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It is important to get protein from a range of sources, whether they are plant or animal-based. Jordan Lye/Getty Images
  • A new study in mice suggests that consuming a moderate amount of protein may be most conducive to metabolic health.
  • In the study, the sweet spot for moderate protein consumption was between 25% and 35% of a mouse’s daily diet.
  • Older people need more protein due to the body no longer being able to process the macronutrients efficiently.

It only makes sense that a person’s nutritional needs change as they go through life from childhood through adulthood. As we grow, reach maturity, and age, our bodies are occupied with different tasks.

As researchers seek to extend our healthy lifespans — periods free of serious disease — they have been hoping to identify the optimal balance of macronutrients that promote good health at each life stage.

A new study of mice investigates the role of protein at different stages of life.

The study found that consuming moderate amounts of protein in youth and middle age may be the key to good metabolic health.

The authors of the study fed young (6-month-old) and middle-aged (16-months-old) mice diets with varying levels of protein for two months. Their diets consist of 5%, 15%, 25%, 35%, or 45% protein. The moderate amounts identified in the study were 25% and 35%.

All mice were fasted for three hours before being euthanized for tissue harvesting and analysis.

In mice, a diet low in protein resulted in the development of fatty liver, and middle-aged mice exhibited higher levels of lipids, or fats, in their systems than younger mice.

The moderate-protein diets lowered lipid and blood sugar levels in the mice.

The study is published in geroscience.

Protein is critical at all stages of life. As Conner Middlemann of Modern Mediterranean noted, “The word ‘protein’ is derived from the Greek word proteiosmeaning ‘first’ or ‘primary,’ which reflects its top-drawer status in human nutrition.”

Dr. Stuart Phillips of McMaster University explained protein’s importance:

“When we’re growing, protein provides the building blocks (amino acids) to make new bones, skin, teeth, muscles, etc. Basically, every tissue requires protein to grow. Once we’re fully grown, protein still provides building blocks — it’s not for growth, but to replace proteins that are being turned over (broken down). Body protein turnover happens during our entire lives.”

In the United States, the required daily amount (RDA) of 0.8 grams of protein per kilogram of body weight falls far short of the body’s actual needs, said Middlemann. She clarified that the figure represents only the required amount of protein to avoid malnutrition, not the amount to promote good health.

How much protein?

Middlemann noted that the RDA is a holdover from a time when nitrogen-balance studies that are no longer considered valid formed the foundation of such recommendations. She said one could get a more accurate understanding of nutritional needs using the Indicator Amino Acid Oxidation (IAAO) technique.

The IAAO technique, said Middlemann, provides a more reasonable daily recommendation. It suggests 1.2 grams of protein per kilogram of body weight is appropriate for healthy young men, older men and older women.

The difference between the two recommendations is significant. The RDA for a 150-pound person is 54 g of protein daily, while according to IAAO measurements, it would increase to 81 g of protein.

But can one consume too much protein?

“We have a high capacity to digest and absorb protein, so I’m not sure you can get so much that it’s ‘too much’,” said Dr. Phillips.

He noted that some have suggested an excess of protein can lead to kidney and bone issues, “but those are largely debunked.”

“For the most part, proteins are relatively equal, but an axiom that’s true is that animal-derived protein is higher quality than plant-derived protein,” Dr. Phillips noted but added “Most work shows that this difference is likely quite small.”

As to whether the study’s findings will carry over to humans, said Dr. Phillips, “Always hard to know, but as short-lived mammals, mice are a proxy for humans, but much of what’s seen in mice may not be easily translatable to humans.”

Middlemann felt the study still had value:

“Even though this is a mouse study, it reinforces my view that most of us — especially anyone over 50 — stand to benefit from getting around 25% of the energy we consume from protein. This is significantly more than the average American currently consumes.”

“Some people need even more protein,” said Middlemann.

Of special note are people practicing resistance training. To maximize lean mass, the average required amount, she said, is about 1.6 grams per kilogram of body weight, and “some people may need 2.2 g/kg or more.”

For people wishing to burn fat while still retaining muscle, 1.6 to 2.4 grams per kilogram may be in order.

While the study found that a moderate intake of protein may be optimal for younger people and middle-aged people, older people still require more protein, Middlemann explained.

“Sarcopenia is the primary age-related cause of frailty, which is associated with a higher risk of disabilities, having to go to a nursing home, and of falls, fractures, hospitalizations, and premature death,” Middlemann said.

It occurs with age-related muscle loss, she said, ranging “anywhere from 0.5% to 2% of total muscle mass per year, starting around age 50 (though in folks who are mostly inactive, it can start even earlier).”

Middlemann also said that her clients have been able to increase “muscle mass, cardio-metabolic health, and overall quality of life” by consuming 25 to 35 grams of protein at each meal and practicing resistance training.

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