution of pathogens in human obesity. Drug News Perspect. 2004 Jun;17(5):307-13.

2. Atkinson RL. Could viruses contribute to the worldwide epidemic of obesity? Int J Pediatr Obes. 2008;3 Suppl 1:37-43.

3. van Ginneken V, Sitnyakowsky L, Jeffery JE. Infectobesity: viral infections (especially with human adenovirus-36: Ad-36) may be a cause of obesity. Med Hypotheses. 2009 Apr;72(4):383-8. Epub 2009 Jan 12.

Source-Medindia