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Google “rapid fat loss” and prepare to be sucked down a rabbit hole of bovine droppings.

I lost a pound a day, for one day, because I thought I was going to die. Conversely, I once saw a man profiled on CNN who also lost a pound a day – for 200 days straight.

Claims of products and programs that will allow you to lose a pound a day are rampant, often accompanied by the smug visage of Dr. Oz and seen in the magazine rack at your grocery store checkout, but in the above case it was actually feasible, because the man in question started at a weight of over 600 pounds, and he spent those 200 days in a tightly controlled “camp” environment where all his exercise and eating was carefully monitored, motivated, and measured.

But what about you?

The Biggest Loser style weight loss has many believing rapidity with dropping fat from your frame is the key. Like the aforementioned case, these people are significantly obese to begin with and are living 24/7 in a tightly controlled environment. Also, they’re engaged in a tortuous competition that is healthy neither for the body nor the psyche.

Your starting weight is going to play a significant role in how fast or slow you lose weight. The more overweight a person is, the faster they can lose. Conversely, if you’re questing to lose only about 10 pounds, the process will be painfully slow. Because metabolism.

People with obesity can lose faster
You can lose weight more quickly if you have significant obesity for a few reasons. One is simply that the body favors using fat as a fuel source if there is a lot of it around. Another is that, barring certain medical conditions, people with obesity often have a high daily caloric burn because it’s a lot of work to keep all that tissue functioning. It generally takes quite a bit of food and sedentary living to create a case of extreme obesity. Once a really heavy person starts moving and rationally restricting calories, they have the ability to create a significant caloric deficit that will lead to rapid weight loss (although note that I’m still not advocating anywhere near the extremes showcased in The Biggest Loser).

And finally, every movement a very heavy person makes is weighed down, making it more challenging. Even a walk is a significant workout if it involves carrying an extra hundred pounds.

For the slimmer it will be slow
For all the reasons mentioned above, a person who is not heavy and only looking to trim down a little – basically, they’re interested in some cosmetic weight loss because it’s only high concentrations of body fat (specifically around the midsection) that pose health problems – it’s going to be a slow process. They have fewer fat stores, and the body will want to protect these stores, as evolution via natural selection has programmed our bodies to preserve fat in case of famine. (Starvation has killed more people throughout human history than any other cause, and this has had a profound effect on our genetics to favor far storage.)

A 2005 article in the Journal of Theoretical Biology explained that not only does metabolism slow down the leaner a person gets, but that fat becomes a less favorable source of fuel to be burned. In other words, you could be losing weight that is not fat, like muscle. Losing muscle is bad.

Running some numbers
Let’s talk about caloric deficits, because that’s what leads to weight loss. First off, a single pound of fat contains 3,500 calories (more or less). Let’s make a test case of a hypothetical woman.

She is 40 years old, 5 foot 6, weighs 190 pounds, and she doesn’t exercise. Her BMI is just barely into the “obese” range. I ran the numbers on her total daily energy expenditure, and it’s about 1,900 calories. So, every day, being inactive, she burns about 1,900 calories. (Get your total daily energy expenditure number here – I’ve found this calculator to be very accurate.)

She could cut her caloric intake down to 1,400 per day and create a daily caloric deficit of 500 calories, which over the course of a week equals a 3,500 calorie deficit, meaning through diet alone she would lose one pound per week.

But let’s add in some exercise, because exercise is awesome.

I ran the numbers on this hypothetical woman, and for her to burn 500 extra calories per day she’d need to run at 6 mph for about 50 minutes. Do that seven days a week and it’s another pound lost each week via exercise.

This doesn’t even get into the issue of how her metabolism is going to begin to slow down as she restricts calories and loses weight, but do you see the problem here?

The problem is: 1,400 calories of food per day isn’t a lot. Any lower and you’re at risk of not only causing a metabolic slow down, but also creating what is called post-starvation hyperphagia, which means you starve yourself and this creates a hormonal response of uncontrollable hunger that causes you to overeat, meaning yoyo weight loss. What’s more, running at 6mph for 50 minutes, seven days a week IS a lot. Overall, it’s a lot of exercise, and a lot of NOT eating, to lose two pounds of fat per week (in this example).

It kind of puts rapid fat loss into perspective, doesn’t it?

And don’t believe all that stuff about weightlifting and adding muscle being some miracle fat burner. I busted that myth years ago. Also, don’t fall for the marketing hype of those exercise programs that proclaim to have a mega caloric “after burn.” I busted that too. More than once.

The most rational weight loss recommendations put it at 1-2 pounds per week, but I just showed you that even for a woman with class 1 obesity, two pounds per week is pushing it. But you don’t want to go too slow either. I mean, you want results, but research shows that being slow-ish but steady has a high success rate weight it comes to sustainable weight loss.

US researchers did a study of 5,145 obese men and women and published it in 2011 in the journal Obesity. The participants qualify as tough cases. The average body weight put them into the second stage of obesity, and all of them had type 2 diabetes. The average weight for women was 209 pounds, and for men it was 240.

After four years, almost a quarter of all the participants in the intervention group maintained a weight loss greater than 10% of their initial weight. Break the numbers down further and it gets even more encouraging. Some didn’t do so well. Thirty-two percent lost almost nothing in the first year, and most of these people weren’t that successful over the next three years either. It’s a fact that a portion of the population won’t succeed at weight loss no matter what. The fact that they didn’t lose weight early on showed they were highly resistant to change. But what about those who did change?

Thirty percent lost about 8% of their body weight in year one (about 17 pounds for women and 19 pounds for men), and 40% of this group either maintained the weight loss over four years or kept on losing more.

The most aggressive losers fared the best. BUT the word “aggressive” does not mean The Biggest Loser–style weight loss. Not even close. In the real world, “aggressive” translates to about 15% of starting body weight in year one. For the women in the study this meant an average of 31 pounds in a year; for men it was 36 pounds.

If you’re having trouble with creating the sustained, rational caloric deficits necessary to lose weight, read my Caloric Deficit Cheat Sheet. And if you’re wondering about the pros and cons of various types of exercise for assisting your weight loss efforts, read my Weights vs. Cardio Cheat Sheet.