About five years ago, Alwyn Cosgrove had an article published called "The Hierarchy of Fat Loss. It's been a pretty influential article, at the very least online in discussions of fat loss.
The short version of the article is, that after correct nutrition, there is a clear hierarchy of value to types of training for fat loss. In the article, they basically go:
1) High intensity metabolic training (think complexes and circuits)
2) Strength training
3) High intensity interval cardio (alternating fast and slow)
4) High intensity steady-state cardio (just fast, straight through)
5) Low intensity steady-state cardio (jogging, going for a walk, etc.)
You pretty much want to spend your time, the article says, in that order. Do 1, if you have htime for two do 1 and 2, etc.
In my experience, however, the most effective combination for steady, long-term body composition change in myself and my clients is a combo of #2 (with a sprinkling of #1), and #5. That's strength training with a little bit of high intensity circuits or interval cardio, plus steady state low intensity cardio. Lift some weights, do a quick short circuit, and then walk around a lot.
I can't prove this with studies, but here's how I see it working.
Strength training. This is your bread-and-butter body composition changer. It's the compound interest in the bank. You lift weights with enough sets and reps and intensity to build strength and muscle mass. This in turn costs more for your body to maintain, and burns more calories. It also feeds your appetite, allowing you to eat more quality food. And it makes you stronger so you can train harder. It keeps paying off workout after workout, and day after day. It doesn't take a lot of time, either - I've seen plenty of clients with results from only 2x a week training.
Circuits. I like to keep these short and hard, and as a supplement to strength training. However they are very difficult to keep doing hard, long. So I cut out the "long" and just go for "hard." This means you can work efficiently but stop before your fatigue starts to cause form errors or inflict so much systemic fatigue that you can't easily recover from it.
Extra movement. Walking, taking the stairs, parking far away, making two trips with the groceries, chasing the kids around the yard, etc. It doesn't sound like much, but coupled with a pedometer to keep track and you'll see it does add up. It's extremely low-intensity, so you can do lots and lots of this without needing extra food or recovery.
Why does this work? Why I think it works is simple - recovery. You're maximizing not your fat loss this minute (or this day, or this week) but your recovery from each workout. The strength training is hard but done correctly won't put you out of commission. The short circuits are the same - hard, but no so long you are systemically drained. You can recover from it and do it again a couple days later without any cumulative long-term fatigue. And the extra movement is just that - extra movement. It's low-intensity but steady. An extra 10% movement (one more flight of stairs a day, another 500-1000 steps, another etc.) adds up day after day. Moving that additional muscle you built also takes more energy than keeping it sedentary, too.
Like I said I can't prove this with studies, but it does work. Combine working hard for your long-term payoff strength with low-intensity additional movement to maximize recover, and you're ramping up your metabolism day after day. It will pay off over time - and methodically gained strength and activity rates are much less prone to drop off suddenly. They're become a habit.
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