Protein guide
Sources and Evidence Base
The peer-reviewed research and official dietary guidelines behind every number on this site, with full citations and how each source informs our calculator.
Every multiplier, adjustment, and recommendation on this site traces back to published, peer-reviewed research or official dietary guidelines. This page lists those sources and explains how each one shapes the calculator.
How we use these sources
Our protein matrix is built from three layers: the USDA Dietary Reference Intakes set the 0.8 g/kg floor; the ISSN position stand and sports-nutrition meta-analyses define the higher ranges for muscle, performance, and weight loss; and the PROT-AGE recommendations drive the older-adult adjustment. Plant-protein digestibility adjustments come from PDCAAS and DIAAS scoring research. We do not invent multipliers or copy them from other calculators.
Cited sources
International Society of Sports Nutrition (ISSN)
International Society of Sports Nutrition Position Stand: Protein and Exercise
The foundational position stand recommending 1.4–2.0 g/kg/day for athletes, with evidence for higher intakes during energy restriction. Underpins our muscle-gain and athletic-performance multipliers.
Campbell B, et al. · 2007 (updated 2017)
USDA / National Institutes of Health
Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids
Sets the 0.8 g/kg RDA for healthy sedentary adults — the floor used in our "General Health" target. Also defines amino acid requirements.
Institute of Medicine, Food and Nutrition Board · 2005
PROT-AGE Study Group
Protein Intake and Exercise for Optimal Muscle Function in Aging
Recommends 1.0–1.2 g/kg/day for older adults, rising to 1.2–1.5 g/kg during stress, to counter anabolic resistance. Basis for our +0.2 g/kg adjustment for ages 60+.
Bauer J, et al. · 2013
Journal of the International Society of Sports Nutrition
A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength
Found that protein supplementation enhances muscle mass and strength gains, with diminishing returns above ~1.6 g/kg/day.
Morton RW, et al. · 2018
Journal of the International Society of Sports Nutrition
The effects of consuming a high protein diet (4.4 g/kg/d) on body composition in resistance-trained individuals
Showed that very high protein intakes well beyond the optimal range do not produce additional benefit, supporting our upper limits.
Antonio J, et al. · 2014
British Journal of Nutrition
Effect of protein supplementation on muscle mass and strength in resistance-trained individuals: a meta-analysis
Confirms the 1.6–2.2 g/kg range for maximizing muscle protein synthesis and the per-meal threshold of ~0.3–0.4 g/kg.
Helms ER, Aragon AA, Fitschen PJ · 2014
Journal of Nutrition
Dietary protein and skeletal muscle: A review of the evidence
Reviews protein quality measures (PDCAAS, DIAAS) and the higher protein needs of older adults.
Phillips SM, van Loon LJC · 2011
USDA FoodData Central
FoodData Central database
The source for all protein-per-serving and per-100 g values in our protein sources reference and food equivalents.
U.S. Department of Agriculture, Agricultural Research Service · Ongoing
Editorial standards
- • Primary sources only. We cite peer-reviewed studies and official guidelines, not blogs or supplement marketing.
- • Reviewed before publishing. Each guide is checked against the cited research before it goes live.
- • Dated and maintained. Articles carry a "last updated" date and are reviewed when new research is published.
- • No conflicts. We sell no supplements and take no sponsorships. The site is funded by advertising, clearly separated from content.
- • Not medical advice. These are population-level guidelines for healthy adults. For personal medical guidance, consult a physician or registered dietitian.
Full DOIs and links are available on each publisher's site; search the title above on PubMed or Google Scholar to find the original paper.