Flexible vs. Rigid Dieting: What Works Best?

by Chris Shugart

The Diet Debate is Over

What works better, a strict diet or a more flexible plan? The answer may surprise you. Or maybe not. Check it out.

Ethan decides to enter his first bodybuilding competition. He has the muscle and he has the discipline. Clearly, it's time to buy some shiny stage panties. The only thing Ethan lacks is a diet.

He hires a coach and gets his plan. He sees he's supposed to eat four ounces of fish every night. White fish, specifically. And not just any white fish, but tilapia. When he asks his coach if he can have his daily Red Delicious apple with peanut butter, his coach says no… sternly. He must only eat Honeycrisp apples with almond butter. These are the rules he must follow if he wants to win.

Ethan's plan is what nutrition researchers called a "rigid" diet. A rigid plan has a narrow list of approved foods, and other foods are off the table.

If Ethan had hired another diet coach, he may have been given a "flexible" diet plan, sometimes called IIFYM or "If It Fits Your Macros." With that plan, Ethan would have to hit his calorie goal and get the right ratio of protein, carbs, and fats. He'd get more food options, though, maybe even an occasional treat, as long as it fits his macros.

The big question: Does any of it matter? If some basic rules are followed, like eating enough protein and getting into a calorie deficit, does it matter if he uses a flexible or rigid diet? That's what the study below tried to figure out.

The Study

Researchers recruited 23 iron-pumping men and women and put them on a 10-week diet. Some got a rigid diet and some got a flexible diet, but all of them ate 20% fewer calories and consumed .9 grams of protein per pound of body weight. All stuck to their usual lifting and cardio plans.

Who Lost More Fat?

Not surprisingly, both groups had about the same results. Averaged out, both groups lost roughly 6 pounds of fat. The rigid group did ever so slightly better, but nothing statistically significant according to the researchers.

What Can We Learn from This?

If you explore the wild Zambian plains of social media, you'll notice that these two diet strategies both have their loyal tribalists, each complete with a charismatic tribal chief selling diet plans. But both flavors of calorie reduction work if you're getting enough protein and working out. The question becomes, what's your favorite "flavor" of eating less?

Some people prefer a strict plan with rigid food rules. They thrive on it. In fact, they don't want choices. The drawback? They're probably following rules that don't matter much, like eating only a certain magical fish. It's anal-retentive. But anal-retentive folks like that. They'll follow someone else's stringent plan and spend their willpower and decision-making dollars elsewhere.

Others thrive on a flexible approach. Paradoxically, a "flexible" diet isn't that flexible. Sure, they can have 1.5 Oreo cookies after dinner every night, but they must carefully measure and manage those macros. It's also anal-retentive; they're just squeezing their butt-cheeks differently. But the variety makes them happier and more compliant.

T Nation contributor Dr. Bill Campbell says that both strategies can work for the same person. In a competitive bodybuilding setting, a person could start with a flexible approach and adopt a rigid approach as the contest draws closer.

But for the rest of us, the real lesson here is about protein.

The Protein Lynchpin

When the average person cuts calories, he or she doesn't focus much on eating a lot of protein. All macros get reduced. But study after study tells us that protein is the lynchpin. All successful diets – defined as losing fat while keeping your metabolism-dependent muscle intact – involve keeping protein intake high.

We saw the same thing play out with the ol' low-fat vs. low-carb debates: as long as calories were in check and protein was high enough, both plans worked for fat loss.

The study above used 0.9g/pound, about the amount shown to support resistance training goals. Other studies show that 1g/pound or even 1.3g/pound does the trick. I like the "gram per pound of body weight" guideline because it's easy to remember and keeps you satiated.

With a Protein-First strategy, you can unclench your butt-cheeks a little. Protein is filling and thermogenic, and it's nearly impossible for protein to get stored as body fat, even if the calories from it exceed your maintenance intake.

A protein shake or two every day using a micellar casein-rich blend, as found in MD Protein (Buy at Amazon), makes it easy to hit your protein goals. The rest of your diet pretty much auto-regulates if you do that. So, you won't have to choose between a rigid or flexible diet, because you won't get too fat to begin with. Now there's an idea.



  1. Conlin, et al. "Flexible vs. rigid dieting in resistance-trained individuals seeking to optimize their physiques: A randomized controlled trial." J Int Soc Sports Nutr. 2021 Jun 29;18(1):52. doi: 10.1186/s12970-021-00452-2.