You are partially correct: I have erred in deeming the thermal efficiency of typical high-T-gradient heat engines greater than that of humans (whose organs exploit more modes of energy conversion than those in a heat engine).
However, the conclusion is robust when comparing from the appropriate baselines. To find the total energy-to-mechanical-energy conversion efficiency, you have to factor in the energy losses in generating the sugar to begin with. This gives sugar cane as having the highest photosynthetic efficiency of 8% (light energy to sugar chemical energy).
That must be applied against the 28% thermal efficiency (sugar energy to mechancial energy) I calculate for humans [1], leaving 2.2% net light-to-mechanical efficiency (neglecting distribution energy costs for the sugar).
This is still inefficient compared to other means of using the same sunlight. Taking a characteristic solar cell efficiency on the low end of 6% (light to electricity), with a characteristic efficiency of 90% (electricity to mechanical) gives a 5.4% net light-to-mechanical efficiency -- still significantly higher than that of growing sugar and feeding it to humans!
[1] Human efficiency estimated from the following assumptions: 816 Cal/hr burned by a 200 lb individual climbing stairs at 0.30 m/s; this gives an energy consumption rate of 952 W and mechanical output of 267 W, or 28% efficiency, though again this is only sugar-to-mechanical efficiency.
Using sugar for human consumption is, in a sense, quite wasteful.
I claim, that given sugar, using it for human consumption is one of the least wasteful things to do with it.
This is still inefficient compared to other means of using the same sunlight.
In the future, if there is an option between powering organic people with sugarcane-produced sugar and powering cybernetic people with solar cells, and we can choose to be either organic or cybernetic, then your argument will be valid—assuming there are no other options, which is silly. For right now, pe...
Related to: Babies and Bunnies: A Caution About Evo-Psych, Superstimuli and the Collapse of Western Civilization.
The main proximate cause of increase in human weight over the last few decades is over-eating - other factors like decreased energy need due to less active lifestyle seem at best secondary if relevant at all. The big question is what misregulates homeostatic system controlling food intake towards higher calorie consumption?
The most common accepted answer is some sort of superstimulus theory - modern food is so tasty people find it irresistible. This seems backwards to me in its basic assumption - almost any "traditional" food seems to taste better than almost any "modern" food.
It is as easy to construct the opposite theory of tastiness set point - tastiness is some estimate of nutritional value of food - more nutritious food should taste better than less nutritious food. So according to the theory - if you eat very tasty food, your appetite thinks it's highly nutritious, and demands less of it; and if you eat bland tasteless food - your appetite underestimates its nutritious content and demands too much of it.
It's not even obvious that your appetite is "wrong" - if you need certain amount of nutritionally balanced food, and all you can gets is nutritionally balanced food with a lot of added sugar - the best thing is eating more and getting all the micro-nutrients needed in spite of excess calories. Maybe it is not confused at all, just doing its best in a bad situation, and prioritizes evolutionarily common threat of too little micronutrients over evolutionarily less common threat of excess calories.
As some extra evidence - it's a fact that poor people with narrower choice of food are more obese than rich people with wider choice of food. If everyone buys the tastier food they can afford, superstimulus theory says rich should be more obese, setpoint theory says poor should be more obese.
Is there any way in which setpoint theory is more wrong than superstimulus theory?