This post came out of a question from someone who asked a question related to whether or not eating a very low calorie diet would lead to a "yo yo" effect caused by lowered metabolism, stoping weight loss and causing weight gain upon eating normally again.
There is no evidence suggesting that a "yo yo" effect would occur from low-calorie dieting, nor would lowering calories too far "stop" weight loss altogether. Truth is, calorie restriction does reduce metabolic rate, you would lose weight at a rate that is lower than normally expected, but if you started eating normally again, your metabolism would speed back up again.
In 2006, Heilbronn et al. studied the effects of calorie restriction (CR) on metabolism. The researchers published in JAMA the results of a six-month randomized controlled trial on CR and how it made an impact on biomarkers of metabolism as well as longevity and oxidative stress in overweight adults.
The subjects were paid and placed in one of the following groups: a control group, a CR group (25% reduction from baseline), a CR group with exercise (12.5% reduction, 12.5% increase activity), and what they called a very-calorie diet (890 kcal/d) followed by weight management at 15 percent weight reduction.
By the third month, metabolism had slowed (measured in part with plasma T3 levels) in both CR and very low-calorie diets. At six months, metabolism had slowed in the CR, CR with exercise, and very low-calorie diet groups.
Everyone lost weight in the intervention groups. Those on the very-low calorie diet lost the most, but they also lost the most muscle. From that same study, the researchers were the first to find reduced oxidative stress and DNA damage from CR in humans.
So, what do we know? We know that if you also drop even to 500-800 kcal per day that, despite slower metabolism (your body's survival mechanism), you still would lose weight albeit at a lower rate (as stated before).
What some researchers have tried to do since then to "trick" the body to not slow metabolism. They do it by staggering the calories with alternate-day CR/fasting or intermittent CR/fasting. These are interesting topics of research and may show up as new weight-loss fads of the future.
I don't recommend people drop calories or lose weight too quickly because it leads to too much loss of hard-earned muscle and, possibly, gallstones (if you're not drinking enough water and eating small meals throughout the day). In my experience, I've also seen quite a few people go lower than 800 calories per day for weeks and end up without energy, getting sick and looking pretty frail.
It's best, I think, to stick with losing only 1-2 pounds per week (by dropping calories steadily and increasing activity to keep muscle up) and then eventually keeping diet within 800-1200 kcal/d range.