The thumb tack hypothesis

There are some interesting numbers in this paper from back in 2005. It's based around the well accepted fact that fat people move less than slim people. Apparently making heavy people move as much as thin people could easily result in 15kg of weight loss per year. That's pretty impressive for hiding the remote or putting drawing pins (thumbtacks?) on fat people's chairs.

The paper looked in great detail at the movement and energy expenditures of mildly obese people (BMI 33) or slim people (BMI 23). They found, as expected, that slim people move far more than fat people.

That's obvious from FIG 1. You really have to click to enlarge before it's readable:





From section A, top left chart, right hand pair of columns, you can see that thin people spent about 510 minutes up and walking.

Fat people were only up and moving for 370 minutes a day.

But now look at chart C, energy expended by activity, left hand pair of columns. The big red blocks on the tops of the columns are energy expended by being up and walking. Ignore the white extension, that's just the projection of what should (but won't) happen under the thumb tack hypothesis.


Thin people spent 800kcal per day on walking.



Fat people spent, guess what: 800kcal per day on walking.


Now, is that neat or is that neat? The lazy fatties were expending EXACTLY as many calories on being up and mobile as the slim people. This point seems to have escaped the authors' attention. Is this anti fat bias? Which group is laziest? Count those calories!

In fact, the only real difference between the groups is that obese people spent MORE calories overall per day and the excess is spent on basal metabolic rate. You cannot argue with a big body. It needs fuel. BMR is life. Obviously they have to eat more to do this.

The projection for 15 kg weight loss per year is based on making fat people mobile for as many minutes per day as thin people. But why should they do this? They are already spending as much energy as the thin person on spontaneous movement. They are spending MORE per day on BMR and an equal amount on odds and sods like the thermic effect of food. They eat more to make up for BMR and because their blood insulin levels steal a little food to store as fat.

Making them move more would simply need more calories. They would be hungrier.

The second phase of the experiment should have tested whether putting drawing pins on the chairs of fatties made them thin. The USA government is, after all, suggesting dance classes to replace TV viewing as the national pastime for its citizens. But I guess they really do know when they are on to a loser and decided not to test this.

Instead they looked at what happens when you make a fatty thin. Drop their weight down to BMI of 31 and look what happens. Well, nothing. A drop of 8kg from BMI 33 gets you down to BMI of 31, not 23. So we are not looking for a conversion from fat to thin, just a small increment, hopefully enough to show the trend. Here's FIG 2:





Weight loss means caloric deficit. BMR requires calories to sustain life so cannot be dropped much. The thermic effect of food etc expenditure makes little difference. If there are less calories spare during weight loss, what has to happen to movement? Look at chart A, right hand pair of columns. It drops from 390 minutes per day to 360 minutes per day, a drop of just under 10% in terms of time expended moving. Not statistically significant, but the trend is that weight loss by caloric restriction DECREASES spontaneous movement. This also was not noted by the authors, but would certainly have been predicted by Gary Taubes.

Get them down to BMI 23 and they would probably stay as still as practical for as long as practical. Then move to steal some food.


Over feeding makes you fat. It does it by increasing insulin levels. Do you then increase your spontaneous movement? The average extra free energy available during an increase of 4kg weight gain is small if insulin is packing most of those calories in to adipocytes, unless you are the outlier who upped their movement time by an hour a day (possibly the most insulin sensitive in the group?). The trend in spontaneous movement doesn't really show, but what hint there is is upward.

As Michael Eades has pointed out, he does see obese people who appear to be insulin sensitive, but they are uncommon. For most obese people the need is to lower insulin levels, then they won't need the thumb tacks on their chairs to either lose weight or become more mobile.

But thumb tacks on chairs is official policy. Without doing the trial.

Oh, I feel another paradox coming on!

Peter