Or in plain English, what happens to your metabolism when you gain or lose weight.
Maintenance of a body weight 10% above or below that “customary” for lean or obese individuals results in respective increases or decreases in the energy expended in low levels of physical activity (nonresting energy expenditure, NREE). These changes are greater than can be accounted for by the altered body weight or composition and are due mainly to altered skeletal muscle work efficiency at low levels of power generation.
We performed biochemical analysis of vastus lateralis muscle needle biopsy samples to determine whether maintenance of an altered body weight was associated with changes in skeletal muscle histomorphology. We found that the maintenance of a 10% reduced body weight was associated with significant declines in glycolytic (phosphofructokinase, PFK) enzyme activity and, in particular, in the ratio of glycolytic to oxidative (cytochrome oxidase, COX) enzyme activity without significant changes in the activities of enzymes relevant to mitochondrial density, respiratory chain activity, or fuel transport; or in skeletal muscle fiber type or glycogen stores.
The fractional change in the ratio of PFK/COX activity in subjects following weight loss was significantly correlated with changes in the systemic respiratory exchange ratio (RER) and measures of mechanical efficiency of skeletal muscle at low workloads (pedaling a bicycle to generate 10 or 25 W of power). Thus, predictable changes in systemic skeletal muscle biochemistry accompany the maintenance of an altered body weight and account for a significant portion of the variance in skeletal muscle work efficiency and fuel utilization at reduced body weight.
One doesn't have to read this entire paper or muddle through all the enzyme names and their purposes to digest the practical implications of this paper. We all know that recidivism rates for weight loss are worse than those for most criminals. I've also long believed that for many of us, women especially since we are more likely to diet earlier in life, it is dieting that makes us fatTER in the long haul.
So let's say as a weight stable well nourished 130 lb woman I require 100 cals/day to do Activity X. Then I gain 30 lbs over the years, so I'm a well nourished 160 lb woman who may actually require 110 cals/day to do Activity X because it involves moving around more weight. My body senses plenty of energy supply ... it can afford a bit of inefficiency. Then I decide to lose 30 lbs. Kid yourselves not, if you lose 30 lbs you have reduced your energy intake and your body senses this. Your body thinks it's a famine period and it knows not when it will all end. So it tightens the belt, so to speak, and becomes more efficient. So now at 130 lbs reduced, I only require 90 cals/day for Activity X.
The finding that fat oxidation during exercise is increased following weight loss is consistent with other ergometric studies (19, 40) and is most likely reflective of the overall biochemical and molecular changes in skeletal muscle that produce a more chemomechanically efficient organ after weight loss. Overall, however, weight-reduced individuals have lower fat oxidation rates (38, 73).Now if I eat like my former 130 lb self, I'll gain weight, and if I return to my 160 lb habits, I'll gain it faster. But this time when I get to 160 lbs I'll may only require only 100 cal/day for Activity X. If I'm eating at former 160lb self levels, I'm still in energy surplus at 160 lbs so I end up tacking on 10 more pounds until the cal/day for Activity X reaches 110.
Rinse, repeat = disaster.
Increasing muscle efficiency during low-level exercise combined with an increased propensity to store ingested calories as fat would function coordinately to favor the regain of lost adipose tissue.