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The ‘glowing cow’ study allows researchers to use real foods that people actually eat, like beef or milk, rather than laboratory-created protein powders, and the results suggest the specific protein source matters less than previously thought.Jeff McIntosh/The Canadian Press

As a postdoctoral physiology researcher with years of training, Dr. Nicholas Burd was entrusted with a crucial task during a recent experiment in the Netherlands: Keep the cows happy.

"My job was to talk to them, brush them and basically keep them in a good mood," recalls Burd, who now leads the University of Illinois Nutrition and Exercise Performance Research Group. "If the animal becomes stressed, milk production declines, so we treated them like princesses."

Burd's foray into animal husbandry was part of a remarkable research project in which cows received a 40-litre injection of amino acids labelled with a rare (and harmless) carbon isotope, in order to produce milk and beef whose fate inside the human body could be tracked after they were eaten.

The latest of these "glowing cow" studies, published in the American Journal of Clinical Nutrition by Burd and his colleagues in Dr. Luc van Loon's muscle research group at Maastricht University in the Netherlands, compares the muscle-building power of milk and beef after a workout. The approach allows researchers to use real foods that people actually eat, rather than laboratory-created protein powders, and the results suggest that the specific protein source you consume matters less than once thought.

The study involved 12 young men who completed sets of leg press and knee extension exercises on two occasions.

After one of the workouts, they drank 350 millilitres of isotope-labelled skim milk, enriched with extra protein to bring the total to 30 grams; after the other workout, they ate 158 grams of ground beef, which contains the same amount of protein. A series of muscle biopsies and blood samples drawn in the hours before and after the workout allowed the researcher to track how quickly the protein was being incorporated into new muscle.

In previous studies comparing different types of protein for muscle growth, milk has emerged as the top performer. That advantage is thought to stem in part from its high levels of leucine, an amino acid that triggers the synthesis of new muscle, and from milk's rapid digestion and absorption.

The initial data suggested that the same might be true in the milk-versus-beef comparison. In the first two hours after the workout, the rate of new protein synthesis was indeed higher for milk than for beef. But after five hours, the groups were statistically indistinguishable, suggesting that milk's fast initial response didn't produce any lasting advantage.

"From my perspective, the real take-home message is that both milk and beef are good choices," Burd says. The subtle differences revealed in the experiment will help scientists understand how the body builds new muscle, but they don't give any strong reason to choose one over the other.

Dr. Keith Baar, a Canadian-born muscle researcher at the University of California, Davis, who was not involved in the study, draws similar conclusions: "The glowing cow experiments have been wonderful and have added a huge amount to our understanding of protein metabolism," he says. But in practice, "if you are not competing at a high level, protein type – as long as it is high-quality protein – is not too important."

So what is "high-quality" protein? Nutritionists sometimes distinguish between animal proteins and plants proteins, since the latter are generally lower in key amino acids needed for protein synthesis. Studies at McMaster University, for example, have shown that milk reliably outperforms comparable amounts of soy protein for muscle-building.

But that view may be too narrow, Burd says, since people seldom consume a single isolated plant protein and nothing else. "Personally, I think plant-based proteins don't get enough credit," he says. "My guess is that when better comparisons are made [such as mixed plant-based protein blends], the disparity between animal and plant-based proteins will become narrower."

There are some situations where protein type could become important. If you can't exercise for a prolonged period of time, for example while ill or recovering from surgery, then your muscles become less sensitive to the muscle-building trigger of protein. In that case, making sure you get leucine-rich protein sources such as milk could help stave off muscle loss.

In most cases, though, the primary focus should be on getting enough, regardless of the source – and on distributing your protein intake throughout the day.

While most Canadians eat plenty of protein, they typically get more than half of it at dinner. Since your muscles can only make use of a limited amount of protein at a time (30 grams, the amount used in the cow study, is a rough estimate of the upper limit for most people), it's more effective to distribute your daily protein in four or more doses throughout the day.

That suggests that starting the day with a couple of eggs might be a good idea. Or perhaps some chicken. With any luck, the scientists at Maastricht will run an isotope study to settle that age-old question too.

Editor's note: An earlier version of this column incorrectly said isotope-labelled skim milk contains 30 grams of protein. This article has been corrected

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