Fat and tuberculosis

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Obesity May Have Offered Edge Over TB:

Over the course of human evolution, people with excess stores of fat have been more likely to survive famines, many scientists believe, living on to pass their genes to the next generation.

But these days, obesity is thought to be harmful, leading to chronic inflammation and metabolic disorders that set the stage for heart disease. So what went awry? When did excess fat stop being a protective mechanism that assured survival and instead become a liability?

A provocative new hypothesis suggests that in some people, fat not only stores energy but also revs up the body’s immune system. This subgroup may have enjoyed a survival advantage in the 1800s, when people were plagued by a disease that decimated Europe: tuberculosis.

The original paper is here. I’m skeptical, but I’d like people who know more about the history and distribution of tuberculosis to weigh in. My working assumption is that excess fat was helpful in most pre-modern contexts (i.e., female fertility) and obesity wasn’t common and simply a modern overshoot.

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8 Comments

  1. I’ve heard TB in America today is relegated to indian reservations and the like. Are native americans skinnier than average? I think the GSS has a variable for that.

  2. > I’d like people who know more about the history and distribution of tuberculosis to weigh in. 
     
    I can’t speak to that other than to say that Thomas Dormandy’s long, brass-tacks but straightforward, concrete, and basically totally old school medical and cultural history of TB was a great read. Fine prose too. I believe he’s an MD PhD, and he does have other books. However, I’m not sure the TB book has much of a section on what you’re looking for. 
     
    You may know, in much of today’s third world 50-100% of people are inactively infected, but 90% of those inactively infected live out their lives without ever having any open cavities, ie active, syptomatic disease. So – might higher inflammatory tone and higher fat reserves help prevent disease activation in the first place, in addition to helping you survive active disease? 
     
    I can’t see JAMA so I don’t know if they discuss that notion. However, as I recall, the 5-year survival rate (or maybe it was 10-year) after diagnosis of active disease was modest; I think it might have been around 10-15%. Most people croaked without going through multiple long remissions. I believe Dormandy has this information.  
     
    I couldn’t understand this sentence: 
    “My working assumption is that excess fat was helpful in most pre-modern contexts (i.e., female fertility) and obesity wasn’t common and simply a modern overshoot.”

  3. Native Americans now are very obese — and have been since they started eating a mostly-carb diet around the turn of the last century. Before that, they were lean. The Pima Indians are the best known case — once they started to eat government rations (mostly flour and other refined carbs), they got fat. 
     
    Gary Taubes, author of Good Calories, Bad Calories, reviews all of this as far back as data go for any population. Here’s a talk he gave, where he talks about the Pima (and others): 
     
    Taubes talk 
     
    If tuberculosis were the main driver behind obesity, then those populations most subjected to its pressures would have the highest frequencies of obesity. In particular, those practicing agriculture for the longest — and especially if they were subject to the 19th C epidemics — should be the most obese, then those that adopted it later, followed last by H-Gs. 
     
    Of course, that’s the opposite of what we observe. 
     
    Obesity is associated with inflammation because eating anything at all causes an inflammatory response — it’s foreign after all. The more you eat, the more inflammation. Carbs leave you hungry since they spike your insulin, which traps fat in fat cells where it can’t be used, spurring you to take in more food to get some usable energy. Fat does the opposite, leaving you sated. So the higher-carb diets that cause obesity will also cause more inflammation. 
     
    This is also because more glucose in the blood — from more carbs in the diet — will lead to a greater formation of Advanced Glycated End-products, screwed-up protein-sugar fusions that the body doesn’t recognize and goes after. A similar process happens when there’s a lot of fructose in the blood, as when we eat lots of high-fructose corn syrup or agave syrup / nectar. The liver can’t handle that much, so some stays in the blood. The process is called fructation, but basically the same idea. 
     
    And carbs in the diet cause your low-density lipoproteins to take on the small, dense shape rather than the large, fluffy shape. (Carbs in diet -> higher triglycerides -> higher VLDL -> which after disposing of their triglyercides turn into the small, dense LDL particles). The small, dense LDL particles are more easily oxidized, so this is another way that carbs are inflammatory. 
     
    Plus most agricultural and modern diets are low in omega-3 fats. The omega-6 kinds found in most vegetable oils are inflammatory. If you’re obese, you’re not stuffing yourself on salmon, but on products made with corn oil. 
     
    Given these mechanisms, it should be no surprise that when you feed people a low-carb diet, their inflammatory markers plummet: 
     
    Press release 
     
    Journal article 
     
    Obesity is not an adaptation to cause inflammation and fight disease — they are both effects of eating a high-carb diet that characterizes agricultural people, and especially the modern world during the past 25 – 30 years, when we began to eat a lot more carbs and less fat.

  4. I don’t know about TB, but I’m a naturally skinny guy who decided to stop being skinny and worked very hard to gain mass. Being sick with stupid little things like a bad cold or food poisoning sure is a lot less unpleasant when you’re carrying more fat. 
     
    I don’t know why – maybe having plenty of energy stored around your stomach and love handles to survive for a few days lets you be less lethargic and weak than when you don’t have much and have to start consuming muscle tissue? Intuitively it makes sense, though I don’t know what medical science says.

  5. Plenty of infos and links about fat/carb/omega-3/omega-6 and lame “modern diet” on this blog.

  6. When I worked with severely mentally ill, homeless, and addict populations, I was given TB test 2x year and provided with hepatitis vaccine.  
     
    Several people I worked with tested positive and took the 6 month (I think that’s what it was) course of treatment for TB.  
     
    Though I have no native American lineage, the proximity of my family to Oklahoma means I have lots of relatives by marriage that do. Anecdotally, they seem to have more problems with high cholesterol and high blood pressure, but I’ve known no one with TB, nor have I read about it in the various local newspapers serving eastern Oklahoma. 
     
    That I’ve not heard of a native American TB problem locally certainly doesn’t mean it doesn’t exist.

  7. A story for you Razib – “Evolution faster when it’s warmer” – http://news.bbc.co.uk/2/hi/science/nature/8115464.stm 
     
    Could explain why there us so much more genetic diversity amongst equatorial Africans as opposed to White Europeans or Asians who live in colder northern climates.

  8. Pseudothyrum, I think you have posted that in a number of places, and so you may have already heard the standard response. But to reiterate, you need to distinguish between temperature and Out-of-Africa demographic effects on genetic diversity in humans.

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