I Tested 3 Muscle Building Diets, Only 1 Was Worth It

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How much muscle can you really build in just 90 days of bulking? And more importantly — how can you do bulk without gaining fat? To find out, I tested the three most popular muscle-building diets (including a lean bulk) on my brother-in-law Dayton. By the end, you’ll see how much muscle he gains, which one builds muscle fastest, which one burns fat while building muscle, and which diet completely backfires so you know how to bulk up without getting fat.

Last year, Dayton dropped from about 33% body fat down to about 17%. After that cut, he wanted to bulk up for a bigger physique — permanently — and not gain fat again. So we ran a 90-day experiment that tested what actually happens when real life hits.

To measure the changes, Dayton got DXA scans before and after each 30-day bulking diet. And because short term DXA’s can be less reliable, we also used ultrasound/skinfold/circumference measurements. He tracked every workout in the Built With Science+ app so we could compare strength gains in the gym to new muscle growth.

Days 1–30: Accidental lean bulk
We started with maingaining: train hard while eating just enough to maintain your weight. Dayton’s maintenance was right around 2300 calories, with at least 100g of protein per day to leave room for carbs to fuel workouts. Training was 5 days a week using dumbbells, a bench, and a pull-up bar, with extra focus on the back and the lats he was convinced would not grow.

But ten days in, stress piled up. At our family’s Filipino Thanksgiving, he ate until he was sick. And he was dealing with losing his grandpa to cancer — his father figure after his dad passed away when he was seven, also from cancer. Even with all that, he did not miss a single workout. At the day 30 check-in, he was up about 1.8 pounds, so we swapped the order and treated this block like a lean bulk. Result: an estimated 1.35 pounds of muscle gained, with some fat gain but not excessive.

Days 31–60: Fast bulk
Next we cranked calories up to about 2800 per day, roughly 500 above what he burns. We mainly added carbs for glycogen and energy so his workouts could really get taken to the next level. Dayton felt so good he started adding sets, and the extra momentum even benefited his business once he started posting more transformation content. But he also started noticing more belly fat.

At the day 60 check-in of our “how to bulk up without getting fat” experiment, he gained about 4 pounds in 30 days, felt bigger and fuller, and his pull-up strength since day 1 had doubled. He gained an estimated 1.8 pounds of muscle — about 30% more than the lean bulk — but he also gained about 3x more fat than during the lean bulk phase.

Days 61–90: Maingaining
For the final 30 days, we brought calories back down to maintenance. Muscle growth needs energy, but that energy does not have to come directly from extra food when you have stored body fat. Dayton got creative with lower calorie treats like a chocolate peanut butter frozen yogurt that tasted like a peanut butter cup with some banana, kept training hard, and finally had a workout buddy after convincing his brother to move in. Training became more than just building muscle — it helped keep him sober. Routine kept him grounded.

At the day 90 check-in of his bulking journey, his bodyweight was essentially unchanged from 30 days earlier. He gained an estimated 1.1 pounds of muscle, and for the first time in the experiment he actually lost a bit of fat as his waist tightened up.

Final results
Over 90 days, Dayton gained roughly 4.4 pounds of muscle — with only dumbbells, a bench, and a pull-up bar. Fast bulking delivered the most gains but came with disproportionate fat gain. Maingaining was surprisingly effective and helped him lean out. But for Dayton’s goal of building muscle before attempting to cut to 12% body fat for summer, the lean bulk made the most sense. And at the end, I surprise Dayton and his brother with a new gym setup so they can keep building.

This is how to bulk according to your body and goals.

Maingain
Who
• Higher body fat lifters (≈20%+)
• Returning to the gym after time off
• Want muscle without bulking
How
• Eat at maintenance calories
• Keep bodyweight the same

Lean bulk
Who
• Novice/intermediate/advanced lifters
• Muscle gains have slowed down
• Want to stay lean while gaining
How
• About 250 calorie surplus
• Gain about 1% bodyweight/month (about 1–2 lb/month)

Fast bulk
Who
• Naturally skinny lifters
• Brand new lifters (newbie gains)
• Struggle to gain weight
How
• About 500 calorie surplus
• Gain 2–3% bodyweight/month (about 3–5 lb/month)

Timestamps:
0:00 - The 90 Day Experiment
0:40 - Diet #1
5:26 - Diet #2
10:00 - Diet #3
15:44 - Which Diet Is Best?

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