Protein per kg: the RDA vs the ISSN range

What an RDA is

The Recommended Dietary Allowance is one of the Dietary Reference Intakes published by the National Academies. It is defined as the average daily intake sufficient to meet the requirement of nearly all, 97 to 98 percent, of healthy individuals.

That definition matters. An RDA is set high enough to cover almost everyone in a healthy population. It is a population benchmark, not a measurement of any one person's needs.

The protein RDA: 0.8 g/kg

The 2005 Institute of Medicine report states that the RDA for protein for adults is 0.8 g per kilogram of body weight per day. Its examples are about 56 g a day for a 70 kg reference man and 46 g for a 57 kg reference woman.

The number has a long history. An earlier allowance started from 0.6 g/kg of high-quality protein and corrected it for 75% efficiency, giving 0.8. In 1989, an average of balance studies gave 0.6 g/kg, and a variability allowance raised it to 0.75. That value was set for both sexes and accepted as the same for older and younger adults.

The ISSN range: 1.4–2.0 g/kg

In 2017, the International Society of Sports Nutrition published a position stand on protein and exercise. The authors, led by Ralf Jäger, wrote in the Journal of the International Society of Sports Nutrition, volume 14, number 20.

The position stand says an overall daily intake of 1.4 to 2.0 g of protein per kg per day is sufficient for most exercising individuals. Its stated aim is building and maintaining muscle.

It also describes per-serving amounts: 0.25 g of high-quality protein per kg of body weight, or an absolute 20 to 40 g per serving. The authors note that these figures vary with age and recent resistance exercise. It reports newer evidence that intakes above 3.0 g/kg may affect body composition in resistance-trained people, mainly through lower fat mass.

Two benchmarks, two questions

The RDA answers a public-health question: what intake covers nearly all healthy adults? The ISSN range answers a sports-nutrition question: what intake is sufficient for most people who exercise to build and keep muscle?

In ratio terms, the ISSN low end of 1.4 is 1.75 times the RDA, and the high end of 2.0 is 2.5 times it. The gap comes from the different goals and populations, not from a disagreement about arithmetic.

The two also rest on different kinds of evidence. The RDA history traces back to balance studies averaged across healthy people, then raised by an allowance for variation between individuals. The ISSN position stand reviews studies of exercise and muscle.

Neither one replaces the other. The calculator shows both, labeled with their sources, so the gap between them is visible instead of hidden.

Linking grams to calories

Protein grams can be compared with the macro split. At 4 kcal per gram, 56 g of protein is 224 kcal, which is 11.2% of a 2,000 kcal day. The ISSN range for 70 kg, 98 to 140 g, is 392 to 560 kcal. That is 19.6% to 28% of the same 2,000 kcal.

All three figures sit inside the adult AMDR for protein of 10–35% of calories. That is a coincidence of these numbers, not a rule. At other weights and calorie totals, the percentages change.

The calculator needs only ‘Units’ and ‘Weight’. Per-kg benchmarks use total body weight, so two people of the same weight get the same grams whatever their build. The benchmarks are for adults and are not made for pregnancy, people under 18 or kidney disease.