Losing fat slowly makes sense. The idea is that through small reductions of calories we can avoid shocking our bodies into holding onto fat and instead gently coax it off over time. An attempt to fly under the radar of our starvation defenses, if you will.
It’s great in theory, but not so great in practice—millions of years of evolution mean the human body isn’t so easily tricked.
Metabolism slows during dieting, leading to higher energy efficiency and making further fat loss more difficult. This becomes more pronounced the more fat we lose and the more we reduce our food intake.
And after fat loss, metabolism can be increased through reverse dieting with no, or at least minimal, fat gain. (Current science is lacking here, but watch this space.)
The important point is that metabolism changes in response to energy intake, potentially even while body composition remains stable.
Bear With Me…
Yes, it’s true that huge caloric reductions are a bad idea. Extremely fast weight loss causes quick and severe metabolic adaptation that makes further progress very difficult. But metabolic adaptation still occurs with slow fat loss from a small caloric reduction.
If we follow generally accepted advice to diet slowly by only slightly lowering calories, our bodies merely adjust to the small reduction, and minimal fat loss is achieved. To sustain the slow progress, many further gradual cuts to the diet are required.
The end result is that energy intake is cut more than necessary over a longer time than was necessary.
To avoid this the caloric deficit must be big enough to take advantage of current metabolic rate and burn the most fat possible before it slows, but not so big that it causes a premature and large metabolic drop.
It all amounts to a tricky balancing act that will be different for everyone, depending on overall body mass, relative amounts of muscle and body fat, activity levels, diet history, and genetic tendency of metabolism in the first place. Phew!
So Is Fast Fat Loss Best?
Of course “fast” is a relative term, but the main point for me is to avoid really slow fat loss. In that sense, fast is best. I used to slowly chip away 50-100 Calories over time, but I could have had the same results sooner and with more food if I’d aimed for a faster fat loss rate.
One critical caveat: the situation changes when you’re very lean and want to get even leaner. At this point high speed weight reduction carries a great risk of muscle loss and severe metabolic adaptation. There’s very little wiggle-room, and no choice but to go slowly and carefully.
However, in general very slow fat loss isn’t the most efficient method. It’s also challenging from a sustainability perspective—for most people, slow progress makes adherence less likely. The most enjoyable and sustainable diet means maximum fat loss over the least time while eating the most calories. The bottom line: moderate caloric reductions are better than tiny ones, and fast fat loss (with some caveats!) is best.