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Animal Testing

Is Everything Getting Fatter?

Here’s a weird and interesting paper to think about. There’s an unfortunately clear trend in human populations towards obesity, which seems to have been especially noticeable over the last thirty years or so. (I believe that the average-thinnest US state today would have been the fattest state in 1980, just to give one vivid example). And it’s not just the US – other areas of the world have shown similar trends. All sorts of causes have been advanced – greater availability of high-calorie-content food can’t be ignored, perhaps less physical activity overall, and there’s been a search throughout all sorts of dietary factors.

But this new paper isn’t looking at the human population at all:

We examined samples collectively consisting of over 20 000 animals from 24 populations (12 divided separately into males and females) of animals representing eight species living with or around humans in industrialized societies. In all populations, the estimated coefficient for the trend of body weight over time was positive (i.e. increasing). The probability of all trends being in the same direction by chance is 1.2 × 10−7. Surprisingly, we find that over the past several decades, average mid-life body weights have risen among primates and rodents living in research colonies, as well as among feral rodents and domestic dogs and cats.

Hmm. There are still a lot of confounding variables here, but this is, um, food for thought. The authors did exclude a number of the more obviously problematic groups:

We excluded datasets (i) consisting solely of terminal or late-life weights because weight loss often occurs towards the end of life, and presages death, and population differences in late-life weights are often not representative of population differences in weight during earlier adulthood; (ii) consisting of animals that, during the period considered, were known or were likely to have been exposed to deliberate selection for phenotypes related to weight or adiposity (effectively ruling out livestock); (iii) consisting of animals that were calorically restricted or had their food intake titrated to maintain relatively constant body weights; and (iv) uniformly exposed to suspected toxins or drugs (e.g. the treatment groups from toxicology programmes).

And after that, all 24 populations studied (12 animal datasets, male and female) still show definite trends towards increasing weight. If you combine the genders and look at the 12 populations, the trend still holds, with a p-value of  0.000049. And it’s true that p-values can be misused and can be misleading, but at that level, you should assume that there’s something going on.

Our findings reveal that large and sustained population increases in body weights can occur in mammalian populations, just as they have occurred among human populations, even in the absence of those factors that are typically conceived of as the primary determinants of the human obesity epidemic via their influence on diet (e.g. access to vending machines) and physical activity (e.g. less physical education classes in schools)

Now, feral rats could be chowing down on a greater abundance of rich food in the available garbage, but that doesn’t explain the upwards creep in ad libitum fed research animal weights. And perhaps better (or more indulgent) care explains the increase in domestic dogs and cats, but that doesn’t apply to the wild rats. And so on – as the authors take care to point out, there’s no reason a priori that a single explanation should apply across all these populations, but when you see this noticeable a trend across so many groups, you would also be remiss not to speculate about one. You can go off track by seeking overarching explanations, but you can go off track by trying to treat everything as a special case, too.

If there is a single cause behind all this, you’re looking at (among other things) the possibility of endocrine disruption from some environmental contaminant (or contaminants), or perhaps some factor of that sort affecting epigenetic signaling pathways, or even an infectious agent. All of these have been thought about in the past, but other explanations have always seemed more plausible. This paper, though, does. . .well, I was about to say “put some more weight behind such theories”, but you know what I mean. What would make an interesting comparison would be the weights of animals over this period that have little or no human contact, but those are probably hard data points to get.

A side effect of this work is that every crackpot in the world will now use it to show why their pet theory is clearly the real explanation. The paper has only been out for a few days, but get braced – it’s coming. But we may still end up with a real explanation that’s not what we would have bet on. I await more research in this area with interest.

49 comments on “Is Everything Getting Fatter?”

  1. MTK says:

    I haven’t read the paper, but domestic cats and dogs doesn’t surprise me in the least.

    The pet food industry isn’t that much different than the human food industry in making food cheaper, tastier, and more calorie rich than ever. Not too mention more pets now are strictly house pets than ever before and don’t run around the yard or neighborhood as much.

    As for lab animals I’d wonder the same thing regarding chow. For many caloric intake is probably highly monitored but I would wonder if where those calories come from has changed in monkey chow for example in the last few decades.

  2. pi* says:

    is it neutrinos?

  3. anon says:

    No dude, it’s chemtrails

  4. physicist says:

    more like dark energy…the fabric of the universe is expanding

  5. sgcox says:

    Good ! So it was not my fault after all, whatever doctor said.

  6. Wile E. Coyote, Genius says:

    It’s global warming. The animals have to expend less energy to stay warm. Therefore those calories are added as fat instead. I think this also largely explains the obesity problem in humans. It all fits together in a perfectly logical manner. No point in fighting it. As global warming progresses, the obesity epidemic will only increase exponentially.

  7. sgcox says:

    #Wile E. Coyote, Genius

    Are coyote dramatically fatter in Mexico than in Canada ? If not, ambient temperature can be ruled out.

  8. Edgar says:

    Regarding, the increase in ad libitum fed research animal weights, the question that immediately comes to mind is whether the type of food given to them has increased over time. In other words, is the product that they are fed just getting better over time – more nutritious and/or more caloric, etc.

    It seems that all the observed trends could simly be traced back in some way to greater availability of food calories.

  9. Hap says:

    I wouldn’t think an infective agent would hit that many species, but maybe a competing bacteria or a virus or phage infecting our gut bacteria might perturb our digestion?

    I hesitate to bring this up, but much of the difference in US health care costs is supposed to be due to conditions specific to the US rather than our systems, particularly our tendency toward obesity. If obesity is increasing elsewhere, are the health outcomes of other countries perturbed proportionately?

  10. Edgar says:

    Oops, in my last comment “has increased over time” should be “has improved over time”.

  11. Magrinho says:

    It’s a response to rising CO2 levels. Obesity is a way of fixing CO2 from the environment. Somebody will discover the CO2-dependent metabolic pathway – I just know it.

    Now if we could only figure out what to do with the bodies…

  12. SomeGuy says:


    Microbiome was the first thing that came to mind for me. Incidentally, our mouse colony has also been seeing weight and fasting glucose creep up over the past several years as well. Been wanting to do a more thorough analysis of this.

  13. Manoj Jadhav says:

    Dear Derek,
    It is interesting observation. When we speak on the developed world it has forgotten basics of life in the quest to make things more perfect, elegant and rich with nutrition.
    People of the developed world had set up their lifestyle into “default mode” where in the human energy is substituted by mechanical energy (machine will do all job!), displacing household food production with industrial food production (ready to eat food- high calories, lots of chemical stabilizers), and instead of health maintenance inclination to the medical dependency.
    If we as human have to address this issue, we need to go back to the basics and get close to the nature to be a lean generation again! This truly needs the real intent of the policy makers and the mighty industry who do not wish to loose on their extra cents…
    We as a scientific community needs to be also blamed for promoting not so healthy science (pill for every health problem!) It is wisely said that anything excess creates problem!

    1. Orv says:

      @Manoj Jadhav: We would certainly all be skinnier if we went back to subsistence farming, yes. It’d also probably make us carbon neutral. But we can’t all do it; there’s too many of us, and not enough land. It also probably wouldn’t make us live any longer. Manual laborers on average have shorter lifespans, and farming is the 8th most dangerous job in the U.S.

  14. PPedroso says:

    Could it be Aliens fattening us to eat us in the near future?

  15. Newbie PI says:

    Epigenetics? Effects of well-fed mothers, having well-fed babies, becoming well-fed mothers, having well-fed babies?

    Also, my understanding was that for many human populations we were also getting bigger in terms of height.

  16. steve says:

    Sugar. It’s not just all the added sugar in processed foods, even our vegetable and fruit selections have been bred to be sweeter and less nutritious. 100 years ago no one would recognize what we now call watermelons, corn, peaches and other fruits and vegetables. People get upset about genetic modification but it’s already done. By making our food sources sweeter we also reduce beneficial compounds like flavonoids and other phytochemicals that are bitter in taste but have numerous health benefits. The more people become addicted to sweets the more they eat sugary foods and the less nutrition they actually receive.

  17. Mark Thorson says:

    GMO corn and soybeans, obviously. Wild rats are eating human food waste, lab rats are eating GMO feed.

  18. A Nonny Mouse says:

    As Steve above says, sugar but specifically high fructose corn syrup.

    I heard a radio program on this a few weeks ago driving home (BBC radio 4- strangely about economics)

  19. Anon says:

    This is really odd, and worth any attempts to explain it…

    But in general, all I can think of is greater availability of higher-calory food, with less competition and less exercise in less space to get it.

    Basically, intensive farming.

    It’s more likely than increase in mass and/or earth’s weight and gravity due to relativity.

  20. NJBiologist says:

    I can’t find a reference right now, but I’ve heard it argued that breeders are responsible for the ever-increasing girth of the Sprague-Dawley rat (and many other strains). The idea is that many animal orders are placed by weight, so faster-growing rats reach market size faster. A handful of breeders have resisted this practice, yielding anomalies like the Wistar Hannover rat (Charles River gives a median size of ~325g–nearly terminal weight–for 3 month old males, vs. a bit over 400g–and still growing–for the CD-branded Sprague-Dawley males at that age).

  21. Sam Adams The Dog says:


    I have never understood the view of those who claim we are not part of “nature.” “Get back to nature”? We never left. I think Bucky Fuller once said, “If it’s not natural, you can’t do it.” And walking to school as a kid in the Bronx, I remember being puzzled by the idea that the sidewalks aren’t nature, but the grass growing in the cracks is.

    I also remember being taken on a class trip as a kid to Washington Irving’s house, “Sunnyside” in the Hudson Valley. The doorways were so low that the adults could not walk through without stooping, We were told that the human population in the U. S. has increased greatly in height since then, as a consequence (or so they said) of better nutrition.

    So the tendency toward greater weight is not the first of these phenomena we’ve seen, at least in the human population. In fact, by the time we figure out what causes it, we’ll probably find at least a few good things to say about it.

    1. Wolf Baginski says:

      There’s a little bit of military history that might be of interest here.

      In 18th Century Europe one of the few sources if height and weight measurements is Army records. As the Industrial Revolution started, Armies could recruit plenty of tall, well-muscled men. Then, as the population shifted from the rural to the urban world, the record show a drop in height and weight. In Britain, the War Office started to get a bit worried. And, a hundred years ago in the Great War, they found themselves having to take men they wouldn’t have accepted because of their heigh and weight (the “Bantams” battalions).

      That was the low point. Vitamins were being discovered and deficiency diseases getting cured. Scurvy, over a century before, had been the first, though the initial understanding was imperfect. Victorian England saw the emergence of branded packaged foods, such as the products of Kellogg and Heinz.

      There were some legendary products used by the British Army, such as McConachies. It was, almost accidentally, the world’s first vegan meat stew. As civilian food has improved, and WW2 food rationing in Britain had positive health effects, the Army had to match that advance.

      This is a fascinating thread in history for some. I spent most of my life in a house built around 1660, and the doors were low. I am told I have a habit of ducking. You can see how the highs and lows of climate affected height. And with longer limbs and more muscles driving them, it’s not so surprising that athletic records keep being broken,

  22. Language Hostile says:

    Edit suggestion – ‘…show why their…’ instead of ‘… show whey their’?

  23. FB says:

    I think the Food Babe has the answer.

  24. flem says:

    Maybe its evolution! same thing that made the dinosaurs so big. the only cure is a giant meteor strike

  25. John Wayne says:

    I really wish I had a pet hypothesis to trot out. Epidemiology should ask questions, and we have to design very careful studies to separate causation from correlation; anybody who skips these steps is trying to sell you something.

  26. Mark Thorson says:

    What has increased enormously since 1980? Cellphone radiation, obviously.

  27. Tuck says:

    “Dietary linoleic acid elevates endogenous 2-AG and anandamide and induces obesity.”

    2-AG and anandamide induce overeating. 2-AG is the endogenous endocannabinoid that THC mimics. So we’ve all got the munchies. In a nutshell.

    High quantities of linoleic acid (via seed oils) are a novelty in the human diet. Sugar is not.

    1. Gibbon1 says:

      Actually I think seed oils are definitely a modern thing as in last 150 years. But also before that sugar as in glucose+fructose is also a modern thing. From memory prior to 1900 consumption of fructose was probably 25gm/day. Neolithic probably 15gm or less. Now you have teenage boys consuming 100-150gm/day.

      Sometime you see graphs that show consumption hasn’t gone up ‘that much’ but those usually begin around 1950-1960 and thus miss the big run up from 1880 to 1950.

  28. Judge says:

    I blame it on Playstation – it keeps people too busy to take the dog/cat/rat/cow/sheep out for a walk.

  29. milkshake says:

    My bet would be on ever-present residues of antibiotics in the food we eat: in dairy products (nisin) but manly in chicken, pigs and cattle – not only the don’t get sick, they actually grow faster by eating more while absorbing nutrients more effectively when given antibiotics, because the antibiotics changes the flora in the gut. And I wonder if changes in the intestinal flora in humans also happens – boosting the absorption and muting the signals of being full.

  30. Sam says:

    Surely BPA deserves a mention?

  31. Luke Muehlhauser says:

    > The paper has only been out for a few days

    What? It was published in 2011.

  32. Morten G says:

    My pet theory:

    It’s carbon dioxide. Even at moderately increased levels for a couple of hours you see distinct impairment in human cognition.
    The typical responses to increased carbon dioxide are head aches and sluggishness. Atmospheric carbon dioxide is much less increased but a constant effect. Opening the windows and airing out is not helpful. So a slightly increased sluggishness for the entirety of the animal’s life leads to increased weight.

  33. steve says:

    @Tuck – The levels of simple sugars in the current US diet are indeed quite novel in human history. I’m not talking about complex carbohydrates but rather glucose and sucrose levels. @Milkshake – I’m sure you’re right. It’s not just antibiotics in food but also antibiotics given to children. The increased understanding of the role of the microbiome in human health has made it quite clear that antibiotic use is a major issue.

    There is a synergy here with diet as high sugar intake also influences the microbiome, particularly after it’s been disrupted with antibiotics. High sugar intake together with other elements of US society (which has been exported to many other countries) such as inactivity, reduced intake of fiber, antibiotic use and increased omega6:omega3 ratios (15:1 in Western diets instead of the 1:1 during human evolution) all synergize in producing obesity.

    Listening to Donald Trump probably doesn’t help either.

  34. Ash says:

    Thanks Obama.

  35. DrSnowboard says:

    Don’t pets just get to look like their owners…?

    But I like the better healthcare / antibiotic argument from Milkshake. However, does this kind of big data exercise ever give a meaningful result?

  36. dams says:

    The next crucial question is : why are the dutch so tall..?? OMG I feel so insignificant in this country..

    1. Sven Norén says:

      To keep their noses above sea level.

  37. colin says:

    Animals evolve to be bigger (less heat loss, more females, less predators, etc.) in stable environments. Then the proverbial meteor hits, and the smallest wiliest survive. Which then evolve to be bigger again. I’m sure the biggest animals have been unequally decimated by humanity. These “fattening” animals are probably the ones doing disproportionated well out of humanity.

  38. steve says:

    @dams – Don’t worry, you’ll have the last laugh; you’ll live longer.

  39. Dude says:

    Nothing new. Animals get bigger. It happened with the dinosaurs millions of years ago.

    Cats and rats will grow to be 20 feet tall, then go extinct.

  40. aairfccha says:

    My first thought was “When speculating about endocrine disruptors, what about hormones in the first place?” but the side effects of what I guess is the most common newcomer in the environment (Ethinyl estradiol from oral contraceptives) include *loss* of appetite.

  41. MoMo says:

    Bingo Milkshake! Low level antibiotics are dosed at grams per ton to fatten livestock prior to slaughter. Those chemicals are also low dose for us and causing weight gain in humans. But wait- thats not all! All of the endocrine disruptors, BPA, PBDEs, glyphosate, and a host of other chemicals spread in the environment plus the organic persistors such as triclosan, prozac and about 20 others are reaking havoc as well, including ethynyl estradiol. Then sperm levels in males have dropped 50%, females are experiencing lower birth abilities, and the makers of HFCS are essentially changing adipose cell ratios in vivo.

    I propose if a molecule doesnt break down in the environment, it shouldnt be released into the general population. But its too late for many!

  42. RM says:

    It looks like all the animals in the study were either domestic, laboratory raised or closely human-associated, meaning that a significant portion of their food was derived from human sources. I’m wondering if similar trends are seen in purely wild animals over the same time period, and if such trends are correlated with how close the wild animal is to human agricultural product food sources. (e.g. if there’s a difference in trends for sparrows on farm fields in Kansas versus moose in rural Alaska.)

    Regarding the ad libitum feeding, I’d also be interested to hear how much mouse chow formulations have changed over the years.

  43. Jim T says:

    Cars sure seem to be getting fatter:

    Seriously, wouldn’t it be more concerning if there were no discernible trends either way?

    1. Orv says:

      I’ve always found it interesting that the trend for any given car model is to get larger over time. Eventually it grows right out of its market niche, and another model gets slotted in below it. A good example is the Honda Fit, which is a subcompact about the size the Honda Civic was in the 1980s; while the Civic has grown out of the subcompact category and is now considered a compact car instead.

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