The standard advice has been clear for years: consume 20–25 grams of protein after your workout to maximize muscle growth. Sports nutrition bodies including the American College of Sports Medicine and the British Nutrition Foundation have built recommendations around this figure. Most protein shakes are formulated around it. Most post-workout protocols repeat it.
New research from the University of Stirling suggests that number may be significantly too low — at least for certain types of training.
The Study
Researchers at the University of Stirling recruited 30 young, trained males and split them into two groups based on lean body mass. The first group had lean mass below 65 kilograms (143 pounds), the second above 70 kilograms (154 pounds).
Both groups completed the same full-body resistance workout:
- Chest press, lat pull-down, leg curl, leg press, and leg extension — performed consecutively
- All leg exercises performed unilaterally
- 75% of 1RM at a tempo of 1 second concentric, 2 seconds eccentric
- Three sets of 10 reps followed by a fourth set to failure
Each participant completed this workout twice. In one session they consumed 20g of whey protein immediately after. In the other, they consumed 40g. Researchers then measured muscle protein synthesis using muscle biopsies and metabolic tracers.
The full study was published in Physiological Reports in August 2016: Macnaughton et al., Physiol Rep. 2016;4(15):e12893.
What the Research Found
Challenge 1: Bigger Athletes Don’t Need More Protein After Training
This was the first study to directly examine the relationship between total lean body mass and muscle protein synthesis following whole-body resistance exercise combined with protein intake.
The result: lean body mass made no difference. Both groups — smaller and larger athletes — showed the same muscle protein synthesis response to the same dose of protein. Body size, as it turns out, is not the right variable for calculating post-workout protein needs.
Professor Kevin Tipton, the senior study author, explained why this matters: previous protein recommendations were based on studies that examined the response to leg-only exercise. His team used a full-body workout instead. The conclusion: it is the amount of muscle mass worked in a single session that has a bigger impact on protein needs, not the total amount of muscle in the body.
In other words, if you train legs only, 20–25g may be adequate. If you train your entire body, you likely need more.
Challenge 2: 40g Outperforms 20g
Contrary to the 25g consensus, 40g of whey protein produced a significantly greater muscle protein synthesis response than 20g after a full-body workout — and this was true regardless of body size.
Tipton stated that the existing consensus — that weightlifters don’t need more than around 25g after exercise to maximally stimulate muscle growth — was based on leg-only training studies. Those studies simply don’t apply to athletes doing full-body sessions, which is how the majority of real-world training actually works.
What This Means Practically
How much protein should you take after a workout?
Based on this research, the answer depends primarily on the type of session you’ve just completed:
Leg-only or single muscle group sessions: 20–25g of high-quality protein remains well-supported by the existing evidence base.
Full-body or multi-joint sessions training large amounts of muscle mass: 40g appears to be meaningfully more effective than 20g at stimulating muscle repair and growth.
The protein ceiling of 25g was derived from isolated exercise studies. Apply it to your full-body training and you may be leaving gains on the table.
Does body size matter at all?
According to this study, not for the post-workout dose — the type and volume of training is more relevant. That said, total daily protein requirements do scale with bodyweight, so larger athletes still need more protein overall. The difference is in how you distribute it, not just how much you eat post-workout.
What about older trainees?
The study was conducted exclusively with young, trained males. Tipton and colleagues note that results may differ for older athletes, women, and those with less training experience. Separately, research consistently shows that older adults are less sensitive to low doses of protein — requiring larger feedings to achieve the same anabolic stimulus. If you’re over 40, erring toward the higher end of the post-workout protein range is a sensible precaution.
Timing: When Should You Take It?
The study doesn’t address the timing window directly, but the broader research literature suggests consuming protein within 1–2 hours post-workout to take advantage of the elevated muscle protein synthesis response that follows resistance training. The window is not as narrow as was once believed — it doesn’t need to be within 30 minutes — but sooner is generally better than several hours later.
Related: What Is the Best Time to Consume Protein for Optimal Muscle Building?
Food or Supplement?
The study used whey protein isolate, which digests quickly and delivers amino acids to the muscle rapidly. Whole food sources work too, but digestion is slower. For the immediate post-workout window, a whey shake is the most practical way to hit 40g quickly without a full meal:
- 1.5 scoops of standard whey protein powder (~35–40g protein)
- Alternatively: 200g of chicken breast (~47g protein) or 6 eggs (~36g protein)
- Greek yogurt with a protein shake is a common combination for those who prefer whole food
The Takeaway
The 25g post-workout protein recommendation was based on leg-only exercise studies. If you’re training your whole body — which most gym-goers are — that number appears to be too low.
The University of Stirling study suggests 40g of protein after a full-body session produces significantly better muscle protein synthesis than 20g, and that body size doesn’t change this recommendation.
Practical summary:
- Full-body workout → aim for 40g protein post-workout
- Single muscle group session → 20–25g remains well-supported
- Timing: within 1–2 hours of training
- Quality matters: whey protein or high-quality whole food sources
- Older trainees: lean toward the higher end regardless of session type
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