Building muscle while staying lean sounds like the holy grail of fitness. But is it actually possible?
One fitness expert decided to find out by putting his brother-in-law through three different muscle-building diets over 90 days, tracking every pound of muscle gained and every ounce of fat burned.
The results reveal which approach builds muscle fastest, which one torches fat while adding size, and which strategy completely backfires for most people.
Spoiler: the answer might not be what mainstream fitness culture has been preaching.
Jump to:
- Starting Point: From Fat Loss to Muscle Gain
- Diet #1: Maingaining (Maintaining Weight While Building Muscle)
- Diet #2: Lean Bulk (Small Calorie Surplus)
- Diet #3: Fast Bulk (Aggressive Calorie Surplus)
- Back to Maingaining: The Final Test
- 90-Day Totals: The Complete Picture
- Which Diet Should You Choose?
- The Bigger Picture
Starting Point: From Fat Loss to Muscle Gain
Dayton had already dropped from 33% body fat down to 17% with his brother-in-law’s guidance. Now he was ready for phase two: building serious muscle without regaining the fat he’d worked so hard to lose.
Now that I’ve cut, I really want to bulk up, get bigger at all. I’m not getting fat again. Like, I refuse to get fat again.
To track progress accurately, Dayton underwent DEXA scans, ultrasounds, skinfold measurements, and circumference tracking before and after each 30-day diet phase. This multi-method approach provided comprehensive data on actual muscle growth versus fat gain.
Diet #1: Maingaining (Maintaining Weight While Building Muscle)
The first approach tested was “maingaining”—training hard while eating just enough calories to maintain body weight. The theory? Force your body to burn stored fat to fuel new muscle growth.
Setting Up Maintenance Calories
Dayton’s maintenance calories were calculated at 2,300 per day. To find yours, multiply body weight in pounds by 13-16 depending on activity level.
For protein, Dayton targeted just 100 grams daily—about 0.7 grams per pound of body weight. Recent research suggests this is sufficient to maximize gains when eating adequate calories and not at extremely low body fat levels.
Training Protocol
Dayton trained five days per week following a structured split. Early results looked promising—he was hitting personal records almost every workout.
But life threw curveballs. Roommate troubles, business stress, and holiday temptations derailed his nutrition consistency.
I got my third plate of the night, a dessert. It’s called ube and then some sort of rice pudding.
Then tragedy struck when Dayton’s grandfather passed away from cancer. Despite the emotional weight, he didn’t miss a single workout.
Unexpected Results
At his 30-day check-in, Dayton had gained 2 pounds—the exact opposite of maintaining weight. His overeating accidentally mimicked the second diet planned for the experiment, so the order was swapped.
The silver lining? Those 2 pounds went to the right places. Based on combined measurements and scans, Dayton gained approximately 1.35 pounds of actual muscle in 30 days.
Compared to a 2020 meta-analysis showing participants gained roughly 2.4 pounds of muscle in 10 weeks, Dayton was performing well above average.
Diet #2: Lean Bulk (Small Calorie Surplus)
Since Dayton accidentally lean bulked during his first phase, that became the official second diet. Lean bulking adds about 250 calories above maintenance, typically resulting in 2 pounds of weight gain monthly.
The Numbers
After 30 days of intentional lean bulking, measurements showed:
- Muscle gained: 1.35 pounds
- Fat gained: Minimal based on belly skinfolds, ultrasound, and waist measurements
- Overall assessment: Successful muscle building with controlled fat gain
This matched Dayton’s training experience perfectly. He was hitting PRs consistently and feeling strong in every session.
Diet #3: Fast Bulk (Aggressive Calorie Surplus)
For maximum muscle-building potential, calories jumped to 2,800 daily—500 above maintenance. Target weight gain: 4 pounds in 30 days.
Fueling Performance
Extra calories came primarily from carbohydrates to maximize workout energy and glycogen stores.
So, I think Jeremy’s right about upping the carbs. I’ve been hitting PR after PR after PR. I have a lot more energy and I’m just feeling way stronger.
The energy boost was so significant that Dayton started adding extra sets to workouts—an indirect benefit of higher calories that controlled studies typically miss.
The Trade-Off
With increased size came noticeable fat gain, particularly around the belly and face. But research suggests fast bulking works differently depending on training experience.
Studies on untrained lifters showed massive muscle gains with minimal fat during fast bulks. However, when tested on experienced lifters, most extra weight came from fat rather than muscle.
As a first-year trainee, Dayton fell somewhere in between.
Final Fast Bulk Results
After gaining the targeted 4 pounds, measurements revealed:
- Muscle gained: 1.8 pounds (30% more than lean bulk)
- Fat gained: Three times more than during lean bulk
- Strength: Pull-up performance doubled since day one
The visible difference was undeniable—fuller muscles, broader shoulders, developing V-taper. But fat accumulation was becoming problematic.
Back to Maingaining: The Final Test
With newfound muscle and some unwanted fat, Dayton returned to maintenance calories. The goal: continue building muscle while actually losing fat.
Can You Really Build Muscle in Maintenance?
Muscle growth requires energy, but that energy doesn’t have to come from food. Most people store over 100,000 calories as body fat that can fuel muscle building.
When Dayton first started training on a fat loss diet, he built muscle at about 0.7 pounds monthly while losing 5 pounds of pure fat each month. Countless studies and real-world examples confirm this is possible, especially for newer trainees following proper programming.
Staying Accountable
Dayton’s brother Cole moved in as his new roommate, bringing powerful accountability. Cole had overcome 10 years of addiction and homelessness through fitness.
I’ve been working out the last year and a half. Before that, I was an addict for 10 years, living on the streets, getting in trouble. Uh, pretty much weighed nothing. Now look at me.
Training together and meal prepping as a team kept both brothers on track.
Maingaining Results (Round Two)
After 30 days maintaining exact body weight:
- Muscle gained: 1.1 pounds (only 15% less than lean bulk, 37% less than fast bulk)
- Fat change: Actually lost fat for first time in experiment
- Waist measurement: Tightened up noticeably
- Belly fat: Visibly reduced
Strength gains during each phase aligned closely with measured muscle growth, confirming that tracking performance is reliable for assessing progress without fancy equipment.
90-Day Totals: The Complete Picture
Over three months using only dumbbells, a bench, and a pull-up bar, Dayton built approximately 4.4 pounds of muscle.
Breaking down each approach:
- Fast bulk: Most muscle gain (1.8 lbs) but disproportionate fat gain
- Lean bulk: Solid muscle gain (1.35 lbs) with controlled fat increase
- Maingaining: Good muscle gain (1.1 lbs) while actually losing fat
Which Diet Should You Choose?
For Dayton’s continued progress, the lean bulk made most sense. It provided enough calories to feel energized and maximize performance without excessive fat gain that would require aggressive cutting later.
But individual circumstances matter tremendously:
Choose Maingaining If:
- You’re above 15% body fat (men) or 25% (women)
- You’re relatively new to proper training
- You’ve never followed structured programming before
- You want simultaneous fat loss and muscle gain
Avoid Maingaining If:
- You’re already very lean (under 15% for men, 25% for women)
- You’re significantly underweight
- You need to gain substantial mass quickly
Choose Lean Bulk If:
- You want steady muscle growth with minimal fat gain
- You’re willing to cut later if needed
- You perform better with more calories
- You’re patient with gradual progress
Choose Fast Bulk If:
- You’re a true beginner to resistance training
- You’re significantly underweight
- You don’t mind cutting substantial fat later
- Maximum muscle gain is your only priority
The Bigger Picture
While diet strategy matters, Dayton made tremendous progress with all three approaches. Consistent training, adequate protein, and progressive overload drove results regardless of exact calorie intake.
Don’t overthink the perfect diet. Pick one that fits your body fat level and lifestyle, then execute consistently for at least 30 days before evaluating.
Amazing. I mean, my whole life changed for so many positive ways. just diet, my energy, everything’s just been overall better.
Track your lifts religiously—if strength increases steadily in the 6-12 rep range while keeping exercises consistent, you’re almost certainly building muscle.
The best diet is ultimately the one you can stick with long enough to see real results.
