That was one of the cases put forth by Dr. Bob Ross during our Forks vs. Feet debate.
He had discussed an as of then unpublished study that concluded that due to changes in occupation-based physical activity, we were all on average burning 100 fewer calories per workday, and that those no longer burned calories have caused us to become obese.
Well, the paper was just published and I had a gander.
Now I do think there are weaknesses to the analysis, in that this study of theoretical energy expenditure lost at work doesn't in fact provide a picture of total daily energy expenditure. Meaning that the authors have no idea what the study subjects energy expenditures were like when they weren't working. That's problematic for a few reasons.
Firstly it's problematic because it's possible that if you take away 100 calories burned through physical labour, perhaps you'll put them back elsewhere. What I mean to say is that in children we've seen evidence for the existence of an Activitystat whereby kids who exercise more at school do less at home and vice-versa. Given such behaviour has been demonstrated to exist in children, I don't think it's an impossible stretch to wonder if it also occurs in adults, especially since we're talking about just 100 calories per day. And even if it didn't translate to intentional exercise, couldn't more sedentary jobs lead to more fidgeting? More fidgeting would mean more Non-Exercise Activity Thermogenesis. Dr. Levine suggests purposeful NEAT can add up to 500-1000 calories a day, how difficult would an unconscious 100 be?
Secondly it's problematic because doubly-labeled water studies suggest that at least between 1983 and 2005, in study populations from Holland and North America, daily energy expenditures haven't changed. Yet in both Holland and North America during that same time period obesity rates have risen dramatically and presumably, we're working progressively cushier jobs.
Thirdly it's problematic because in doubly labeled water studies that look at total daily energy expenditure of folks living in developing nations vs. developed nations, for instance subsistence farmers in Nigeria vs. urban Chicagoans, there's been no difference in calories burned, and that total daily calories burned in both populations didn't correlate with weight. I would certainly imagine that being a subsistence farmer in Nigeria would be quite physically demanding work.
Lastly, in a massive study of 98 doubly labeled water studies representing 183 cohorts including 14 from countries with low or middle "human development index" (and hence more likely to have physically demanding jobs), again there was no difference in total daily energy expenditures.
But even putting aside those concerns, I think the paper's conclusions are telling in regard to the Forks vs. Feet debate.
Clearly both forks and feet are thermodynamically implicated in obesity.
In one corner, taking this paper to be true (true despite the fact doubly labeled water studies that actually measure total daily energy expenditures suggest otherwise), we've now got 100 calories a day we're not burning due to less physically demanding jobs.
In the other corner we've got energy intake data suggesting that since 1970, based off of plate-waste disappearance data, adults are consuming 500 more calories daily now as compared with 1970.
Basically we're looking at a 600 calorie surplus. 100 from fitness and 500 from food. Put another way, our modern caloric excess is 83% food and 17% fitness, not exactly a home run for the Feet camp, but damn close the 80/20 rule I tend to believe is true.
Church, T., Thomas, D., Tudor-Locke, C., Katzmarzyk, P., Earnest, C., Rodarte, R., Martin, C., Blair, S., & Bouchard, C. (2011). Trends over 5 Decades in U.S. Occupation-Related Physical Activity and Their Associations with Obesity PLoS ONE, 6 (5) DOI: 10.1371/journal.pone.0019657
Wilkin, T., Mallam, K., Metcalf, B., Jeffery, A., & Voss, L. (2006). Variation in physical activity lies with the child, not his environment: evidence for an ‘activitystat’ in young children (EarlyBird 16) International Journal of Obesity, 30 (7), 1050-1055 DOI: 10.1038/sj.ijo.0803331
Luke, A., Dugas, L., Ebersole, K., Durazo-Arvizu, R., Cao, G., Schoeller, D., Adeyemo, A., Brieger, W., & Cooper, R. (2008). Energy expenditure does not predict weight change in either Nigerian or African American women American Journal of Clinical Nutrition, 89 (1), 169-176 DOI: 10.3945/ajcn.2008.26630
Lara R Dugas, Regina Harders, Sarah Merrill, Kara Ebersole, David A Shoham, Elaine C Rush, Felix K Assah, Terrence Forrester, Ramon A Durazo-Arvizu, & Amy Luke (2011). Energy expenditure in adults living in developing compared with industrialized countries: a meta-analysis of doubly labeled water studies The American Journal of Clinical Nutrition, 93 (2), 427-441 : 10.3945/ajcn.110.007278
Westerterp, K., & Speakman, J. (2008). Physical activity energy expenditure has not declined since the 1980s and matches energy expenditures of wild mammals International Journal of Obesity, 32 (8), 1256-1263 DOI: 10.1038/ijo.2008.74
Swinburn, B., Sacks, G., & Ravussin, E. (2009). Increased food energy supply is more than sufficient to explain the US epidemic of obesity American Journal of Clinical Nutrition, 90 (6), 1453-1456 DOI: 10.3945/ajcn.2009.28595