Pull Up A Stool And Let's Talk About Your Microbiome

A medical case report recently made headlines supporting the notion that the bacteria inside our gut have something to do with the size of our gut.

Clostridium is a nasty strain of bacteria that is resistant to many antibiotics. Normally, the many other species of bacteria in the gut keep Clostridium in check, but when those friendly bacteria are wiped out during antibiotic therapy, Clostridium can thrive and produce severe inflammation (colitis) and diarrhea. This can develop into a serious illness that claims the lives of 14,000 Americans every year.

A novel way to treat this condition is through use of fecal microbiota transplant (FMT). In other words, the patient ingests the intestinal bacteria from a healthy person to replenish their own stock and get Clostridium back under control. We will leave it to your imagination as to how doctors collect the good bacteria, but let's just say you can make some decent money if you're willing and able to donate. Thankfully for patients, FMT is available in pill form.

As unappealing as it sounds, fecal transplants - which repopulate intestinal bacteria in the recipient - are proving to be very effective in treating some serious ailments. 

As reported recently, a young woman with a stable weight of ~130 pounds had to undergo FMT to fight a Clostridium infection. The good news is that she beat the infection, but the bad news is that she gained 34 pounds in 16 months, classifying her as obese with a BMI of 33. Even more alarming is that she could not lose weight despite being on a supervised liquid diet and exercise program. The donor for the FMT (her teen daughter) was overweight, but otherwise in good health, so doctors are now recommending that FMT donors be of normal weight.

In light of this news, here's a beginner's guide to the tiny creatures calling you "home"...

You are not just a person – you are an ecosystem. Your body is home to trillions of microscopic critters, including viruses, bacteria, and fungi, living on or inside you. Collectively, these communities of microbes constitute what is called your “microbiome”.

And there are more of “them” than “you” – the number of microbes inhabiting your body is larger than the number of cells making up your body! To put this in perspective, it has been estimated that your microbiome weighs about 3 pounds. Good news if you’re on a diet – when you step on the scale tonight, feel free to subtract 3 pounds of stuff that isn’t “you” per se.

A new study concerning our microbiome seems to be coming out each week, so it is time we get to know our microbial roommates.

1. Where does your microbiome come from?

We are born virtually sterile, but quickly receive an infusion of bacteria from our mom, first through the birth canal and then through the milk. Over 900 species of bacteria have been found in breast milk, and these are the pioneers that settle into your gut, which appears to stabilize by the age of 3. Of potential interest are babies born by caesarean section or those who are fed formula instead of breast milk. Babies delivered via C-section do in fact have a different microbiome and may be at higher risk for certain types of allergies and obesity (more on this below). Our microbiome continues to receive fresh new imports as we move through, inhale, and ingest our environment.

How much of you is really you? There are more microbes in your body than the number of cells making up your body. We are just now beginning to appreciate the many things they do for us.

2. Your microbiome is like your own personal “germ cloud”.

You’ve probably noticed that everyone’s home smells a little different. Sometimes this is due to cooking, pets, or the amount of trash they let accumulate, but it is also due in part to the microbiome of the inhabitants. Researchers have found that you are surrounded by a “germ cloud”, and you leave pieces of your microbiome wherever you go like a trail of breadcrumbs. It might even be possible for police to use microbiomes to track people one day like they currently use fingerprints or DNA. In other words, you have a “microbiome fingerprint” that is left behind like a germ echo wherever you go.

This “germ cloud” may also explain how dogs can track people so easily. The byproducts generated by the millions of bacteria living on your skin are aromatic (odorous), producing a scent that is released into the air as you move. Animals with a keen sense of smell can get a whiff of these aromatic compounds and follow them to the source.

Speaking of “germ clouds”, if you ever wondered if it is possible to fart out germs, some brave scientists have sniffed out the answer to this question. You can read about the results here.

3. Antibiotics substantially alter your microbiome.

We take antibiotics to get rid of pathogenic bacteria that make us sick. The problem is they are not selective, so they destroy a lot of our friendly bacteria in addition to the bad guy. We need these friendly bacteria to do all sorts of things – to name just a few:  they help us digest food, make vitamins, and build anti-inflammatory compounds.

Another important thing our microbial friends do is keep infections in check. For example, yeast infections from pathogenic fungi can arise if good bacteria are not around competing for resources. And some bacteria, like the nasty Clostridium difficile, are naturally resistant to many antibiotics. When good bacteria are killed as collateral damage in an antibiotic treatment, the growth of Clostridium can run amok. These bacteria secrete a toxin that causes diarrhea and they can lead to a life-threatening superinfection in some patients.

4. Your microbiome may protect you from allergies or obesity.

Several recent studies have correlated unusual microbiome composition with the presence of certain allergies. Dr. Hans Bisgaard has shown that infants harboring fewer species of gut bacteria have an increased risk of developing certain allergies as they grow up. More recently, Dr. Catherine Nagler has shown that certain bacterial species offer protection from peanut allergies.

Dr. Martin Blaser has found that administration of penicillin to mice soon after birth altered their gut microbiome in such a way that it made them more prone to obesity as adults. Remarkably, the tendency to grow obese is transferrable to germ-free mice – in other words, by transplanting the microbes from the penicillin treated mice to normal mice made the normal mice more susceptible to weight gain.

Studies such as these make it tantalizing to speculate that we may be able to treat certain ailments in humans by altering our microbiome with specific probiotic regimens. Maybe they could even slip these bacteria into our peanut butter instead of deadly Salmonella.

5. How do scientists study the microbiome?

Advances in DNA sequencing have allowed scientists to rapidly map the genomes for many microbial species, which provides us with a “genomic fingerprint”. We can process samples swabbed from the skin or body cavities, or process stool samples, for DNA sequencing. Usually just sequencing the 16S ribosomal RNA gene is enough to distinguish one bacteria species from another.  

It should be mentioned that some scientists are issuing cautions about over-interpreting microbiome studies. Many of the studies altering the microbiome have been performed in mice, so it remains to be determined to what extent the findings can be extrapolated to humans. Furthermore, many of the methods used to alter the microbiome in lab animals do not faithfully mimic what humans do with antibiotics. For example, in some studies the investigators give large doses of antibiotics over unusually long periods of time to see an effect in lab animals, which does not equate to the typical dosing of antibiotics in humans. Finally, many of these studies are correlative and have not yet definitively demonstrated causation. There is a big difference between correlation and causation.

6. So should I take my microbiome into my own hands?

Much more research needs to be done to assess the true impact of the microbiome versus other factors that come into play, such as host genetics, diet, and the environment. It is argued that some microbiome studies are hyped up and way overblown. Long story short:  if you or your child becomes sick with an infectious agent, it is not wise to withhold antibiotic treatment out of fear that it will cause allergies or obesity. If you are overweight, a healthier diet and plenty of exercise is going to do much more than any probiotic pill. In fact, there is little evidence that the popular probiotics on the market do anything to remedy the wide-ranging health problems some claim to treat, although there is data showing potential benefit in treating some gastrointestinal maladies, especially acute diarrhea caused by rotavirus.

Go here to learn more about the NIH human microbiome project.

Contributed by:  Bill Sullivan, Ph.D.

Follow Bill on Twitter.

Lax S, Smith DP, Hampton-Marcell J, Owens SM, Handley KM, Scott NM, Gibbons SM, Larsen P, Shogan BD, Weiss S, Metcalf JL, Ursell LK, Vázquez-Baeza Y, Van Treuren W, Hasan NA, Gibson MK, Colwell R, Dantas G, Knight R, & Gilbert JA (2014). Longitudinal analysis of microbial interaction between humans and the indoor environment. Science (New York, N.Y.), 345 (6200), 1048-52 PMID: 25170151

Bisgaard, H., Li, N., Bonnelykke, K., Chawes, B., Skov, T., Paludan-Müller, G., Stokholm, J., Smith, B., & Krogfelt, K. (2011). Reduced diversity of the intestinal microbiota during infancy is associated with increased risk of allergic disease at school age Journal of Allergy and Clinical Immunology, 128 (3), 646-65200000 DOI: 10.1016/j.jaci.2011.04.060

Cox, L., Yamanishi, S., Sohn, J., Alekseyenko, A., Leung, J., Cho, I., Kim, S., Li, H., Gao, Z., Mahana, D., Zárate Rodriguez, J., Rogers, A., Robine, N., Loke, P., & Blaser, M. (2014). Altering the Intestinal Microbiota during a Critical Developmental Window Has Lasting Metabolic Consequences Cell, 158 (4), 705-721 DOI: 10.1016/j.cell.2014.05.052

Stefka, A., Feehley, T., Tripathi, P., Qiu, J., McCoy, K., Mazmanian, S., Tjota, M., Seo, G., Cao, S., Theriault, B., Antonopoulos, D., Zhou, L., Chang, E., Fu, Y., & Nagler, C. (2014). Commensal bacteria protect against food allergen sensitization Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1412008111

Williams NT (2010). Probiotics. American journal of health-system pharmacy : AJHP : official journal of the American Society of Health-System Pharmacists, 67 (6), 449-58 PMID: 20208051

Alang, N., & Kelly, C. (2015). Weight Gain After Fecal Microbiota Transplantation Open Forum Infectious Diseases, 2 (1) DOI: 10.1093/ofid/ofv004

6 Things You Need To Know About Your Microbiome

You are not just a person – you are an ecosystem. Your body is home to trillions of microscopic critters, including viruses, bacteria, and fungi, living on or inside you. Collectively, these communities of microbes constitute what is called your “microbiome”.

And there are more of “them” than “you” – the number of microbes inhabiting your body is larger than the number of cells making up your body! To put this in perspective, it has been estimated that your microbiome weighs about 3 pounds. Good news if you’re on a diet – when you step on the scale tonight, feel free to subtract 3 pounds of stuff that isn’t “you” per se.

A new study concerning our microbiome seems to be coming out each week, so it is time we get to know our microbial roommates.

1. Where does your microbiome come from?

We are born virtually sterile, but quickly receive an infusion of bacteria from our mom, first through the birth canal and then through the milk. Over 900 species of bacteria have been found in breast milk, and these are the pioneers that settle into your gut, which appears to stabilize by the age of 3. Of potential interest are babies born by caesarean section or those who are fed formula instead of breast milk. Babies delivered via C-section do in fact have a different microbiome and may be at higher risk for certain types of allergies and obesity (more on this below). Our microbiome continues to receive fresh new imports as we move through, inhale, and ingest our environment.

How much of you is really you? There are more microbes in your body than the number of cells making up your body. We are just now beginning to appreciate the many things they do for us.

2. Your microbiome is like your own personal “germ cloud”.

You’ve probably noticed that everyone’s home smells a little different. Sometimes this is due to cooking, pets, or the amount of trash they let accumulate, but it is also due in part to the microbiome of the inhabitants. Researchers have found that you are surrounded by a “germ cloud”, and you leave pieces of your microbiome wherever you go like a trail of breadcrumbs. It might even be possible for police to use microbiomes to track people one day like they currently use fingerprints or DNA. In other words, you have a “microbiome fingerprint” that is left behind like a germ echo wherever you go.

This “germ cloud” may also explain how dogs can track people so easily. The byproducts generated by the millions of bacteria living on your skin are aromatic (odorous), producing a scent that is released into the air as you move. Animals with a keen sense of smell can get a whiff of these aromatic compounds and follow them to the source.

Speaking of “germ clouds”, if you ever wondered if it is possible to fart out germs, some brave scientists have sniffed out the answer to this question. You can read about the results here.

3. Antibiotics substantially alter your microbiome.

We take antibiotics to get rid of pathogenic bacteria that make us sick. The problem is they are not selective, so they destroy a lot of our friendly bacteria in addition to the bad guy. We need these friendly bacteria to do all sorts of things – to name just a few:  they help us digest food, make vitamins, and build anti-inflammatory compounds.

Another important thing our microbial friends do is keep infections in check. For example, yeast infections from pathogenic fungi can arise if good bacteria are not around competing for resources. And some bacteria, like the nasty Clostridium difficile, are naturally resistant to many antibiotics. When good bacteria are killed as collateral damage in an antibiotic treatment, the growth of Clostridium can run amok. These bacteria secrete a toxin that causes diarrhea and they can lead to a life-threatening superinfection in some patients.

4. Your microbiome may protect you from allergies or obesity.

Several recent studies have correlated unusual microbiome composition with the presence of certain allergies. Dr. Hans Bisgaard has shown that infants harboring fewer species of gut bacteria have an increased risk of developing certain allergies as they grow up. More recently, Dr. Catherine Nagler has shown that certain bacterial species offer protection from peanut allergies.

Dr. Martin Blaser has found that administration of penicillin to mice soon after birth altered their gut microbiome in such a way that it made them more prone to obesity as adults. Remarkably, the tendency to grow obese is transferrable to germ-free mice – in other words, by transplanting the microbes from the penicillin treated mice to normal mice made the normal mice more susceptible to weight gain.

Studies such as these make it tantalizing to speculate that we may be able to treat certain ailments in humans by altering our microbiome with specific probiotic regimens. Maybe they could even slip these bacteria into our peanut butter instead of deadly Salmonella.

5. How do scientists study the microbiome?

Advances in DNA sequencing have allowed scientists to rapidly map the genomes for many microbial species, which provides us with a “genomic fingerprint”. We can process samples swabbed from the skin or body cavities, or process stool samples, for DNA sequencing. Usually just sequencing the 16S ribosomal RNA gene is enough to distinguish one bacteria species from another.  

It should be mentioned that some scientists are issuing cautions about over-interpreting microbiome studies. Many of the studies altering the microbiome have been performed in mice, so it remains to be determined to what extent the findings can be extrapolated to humans. Furthermore, many of the methods used to alter the microbiome in lab animals do not faithfully mimic what humans do with antibiotics. For example, in some studies the investigators give large doses of antibiotics over unusually long periods of time to see an effect in lab animals, which does not equate to the typical dosing of antibiotics in humans. Finally, many of these studies are correlative and have not yet definitively demonstrated causation. There is a big difference between correlation and causation.

6. So should I take my microbiome into my own hands?

Much more research needs to be done to assess the true impact of the microbiome versus other factors that come into play, such as host genetics, diet, and the environment. It is argued that some microbiome studies are hyped up and way overblown. Long story short:  if you or your child becomes sick with an infectious agent, it is not wise to withhold antibiotic treatment out of fear that it will cause allergies or obesity. If you are overweight, a healthier diet and plenty of exercise is going to do much more than any probiotic pill. In fact, there is little evidence that the popular probiotics on the market do anything to remedy the wide-ranging health problems some claim to treat, although there is data showing potential benefit in treating some gastrointestinal maladies, especially acute diarrhea caused by rotavirus.

Go here to learn more about the NIH human microbiome project.

Contributed by:  Bill Sullivan, Ph.D.

Follow Bill on Twitter.

Lax S, Smith DP, Hampton-Marcell J, Owens SM, Handley KM, Scott NM, Gibbons SM, Larsen P, Shogan BD, Weiss S, Metcalf JL, Ursell LK, Vázquez-Baeza Y, Van Treuren W, Hasan NA, Gibson MK, Colwell R, Dantas G, Knight R, & Gilbert JA (2014). Longitudinal analysis of microbial interaction between humans and the indoor environment. Science (New York, N.Y.), 345 (6200), 1048-52 PMID: 25170151

Bisgaard, H., Li, N., Bonnelykke, K., Chawes, B., Skov, T., Paludan-Müller, G., Stokholm, J., Smith, B., & Krogfelt, K. (2011). Reduced diversity of the intestinal microbiota during infancy is associated with increased risk of allergic disease at school age Journal of Allergy and Clinical Immunology, 128 (3), 646-65200000 DOI: 10.1016/j.jaci.2011.04.060

Cox, L., Yamanishi, S., Sohn, J., Alekseyenko, A., Leung, J., Cho, I., Kim, S., Li, H., Gao, Z., Mahana, D., Zárate Rodriguez, J., Rogers, A., Robine, N., Loke, P., & Blaser, M. (2014). Altering the Intestinal Microbiota during a Critical Developmental Window Has Lasting Metabolic Consequences Cell, 158 (4), 705-721 DOI: 10.1016/j.cell.2014.05.052

Stefka, A., Feehley, T., Tripathi, P., Qiu, J., McCoy, K., Mazmanian, S., Tjota, M., Seo, G., Cao, S., Theriault, B., Antonopoulos, D., Zhou, L., Chang, E., Fu, Y., & Nagler, C. (2014). Commensal bacteria protect against food allergen sensitization Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1412008111

Williams NT (2010). Probiotics. American journal of health-system pharmacy : AJHP : official journal of the American Society of Health-System Pharmacists, 67 (6), 449-58 PMID: 20208051

The Friday Five

Highlighting some of the coolest science news we’ve seen lately.

1. Remember the classic 80s B-movie, "Tremors", with Kevin Bacon? It looks like life is imitating “art” in the case of Eunice aphroditois (aka the Bobbit worm). Click here to watch this critter leap from out of the ground to gobble up an unsuspecting fish.

 

 

2. Cinnamon, Boris, and Sylvester. These were the names of the three cats that have helped scientists map the feline genome (no cats were harmed in these experiments!). 

 

3. An excellent video on the development of antibiotic resistance and evolution of scary “super bugs” like MRSA.
 

 

 

4. Also in antibiotic news, new studies indicate that some of these medications may have unintended consequences. In mice, low dose penicillin altered the normal gut bacteria, which had a long lasting effect on metabolism that predisposed them to obesity. But the research here is also in its “infancy”, so no one should withhold antibiotics to treat serious infections.

 

Additionally, another study has suggested that certain antibiotics given early in life may alter immunity in the long-term, again by impacting the normal bacteria inhabiting the gut.

5. While most people try to get rid of parasites, there is at least one man who invites them into his gut. Parasitologist Julius Lukes is using himself as a human guinea pig to convince others that parasites are not always bad and may actually have co-evolved to do good things for us. To prove they can be our friends, he’s infected himself with tapeworms. It will be interesting to see how this turns out “in the end”!      

Science quote of the week:

“The most exciting phrase to hear in science, the one that heralds the most discoveries, is not ‘Eureka!’ (I found it!) but ‘That's funny’..."  --Isaac Asimov

Contributed by:  Bill Sullivan

Follow Bill on Twitter: @wjsullivan

Tamazian, G., Simonov, S., Dobrynin, P., Makunin, A., Logachev, A., Komissarov, A., Shevchenko, A., Brukhin, V., Cherkasov, N., Svitin, A., Koepfli, K., Pontius, J., Driscoll, C., Blackistone, K., Barr, C., Goldman, D., Antunes, A., Quilez, J., Lorente-Galdos, B., Alkan, C., Marques-Bonet, T., Menotti-Raymond, M., David, V., Narfström, K., & O’Brien, S. (2014). Annotated features of domestic cat – Felis catus genome GigaScience, 3 (1) DOI: 10.1186/2047-217X-3-13

Cox, L., Yamanishi, S., Sohn, J., Alekseyenko, A., Leung, J., Cho, I., Kim, S., Li, H., Gao, Z., Mahana, D., Zárate Rodriguez, J., Rogers, A., Robine, N., Loke, P., & Blaser, M. (2014). Altering the Intestinal Microbiota during a Critical Developmental Window Has Lasting Metabolic Consequences Cell, 158 (4), 705-721 DOI: 10.1016/j.cell.2014.05.052’’

Russell, S., Gold, M., Reynolds, L., Willing, B., Dimitriu, P., Thorson, L., Redpath, S., Perona-Wright, G., Blanchet, M., Mohn, W., Brett Finlay, B., & McNagny, K. (2014). Perinatal antibiotic-induced shifts in gut microbiota have differential effects on inflammatory lung diseases Journal of Allergy and Clinical Immunology DOI: 10.1016/j.jaci.2014.06.027

Dying To Make Us Laugh

It is the height of irony that Robin Williams (left) played Dr.

Patch Adams (right) in the 1998 film of the same name.

Adams used humor to heal and promote health, while new

research is showing that humorists are susceptible to more

harm when they improve our health with laughter.

One of the five funniest people to ever live died recently. Robin Williams was at least as funny as Jonathan Winters, Bill Cosby, Steve Martin, and Groucho Marx. He was 63 years old at the time of his death, and the others on my list have lived good long lives as well. Jonathan Winters was 88 when he died in 2013, Groucho Marx was 87, and Steve Martin and Bill Cosby are 68 and 77, respectively.

But a long-lived comedian is more of an exception than a rule. John Belushi, Gilda Radner, John Candy, Lenny Bruce, Patrice O’Neal, Madeline Kahn, Chris Farley, Bernie Mac, and Andy Kaufman all died in their 30’s or 40’s. This is sad to be sure, but it's even sadder when you consider that comedians hasten their own deaths while improving our health.

“Laughter is the best medicine.” The saying has been around for years – and it has merit. Laughter reduces cortisol production, which is a stress hormone that taxes our health and makes us gain weight. Laughter may improve our immune system function as well, and this fights off or prevents infection. A 2009 review showed that several studies indicated that laughter improved immune cell function (natural killer cells) and increased antibody levels (sIGA).

Laughter improves respiratory function and cardiac function because it increases respiratory rate and requires increased blood flow. These were reviewed in a very funny Christmas article in the British Medical Journal in 2013.

Laughter works on our brain too. Besides relieving stress, laughter triggers the release of endorphins that help our mood – and a good mood is a definite benefit to our health. Laughter also works on the neural pathways of resilience, so that we bounce back from disappointment better.

Gelototherapy is a pseudoscientific term for laughter therapy.

I say pseudo- because I can’t really find anyone who uses the

 term. But other health ideas have sprung up around laughter

as well. Laughter yoga (hasyayoga) is done in groups with eye

contact. It may start out as forced laughter, which is said to

have much less effect on health, but forced laughter turns

readily to spontaneous or mirthful, laughter, which lengthens

our life so we can go out to more yoga classes.

As far back as the 1970’s, journalist Norman Cousins advocated laughter therapy (gelototherapy) after he was said to have healed himself of heart disease and ankylosing spondylitis using Marx Brothers movies. He was an adjunct professor of Medical Humanities at UCLA where he did more research on the healing aspects of spontaneous laughter. But most of his results were on a case description basis – anecdotal at best.

There have been few if any studies that link laughter to extended longevity; however, a 2013 study of centenarians showed that they do tend to laugh more, as part of the PATL (positive attitude toward life). Linking laughter directly to longevity would be very difficult in the scientific sense, but the above stated health benefits can’t be hurting us, can they?

Apparently all this benefit comes at a cost to those giving us their gift. Comedians don’t just seem to die young, they are dying younger. Several studies have looked longitudinally at the health problems and obituaries of people in different professions, and that funny kids have more health problems later in life.

As part of a study of high intelligence individuals called the Terman Life Cycle Study (1922-1991), those kids rated by their parents as having a good sense of humor tended to have more health problems as adults, including alcoholism and lung disease from smoking.

People say that fat guys are funny. I’m fat and my kids say I’m

not that funny. But the question remains, do fat boys become

funny to survive, or does being funny make guys fat? Curly

Howard of the Three Stooges got fat after a bout of depression

at the end of his first marriage. He had a series of strokes late in

his movie career and died at the age of 49.

In a three-year study of police officers in Finland, those that were rated funnier or more humorous tended to be overweight, smoked more, and have more cardiovascular disease. For funny people who choose to become comedians then numbers just get worse. A 1992 study showed that comedians and humor writers died at younger ages. They tended to have more physical and mental health problems. This might relate to the environments in which they work – smoke and alcohol filled clubs, or it might reflect their tendency to see the humor and positive in things and not pay attention to risks of the unhealthy habits in which they engage.

A recent review of deaths by profession shows that performers of all sorts, including comedians, tend to die younger – health problems most certainly playing a role in their earlier demise. Robin Williams had suffered for years with substance abuse problems, bouts of severe depression, and heart disease. Comedians tend to have more depression than the general population and the suicide rate of performers, including comedians, is twice that of the general population.

But which comes first, do depressives become comedians because their altered thinking lends itself to looking at the world differently, or perhaps it is a way of self-medicating (as is drug abuse they tend to fall into) and fitting in with the world. Or, does comedy and the rejection that often comes with it, lead to more bouts of depression? Whichever it is, the physical and mental health problems of humorists seem all the more tragic when it is considered how much good these people do for us.

Contributed by Mark E. Lasbury, MS, MSEd, PhD

As Many Exceptions As Rules

C.R. Epstein, R.J. Epstein (2013). Death in The New York Times: the price of fame is a faster flame QJM: monthly journal of the Association of Physicians, 106 (6), 517-521

Greengross G (2013). Humor and aging - a mini-review. Gerontology, 59 (5), 448-53 PMID: 23689078

Ferner RE, & Aronson JK (2013). Laughter and MIRTH (Methodical Investigation of Risibility, Therapeutic and Harmful): narrative synthesis. BMJ (Clinical research ed.), 347 PMID: 24336308

Bennett MP, & Lengacher C (2009). Humor and Laughter May Influence Health IV. Humor and Immune Function. Evidence-based complementary and alternative medicine : eCAM, 6 (2), 159-64 PMID: 18955287