Take your average guy or gal that decides they’re committed to finally losing that extra weight that’s crept on over the years. They’re going to eat healthy (primally , of course), start working out, and stop all the nonsense they know to be unhealthy. This might be you.
A few days go by, then a week. The scale is budging, but barely. “This is going to take forever! How long is this going to take?” We all want instant results, right? Well, what is realistic? What is safe? What is effective? And what can you expect when you attempt to lose excess body fat and reach your ideal body composition ?
Everyone knows that slow, gradual weight loss produces the best long-term results and fast weight loss is unsafe and unhealthy. People you know have probably clucked “Oh, you’re losing weight fast now with that low-carb fad diet, but just wait a few weeks and it’ll all come rushing back!” And when you go somewhere like the CDC’s weight loss page , they pat your head for “want[ing] to lose it very quickly” and reassure you that “people who lose weight gradually and steadily are more successful at keeping it off.” It’s become an article of faith that slow and steady weight loss wins the race.
But is it actually true?
I searched the literature for support of this widely-accepted weight loss truth. If folks like the Center for Disease Control were saying it, there had to be some evidence for it. Right?
I came up empty. What little evidence I could find seemed to support the opposite contention: that rapid initial weight loss is associated with better long term weight maintenance than slower weight loss. Just look:
What Research Reveals About Rapid Weight Loss
- A 2000 review  concluded that “greater initial weight loss” improves long term weight loss maintenance, even when that weight is lost using extreme diets.
- A 2001 review  concluded that the use of very low calorie diets to spur rapid short term weight loss can be highly effective for long term weight maintenance, provided subjects follow up with a “weight-maintenance program” including physical activity, nutritional education, and behavioral therapy.
- A 2004 review  of the effect of “lack of realism” in weight loss goals on long term weight maintenance found that “higher dream weight loss goals” were linked to greater weight loss at 18 months.
- There was the paper from 2010 showing that among middle-aged obese women , those who lost weight the fastest were the most likely to keep it off after 18 months.
- There was also a more recent paper  where people who lost weight quickly were no more likely than people who lost it slowly to regain the weight in the long term. Members of the fast weight loss group were more likely to hit their short term weight loss goals (12.5% reduction in body weight) and stick with the program. Even though both groups had regained about 70% of the lost weight after three years, the net weight loss in the fast weight loss group was greater.
Across most of the available literature, slow and steady did not win the race. The hare usually beat the tortoise. This actually makes sense. Why is the ancestral health community so strong? Why does this site attract so many readers? In part, because of the ease, simplicity, and early efficacy of this way of eating. Right off the bat (or in just 21 days ), you lose weight, feel better, and regain lost energy. Why wouldn’t you keep doing it? You’re more likely to stick with a diet if you’re wildly successful right away.
Okay, okay. But is rapid weight loss safe?
It depends on who you are.
If you’re obese, rapid weight loss is safe, since as much as 87% of the total weight you lose  will be body fat. And just as dietary fat is an excellent fuel source that burns cleanly , the animal fat hanging off your body is good to burn. That’s why rapid weight loss in the obese is consistently associated with improved health markers. Insulin sensitivity increases and belly fat decreases . Blood lipids normalize. Testosterone increases  and sexual function is restored. Oxidative stress biomarkers are reduced . All sorts of wonderful things happen when you’re overweight or obese and start losing weight.
If you’re lean, rapid weight loss looks a little different. The leaner you are, the more muscle mass you’ll lose during weight loss  and the more negative effects you’ll accrue. An extreme example of this is the bodybuilder preparing for competition. He’s reducing calories. He’s exercising. He’s doing everything he can to drop weight as quickly as possible. And in dropping from 14.8% body fat to 4.5%, he loses strength, his testosterone plummets, and his mood worsens  — the opposite of what happens to the obese when they drop weight fast.
Another population for whom rapid weight loss might be contraindicated is the elderly. If you’re elderly, rapid weight loss is usually associated with illness and accelerated muscle loss , and it’s a frequent complication  of Alzheimer’s disease. Slower weight loss using a diet rich in protein (to stave off muscle loss) and regular physical activity is a better option for older people.
It also depends on how you do it. Let’s look at some of the methods used to cause rapid weight loss.
Pros and Cons of Common Weight Loss Strategies
Dehydration-Based Weight Loss
Athletes who need to make weight to qualify for competition, like MMA fighters or bodybuilders, often do so by quickly dropping water weight. They’ll go jogging in full black sweatsuits. They’ll sit in saunas. They’ll remove all salt from their diet (since sodium helps us retain water). They’ll go super low-carb (since stored glycogen brings water along for the ride). They’ll take hot baths and stop drinking water altogether on the day of the weigh-in. That’s how a fighter who normally walks around at 195 pounds can qualify for the 170 pound division — by dehydrating himself.
Verdict: Not safe. Dehydration impairs physical performance , cognitive function , and connective tissue elasticity . Plus, it’s literally just water weight that will come screaming back once you start eating carbs, salt, and drinking water again.
Very Low-Calorie Dieting
This might be the most common method people employ to achieve fast weight loss: eat barely anything. And there are studies supporting the efficacy of very low-calorie dieting (VLCD), but when your average person with weight to lose hears that and just stops eating (usually supplemented with lots of cardio), it’s disastrous. Clinical VLCDs have very specific guidelines . Before patients are selected, they undergo a physical and go over their medical history. Once on the diet, they receive counseling, supplements, premade food (often liquid), and regular checkups to identify nutrient deficiencies and side effects. They’re meant for the obese, not someone who wants to lose a few stubborn inches off their belly.
Verdict: Safe, but you’d better know what you’re doing. Professional supervision is probably a good idea if you intend on making this work long-term.
Protein-Sparing Modified Fasts
Protein-sparing modified fasts (PSMFs) are a type of very low-calorie diet, but calories aren’t the express focus; getting enough protein and then stopping is the focus. You eat as much protein, preferably from animal sources, as you require to curb loss of lean mass and maximize fat loss, then add heaps of low-carb vegetables. PSMFs are high-protein, low-carb, and low-fat diets. A PSMF might look this:
- Minimum 1.5 grams of protein per kg of lean body mass (if sedentary; closer to 2 g/kg if strength training) from lean meats and protein powders like whey isolate.
- Unlimited fibrous vegetables (spinach, lettuce, kale, chard, broccoli, asparagus, etc; anything without an appreciable number of digestible carbs).
- Three to four grams of fish oil.
- Multivitamin/mineral supplement.
- Cook with little to no added fat. Eat no fruit, sugar, or starches.
In the fitness/weight loss community, people will typically maintain this for 1-2 weeks, then do a refeed  and hop back on it, or resume a more normal diet. Clinical use of the PSMF in severely obese people usually lasts longer than one or two weeks and is quite effective:
- In obese patients , a PSMF allowed 47 +/- 29 pounds of weight loss. By the end of the maintenance period, most of the weight was still missing, so it was pretty successful (particularly in those who had the most to lose).
- A 2 week 400 calorie PSMF was safe and effective  in obese patients, especially compared to a 400 calorie liquid protein diet.
Every study I could find on PSMFs found they worked and were safe, with some caveats:
In obese teens , a 3-month long PSMF supplemented with potassium, calcium, and magnesium resulted in weight loss and maintenance of normal serum concentrations of the supplemented minerals. But when researchers looked at red blood cell levels of the minerals — which offers a more accurate glimpse of mineral status than serum level — they found that RBC magnesium had plummeted. It’s likely that other micronutrient-related biomarkers could also suffer.
Verdict: Safe and effective, provided you don’t remain on the diet for too long. Extended PSMFs are more likely to cause harm and nutrient deficiencies (that may not show up in standard serum tests) than shorter PSMF bursts. Obese people in clinical settings with professional support can probably safely benefit from longer PSMFs, but the average Primal reader just trying to lean out a bit or get over a plateau should stick to 1-2 weeks.
Low-Carb, High-Fat Diets
In the population most in need of safe, effective weight loss — the millions of insulin-resistant obese and overweight individuals who do not participate in clinical weight loss trials helmed by doctors and technicians and supported by supplements and formulated diets — a basic low-carb, high-fat diet is the simplest and most effective method. Comparisons  between ad libitum (where you eat until sated) low-carb diets and calorie-restricted (where you weigh and measure) diets find that the former result in faster weight loss.
Low-carb, high-fat approaches also sidestep another potential downside to rapid fat loss in general: the risk of gallstone formation . Research shows that adding some fat in the diet to promote gallbladder emptying  takes care of the gallstone problem. One study found that 4 of 6 subjects on a 520 calorie liquid low-fat diet developed gallstones, while none of the 7 subjects on a 900 calorie liquid diet with 30 grams of fat developed them, even though both diets resulted in the same amount of weight loss.
One potential downside of rapid weight loss are elevations in liver enzymes. The rise is usually transient, resolving shortly after cessation of the diet, and it seems to happen more often in women than men . But the macronutrient ratio is perhaps the biggest determinant. When these very low-calorie diets are high in carbohydrates, liver enzymes are higher. When the diet is carb-restricted, the liver enzymes are lower .
Verdict: Safe and effective.
If there’s a neat and tidy answer (and there never is in a topic as complex as human physiology), it’s this: rapid weight loss is safe as long as you’re losing (mostly) fat and not lean muscle mass. If you’re dropping weight quickly (or slowly!) but you feel good, your physical performance is improving or staying the same, and you’re losing inches from the waist, your weight loss is probably mostly fat. If you’re dropping weight quickly but troublesome side effects occur, your weight loss may be drawing on more lean mass than you’d like, and you should probably slow down. Weight loss should feel good.
There’s another commonality among all the “crash” diets that end up leading to long term maintenance: they combine rapid weight loss and education. It shouldn’t just change what you put in your mouth, but how you think about what you put in your mouth. Without learning about food and how it affects you and how to eat long term in the real world, the weight’s just going to come roaring back.
With the Primal Blueprint , I’ve tried to pair education with results for a sustainable way of eating and living. There are other ways to get there, as I’ve mentioned above, but this one seems to work well for nearly everyone I know that’s tried it.
Thanks for reading, everyone. I’m curious about your thoughts on rapid weight loss.
Prefer listening to reading? Get an audio recording of this blog post, and subscribe to the Primal Blueprint Podcast on iTunes for instant access to all past, present and future episodes here .