Obesity, delayed gratification and the Marshmallow Test


As of September 2017, new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Check out my bestselling new book on the science of endurance, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance, published in February 2018 with a foreword by Malcolm Gladwell.

- Alex Hutchinson (@sweatscience)


A new follow-up to the famous Marshmallow Test study on delayed gratification has just been published. Back in the 1960s, researchers tested a group of pre-school children on how long they could resist the temptation of an immediate reward (e.g. a marshmallow) in favour of a “larger, later” reward (e.g. two marshmallows). They followed these kids for decades, and found that the kids who were able to hold out the longest ended up less vulnerable to outcomes ranging from obesity to divorce to crack cocaine addiction.

The newest update, just published in Proceedings of the National Academy of Sciences (full text freely available here, press release here), with the subjects now in their 40s, confirms that the kids who were “high delayers” are still “high delayers,” and the kids who were “low delayers” are still “low delayers.” For the first time, they used brain scanning to determine that the high delayers showed greater activation in the prefrontal cortex while the low delayers had greater recruitment of the ventral striatum. This may reflect the differing use of of different “cold” and “hot” modes of cognition in choosing between competing impulses.

Anyway, I’m not going to go into great depth about the neuroscience here (as noted above, those who are interested can read the full paper freely). What caught my attention was the following quote in the press release:

“This is the first time we have located the specific brain areas related to delayed gratification. This could have major implications in the treatment of obesity and addictions,” says lead author Dr. B.J. Casey, director of the Sackler Institute for Developmental Psychobiology at Weill Cornell Medical College and the Sackler Professor of Developmental Psychobiology.

One of the interesting debates that I’ve become more attuned to in following the blogs of people like Yoni Freedhoff and Arya Sharma is the tendency to ascribe moral failings — a lack of willpower and unwillingness to make the “right” choices — to obese people. Dr. Sharma frequently argues that “Eat Less, Move More”-type advice is useless for losing weight, because it fails to understand the “countless ways in which the psychoneurobiology, energy physiology and metabolism in anyone who has lost weight” drive you to regain that weight.

So in this picture, does increased power of delayed gratification have any role in treating or avoiding obesity? Or are the biological imperatives too strong for anyone’s self-control? Dr. Sharma had a very interesting post a couple of weeks ago about the role of personal choice in weight loss, responding to a recent paper in the Journal of the American Dietetic Association. He doesn’t reject the role of impulse control in weight loss — in fact, he suggests it should be considered:

Recognising and fully acknowledging how the brain’s neural circuitry that underlies these behaviours interacts with (and is thus ultimately responsive to) environmental situations and cues can perhaps provide a far more realistic and effective counseling strategy.

Of course, losing weight and avoiding weight gain in the first place are two distinct questions — and in the long term, any success we have in tackling society’s growing levels of obesity will probably come from helping future generations avoid obesity in the first place. The Marshmallow Test data does tell us something interesting: that you can predict who’s most likely to become obese based on tests of brain function in pre-school. That has nothing to do with resting metabolic rate, aptitude for sports, or even what they’re being fed at home.

Obviously, this trait isn’t the root of the problem. Presumably humans have always been born with varying degrees of delayed gratification; it’s only in our modern society that low delayers are at risk of obesity. This is consistent with the idea of an “obesogenic environment” — a world with a copious oversupply of calorie-dense food, convenient labour-saving devices always available, ubiquitous advertising to tempt us into taking the first marshmallow.

But still… it suggests that choices matter. I realize this starts to sound like a moral judgement (i.e. obese people must have made the “wrong” choices), but I don’t mean it that way. In fact, the Marshmallow Test tells us these choices are, to some extent, hardwired into us. But by acknowledging the role of choices, and understanding how and why the “wrong” choices are made, perhaps we can increase our odds of making the right choices. Dr. Sharma suggests a few ways this might work in the post I quoted from above. Another option: the idea of “brain training” is in disrepute right now, partly because it was so dramatically overhyped and oversold a few years ago, but maybe it’s something to consider. It’s a topic that comes up (peripherally) in the Jockology column I just wrote for next Monday’s Globe, and I’m looking forward to seeing more research on it.

Running on grass bursts more red blood cells than asphalt


As of September 2017, new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Check out my bestselling new book on the science of endurance, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance, published in February 2018 with a foreword by Malcolm Gladwell.

- Alex Hutchinson (@sweatscience)


Interesting new study in the September issue of Journal of Sports Sciences that Amby Burfoot recently tweeted about, which appears to show that running on grass causes more hemolysis — the rupture of red blood cells — than running on asphalt.

It’s been known for a long time that prolonged running causes hemolysis. In fact, most forms of prolonged exercise can cause some hemolysis, simply because blood is being pumped around so vigorously and exposed to high oxygen fluxes. But running is particularly susceptible because of the footstrike: the cells get squashed as they pass through the capillaries of the foot at the moment of impact. Is this a problem? Well, excessive hemolysis can play a role in iron insufficiency — but this is generally only a problem if there are other exacerbating factors like overtraining. In this case, hemolysis is mainly interesting not because it’s a serious problem, but because it can tell us something about how different surfaces affect footstrike impacts.

The new study, by researchers in India, is very simple. Ten runners ran for an hour at 60-70% max heart rate on grass, while another 10 did the same on asphalt. Blood samples were taken before and after. The researchers expected to find more hemolysis from the harder asphalt surface, but they instead found the opposite. So what’s going on? Although the grass was superficially smooth and even, they speculate that its underlying unevenness affected the runners’ strides:

Running on uneven and inconsistent surfaces like the beach or grass can cause more injuries because each step creates varying pressures and forces in the feet, ankles, knees and hips as runners most constantly adjust to the surface. These natural surfaces also tend to slope and create a dangerous off-centre force on the ankles and feet while running. Even though the grass surface appeared to deform relatively more than asphalt, it was assumed that the uneven nature could have led to inappropriate pressure distribution and impact forces on the foot, which could have resulted in an increased haemolysis in these runners.

As Burfoot points out, this is reminiscent of Benno Nigg’s ideas: whether a surface is hard or soft, your leg automatically adjusts to cushion the impact. But on surfaces where you’re unable to correctly predict exactly how your foot will land — i.e. grass — that automatic adjustment can’t take place, and that’s when strong uncompensated forces shoot up your legs.

So it all fits together, right? Well, I have one caveat. Here’s the data from the new study for the two key measures of hemolysis (unconjugated bilirubin and lactate dehydrogenase, for those keeping score at home):

As you can see, the pre-run differences between the two groups (which were assigned randomly) are greater than the change from pre- to post-run! So statistical analysis may suggest that the two groups responded differently, but for now I’d treat this finding very cautiously. This study really should have been conducted as a randomized crossover trial, so that each runner was measured under both conditions. Bottom line: great idea for an interesting study, but until the results are replicated somewhere else I’d take the conclusions with a grain of salt.