Thursday 23 August 2012

Does giving antibiotics to animals cause obesity in humans?

This sounds like a load of nonsense to me, but who knows?

Livestock drugs 'link' to obesity epidemic

Farmers may have fuelled the obesity epidemic by using antibiotics to fatten up livestock, a new study suggests.

This is based on a study published in Nature. I haven't read it because it's behind a paywall and I don't really care, but the salient points seem to as follows...

For decades since the 1950s, farmers have used low non-therapeutic doses of antibiotics to increase the body weight of cows, sheep, pigs and chickens.

They have indeed, although the EU banned the practice in 2006.

"The rise of obesity around the world is coincident with widespread antibiotic use, and our studies provide an experimental linkage," said lead researcher Professor Martin Blaser, from New York University School of Medicine in the US.

Hmm. Not the most compelling start. Correlation/causation and all that. Quite a lot of things coincided with the rise of obesity. Falling smoking rates and rising alcohol consumption fit pretty well, to take just two examples off the top of my head.

"It is possible that early exposure to antibiotics primes children for obesity later in life."

Anything's possible, Professor. So what is this research of which you speak?

The scientists administered common antibiotics such as penicillin and vancomycin to weaning mice at similar doses as those used in agriculture.

The treatment altered the composition of gut bacteria in the mice which in turn led to metabolic changes, such as increased production of fatty acids. After about six weeks the mice had gained about 10% to 15% more fat mass than untreated mice.

That's what one would expect to happen since the antibiotics are known to produce weight gain. What happened next? Were human subjects given a diet of these mice and carefully monitored for years to track changes in their body mass index?

They were not. It seems that fattening up the mice was the start and end of the experiment. So what does that tell us?

"By using antibiotics, we found we can actually manipulate the population of bacteria and alter how they metabolise certain nutrients," said co-author Dr Ilseung Cho, also from New York University.

Although it was known that antibiotics could fatten up animals, previously the mechanism involved was unclear.

Well, that's great and all, but identifying the biological mechanism behind a known effect is a little different from saying that antibiotics fed to livestock caused an international epidemic of human obesity.

All that's been shown here is that if you give certain drugs—which we know make large animals fatter—to small animals, then those animals will also put on weight. From this, you might speculate that if the same drugs were given to humans in equivalent quantities, they would also put on weight. That would depend on the doses, the type of antibiotics used and the length of time they were used for. It also relies on the assumption that humans will react to the drug in the same way as cows and mice.

If you were a reckless person—such as a Telegraph or Independent journalist—you could go out on a limb and imagine that eating slices of these animals might involve trace chemicals from the antibiotics entering the body and somehow having the same effect on weight gain, but that would be to indulge in wild speculation that is not supported by the study.

To repeat, I have not read the study, although this video of the author summarising its findings suggests that my description is fair. In it, he raises the possibility of antibiotics used in childhood being a possible explanation for childhood obesity. Essentially, he thinks that bacteria in the gut help prevent weight gain. That is a very tentative theory (although that doesn't stop the ever-irresponsible Daily Mail calling it "another very good reason for avoiding antibiotics") but it is not incompatible with the available evidence. What he does not do is suggest that there is some "passive" effect flowing from animals to humans. That appears to be an invention of the media and it should not be taken seriously.

Besides, everybody knows that obesity is caused by advertising. Didn't this guy get the memo?


Ivan D said...

I thought it was all down to Coca Cola Chris. The LibDems seem to think so and that nice chap over in New York seems certain.

Anonymous said...

My guess on this is whether there are sufficient antibiotics left in the animal to affect a human. I do not know for sure but I believe that there was a cut off period of 6 weeks when animals are stopped being fed antibiotics so they do not go to humans. This article says:

"An estimated 90 percent of antibiotics fed to food animals are excreted in their waste, and that manure is often sold as fertilizer products, some of which are marketed as “organic compost.”

It is implied that little or none is ingested by us.

If I can add my own junk science to the conversation, if sufficient antibiotics were ingested it might also be a clue why asthma may have risen since 1950. Of course you have confounders like increased industrialisation and vehicle exhaust emissions, or possibly even less cigarette smoke exposure. In the UK the rate of asthma did flatten off in mid 2000s. Multiple sclerosis and asthma have been linked to over clean households and under active immune systems.

Of course correlation/causation here too and I could be talking as usual complete b0llocks.

Carl V Phillips said...

Further to Dave's comment, most antibiotic residue that are ingested is digested so that it is denatured before it enters the body. Use of antibiotics in ag creates nasty externalities by creating resistance in the buggies, but the doses of antibiotics kids get intentionally are going to dwarf the residue.

Also (working from memory and not claiming to be an expert on this topic), I recall that the evolution time to replace your gut fauna with whatever is "optimal" for your particular diet is a matter of a year or two. So childhood exposure will not create a lifetime change. Also, the evolution tends toward being maximally efficient use of the energy in the food (selection pressure favors the buggies that can eat whatever ends up in your gut).

So if you move to a different food culture or radically change your diet, you will not make as efficient use of the food as you were doing on your old diet (or you will in a couple of years if you stick with the new). That suggests that anything that messes up the optimal evolutionary equilibrium in your gut will *reduce* your energy use efficiency.

In other words, this theory just does not make any sense.

Unknown said...

I'm with you on this one Chris. Can't rule it out, but it's unlikely. Considering antibiotics stop getting administered before the animals are slaughtered, and that smaller animals (so i'm not talking about cows) would presumably be given relatively low doses, in terms of what a human would require anyway.

Add to that the fact that we wouldn't eat the whole animal, but a slice of ham, or a cut of steak or a burger, and it seems to diluted as to not be there. I could see some sense in this if we ate a whole cow moments after it had antibiotics, but then again i would argue it would be the eating of a whole cow that made someone obese.

But hey, that's just me. As you say, they'd only get fat if the cow was wearing a sandwich board advertising something.

Anonymous said...

Is this a silly question? Would cooking not destroy the antibiotic?

zdk said...
This comment has been removed by the author.
Anonymous said...

I know you didn't read the paper (the paywall thing) but if that's the case maybe you should, you know, not make assumptions based on something you didn't read... ?

The idea is that antibiotics directly perturb the microbiome, changing the way mammals metabolize food, changing the kind of molecules we're getting from food. The point was NOT that we're getting antibiotics through meat, but that what happens in cows, pigs and mice is also like to happen in people.

In the study antibiotic treatment clearly correlates (though possibly not causative) with changes in the microbiome structure and genes in the liver responsible for nutrient digestion.

No other claims are made. So antibiotics are postulated as contributing to the obesity epidemic... with no stipulations about it being the most important cause.

Really, if you're going to do research blogging at least make sure you READ the paper before attacking it.

Anonymous said...

Blaser, who did the mouse experiment, is not ruling out anything:

He's also looking at whether extremely low antibiotic doses, such as people might receive by eating meat and dairy products from antibiotic-fed animals, have metabolic effects.

Interesting observation from the mouse study:

Mice that got the antibiotics piled on more fat than other mice, even though the fatter mice did not eat more. Also, their poop had fewer calories – suggesting they were absorbing more and eliminating less.

The Tresande study in the above link is also interesting. Infants 0-5 months who received antibiotics were fatter after 3 years than infants treated later or not at all.
Not an experiment of course, but an interesting and maybe even scary finding.