Let’s start this blog series with a pop quiz! What is the Food and Nutrition Board’s recommendation for women’s protein requirements?*
- 75 grams per day
- 70 grams per day
- 50 grams per day
- 46 grams per day
If you chose option 4, you are correct.
*This number is based on the FNB’s reference body weight for females, 125 pounds. (125 lbs/2.2 = 57 kg x 0.8 g = 46 grams or 184 calories per day from protein)(1).
Now, choose approximately how many grams of protein per day is needed for basic physiological functions in females.*
- 70 grams per day
- 75 grams per day
- 50 grams per day
- 46 grams per day
If you chose option 1, you are correct.
*Functions include fetal growth and survival, immune formation and function, skeletal muscle maintenance and growth, enzyme creation and function, albumin formation, hormone synthesis and endocrine function, metabolic reactions, redox reactions, absorption, transport, and storage of nutrients, oxygen transportation and utilization, cardiovascular function, lymphatic function, neurotransmitter formation and function, libido, skeletal remodeling, skin and hair health, etc. (2, 3).
Notice that women’s protein requirements from the FNB are 1.5 times less than what is needed for physiological function.
Women’s protein requirements and national defense
Women’s protein requirements via the Food and Nutrition Board (FNB) of the Institute of Medicine are technically called recommended dietary allowances or RDAs. RDAs are “the average daily dietary intake level sufficient to meet the nutrient requirements of nearly all (97–98 percent) healthy individuals in a group” (1).
But why were the RDAs created? And what, exactly, do they mean?
Accounts of when the RDAs were formed differ: some papers report 1941 (4. 5), while others report 1943 (6). Regardless, near the middle of World War II, the US National Research Council set out to create nutritional guidelines to curb “nutrition problems in connection to national defense” (6).
Food shortages during the war led to relief efforts for those suffering from malnutrition and starvation. The food provided in these campaigns was based on the information provided by the newly formed RDAs.
The RDAs were updated every five to 10 years from 1943 to 1989. In the mid-1990s, they were joined by more specific nutritional guidelines: the estimated average requirements (EAR), adequate intakes (AI), and upper limits (UL) under the umbrella of dietary reference intakes (DRI).
Guidelines that were once used to solve malnutrition in a time of economic depression and war were then adopted for multiple other needs.
“[RDAs are now used] for planning and procuring food supplies for population subgroups; for interpreting food consumption records of individuals and populations; for establishing standards for food assistance programs; for evaluating the adequacy of food supplies in meeting national nutritional needs; for designing nutrition education programs; for developing new products in industry; and for establishing guidelines for nutrition labeling of foods. In most cases, there are only limited data on which estimates of nutrient requirements can be based” (6).
While the original purpose of the RDAs was to avoid deficiency in most people—not to achieve health in individuals—the 10 years-long updates to DRIs marked a shift toward criteria associated with health benefits for multiple groups of people.
For instance, the DRIs are based on specific biochemical functions of nutrients (when research is available), functional criteria that are related to health benefits, and the inclusion of food components that play a role in maintaining health, such as fiber. The effects of excessive nutrients are also considered in light of (the largely beneficial) food fortification programs that began in the early 1920s (7, 8).
But even with the shift toward DRIs, women’s protein requirements changed only slightly. Since 1941, the RDA has actually decreased from the original requirement of 1.07 g/kg. However, the 1941 reference body weight for women, 56 kg, has increased to 57 kg…from 123 pounds to 125 pounds (9, 10).
Keep these changes in mind: we’ll touch on them again later in the blog.
Why are women’s protein requirements so important?
Consider each physiological function that relies on dietary protein:
- fetal growth and survival
- immune formation and function
- skeletal muscle maintenance and growth
- enzyme creation and function
- albumin formation
- hormone synthesis and endocrine function
- metabolic reactions
- redox reactions
- absorption, transport, and storage of nutrients
- oxygen transportation and utilization
- cardiovascular function
- lymphatic function
- neurotransmitter formation and function
- skeletal remodeling, skin and hair health
One frequently overlooked function that I’d like to emphasize is the maintenance and growth of skeletal muscle.
Why would skeletal muscle maintenance be more important than the others?
Sarcopenia is a decidedly untrendy but widespread syndrome characterized by loss of muscle mass, strength, and function. It’s usually related to advanced age. However, about 10% of women experience sarcopenia beginning in their early 20s, with some women in their 30s and 40s experiencing more advanced, class II sarcopenia (11).
Sarcopenia doesn’t sound particularly problematic. But just a shallow dive into the research literature reveals how impactful sarcopenia can be. It greatly increases:
- the risk of fractures, falls, and frailty
- increased time in the hospital (12)
- residency in a nursing home
- decreased quality of life
- premature death (13)
But even with the massive amount of human studies examining sarcopenia and its effects, a current nutritional trend, particularly prevalent in the holistic community, is purporting the benefits of a low protein diet for longevity, reduction of biological age, reducing the risk of cancer, etc.
And while some of the studies on low protein intake are incredibly interesting, most are conducted on yeast, flies, and mice*, are observational (inferring correlation, not causation), or, perhaps most importantly, don’t consider the tradeoff of healthspan for lifespan (14).
What does that mean, exactly?
It means that while protein restriction (and caloric restriction, generally) MIGHT increase the lifespan in humans or reduce the risk of certain diseases, there is stronger and more established evidence that it decreases quality of life and function.
A better option, suggested by multiple research teams, is time-restricted feeding protocols, such as intermittent fasting or the strategic use of fasting-mimicking diets. As one astute author notes:
“It is important to highlight that (i) an increase in health span (induced by fasting or other anti-aging interventions) does not always associate with increased longevity  and (ii) it remains to be fully established if increased longevity is associated with all forms of fasting. Conversely, increased lifespan is not always associated with increased health span or delayed anti-aging symptoms” (15).
Mice’s metabolic rate is 7 times faster than humans—mice use approximately 14,000 calories per day in relation to our 2000 calories per day. Metabolic studies on mice are useful, but results cannot be extrapolated to humans.
Are current women’s protein requirements accurate?
Where did the 0.8 g/kg/day RDA come from, anyway?
Let’s first examine a crucial point about protein.
Protein is used for biosynthesis and structure, not energy
Dietary proteins are primarily used for biosynthesis (creation of enzymes, hormones, antibodies, neurotransmitters, etc.) and structure building. Carbohydrates and fats, on the other hand, are primarily used for the creation of energy (ATP).
So, instead of women’s protein requirements being based on energy requirements, as carbohydrates and fats are usually expressed, they’re based on our biosynthetic needs and the amount of body mass, specifically lean mass, we need to maintain.
What does this mean in real life?
Generally, we see carbohydrate and fat recommendations in the context of energy being consumed. For example, as a percentage of a 2,000-calorie diet.
Protein recommendations, however, are calculated according to an individual’s body weight. (Remember, proteins are not primarily used for energy needs.) In a perfect world, daily protein requirements would be based on lean mass (16). But most research studies, and most people, use body weight in lieu of lean mass due to the difficulties (such as cost and availability) of acquiring body composition calculations.
So, for now, body weight is the best overall way to calculate daily protein needs.
Now, think back to this passage from earlier in the blog:
“But even with the shift toward DRIs, women’s protein requirements changed only slightly. Since 1941, the RDA has actually decreased from the original requirement of 1.07 g/kg. However, the 1941 reference body weight for women, 56 kg, has increased to 57 kg…from 123 pounds to 125 pounds (9, 10).”
Now you can see why this shift is so impactful. Even though the reference body weight only slightly increased from 123 pounds to 125 pounds (not to mention the population’s body weight increase from an average of 125 pounds to 170 pounds!), the protein recommendations actually decreased instead of increasing along with body weight (17).
The problem with nitrogen balance studies
To get back to our initial question regarding the origin of the current protein RDA of 0.8 grams per kilogram of body weight—this figure is sourced from studies using nitrogen balance as a way of calculating protein balance, i.e., whether the amount of ingested protein is enough to prevent a net loss of protein resulting in negative effects on biosynthesis and structure.
“…the RDA of a protein was recommended to meet [nitrogen] balance, and should not be considered as the optimal amount for maintenance, optimal health or specific functions of organs” (2).
But, using nitrogen balance to determine protein requirements may lead to inaccurate recommendations. The dietary timing, collection methods, and wide variability in results have led many researchers to criticize this technique.
“The drawbacks with nitrogen balance are that a minimum of 3 days is needed per level of test intake and 7–10 days of adaptation are needed to each intake of protein or amino acid. In addition, complete collection and quantification of all sources of nitrogen excretion (mostly in urine and feces) is difficult. More fundamentally, the nature of the calculation of nitrogen balance is likely to result in wide variability because nitrogen intake and nitrogen excretion are much larger numbers than the difference between them (i.e., nitrogen balance). Hence alternative methods have been developed based on carbon oxidation” (18).
Thankfully, there is a more accurate method. The Indicator Amino Acid Oxidation (IAAO) method bypasses the pitfalls of nitrogen balance studies thanks to shorter adaptation times and breath testing, for instance (18).
So, what have studies using IAAO found regarding the current 0.8 g/kg/day RDA for protein?
Are current women’s protein requirements adequate?
This is the million-dollar question that nutritionists, doctors, and physiologists everywhere are pondering.
Every paper I could find that examined physiological protein requirements concluded that, no, the RDA is much too low for women’s protein requirements (19).
These papers ranged from narrative reviews on protein and human health across the lifespan (3, 9, 20), to protein requirements for athletes (21), to protein requirements for female athletes, specifically (22), to, somewhat surprisingly, protein requirements for those with advanced disease states (23, 24) ⬅️ citation 24 is especially eye-opening.
There are some technical caveats.
AMDR v. MyPlate v. RDA
The RDA for protein is the most popular nutritional recommendation in the US. However, two other guidelines, the Acceptable Macronutrient Distribution Range (AMDR) and the more well-known MyPlate chart, also provide women’s protein requirements.
What do they recommend?
The AMDR for protein is 10-35% of daily caloric intake, equaling between 50-175 grams of protein per day based on the standard 2,000-calorie diet.
The MyPlate protein recommendation is approximately 17-21% of daily calories, equaling approximately 87.5 grams of protein per day. However, MyPlate recommendations are individualized based on age, sex, weight, height, and physical activity level (25).
Remember, the RDA for protein equates to approximately 46 grams of protein per day.
So, depending on your source, you may consume adequate amounts of protein for physiological functions.
DRIs v. MDRIs
Did you know that there are actually two separate sets of DRIs?
Yes, it’s true!
You may be familiar with the lesser-known DRIs if you’re a veteran or currently in the US military. Military personnel follow nutritional guidelines set by the MDRIs, or Military Dietary Reference Intakes.
“For most nutrients, the MDRI is the highest gender-specific reference value or RDA” (26).
According to MDRIs, the average caloric intake for females is 2300 calories per day but ranges from 2200 to 3150 based on activity levels. The reference body weight for military females is 175 pounds. And the MDRI for protein ranges from 0.8 (the standard RDA) up to 1.5 g/kg body weight, equaling between 64 and 120 grams of protein per day.
The MDRIs account for the extraordinary physical demands of active military service that warrant higher amounts of nutrients, including protein.
Pregnancy and lactation
The final caveat is that of pregnancy and lactation. During these heavily nutrient-dependent times, the RDA for protein increases to 71 grams per day, just over the approximated amount for basic physiological functions discussed earlier in the blog.
Interestingly, the most nutrient-demanding time of life is lactation, not pregnancy. However, women’s protein requirements are the same in both periods. One recent study, however, found that exclusively lactating women likely need between 1.7-1.9 g/k/day of protein to provide nutritionally adequate breastmilk and maintain muscle mass (27).
For the average American woman weighing 170 pounds (much closer to the MDRI reference weight v. the 125-pound standard DRI reference weight), this equates to 131-146 grams of protein per day.
Given these caveats, it stands to reason that females who lead active lives, like:
- athletic females (a phrase coined by our friend Steph Gaudreau for women who do athletic activity but don’t necessarily consider themselves athletes)
- those in high-stress or physically demanding jobs (ahem, moms)
should also ingest higher amounts of protein.
Stay tuned for the next blog in the series, where we’ll cover protein thresholds (how much can we actually use?), protein needs in special populations such as children and those with connective tissue disorders, and the question of the negative effects of too much protein on the kidneys and GI tract…is protein damage a thing?