Why isn't phosphorus or nitrogen a limiting nutrient for animals?

Question

Nitrogen and Phosphorus are usually the limiting nutrient for plants, especially for algae. Phosphorus is used for DNA, ATP and phospholipids, and Nitrogen is used for pretty much every protein a cell might want to produce. That is, their need for biological processes is not tied specifically to photosynthesis: anything that lives is going to need them, pretty much for anything it might want to do. It would make sense for them to be a limiting nutrient for almost anything that's trying to grow, plant or animal.

Yet for animals the limiting "nutrient" seems to always be energy, ie: food. Why aren't animals limited by lack of nutrients in the same way that plants are? Obviously animals need these nutrients, too. Or to reverse the question, why do plants need so much more phosphorus/nitrogen than animals do?

My best guess is that an animal's digestion of plant material is relatively inefficient energy-wise but relatively efficient nutrient-wise. So for an animal to eat enough food to have sufficient energy to survive, it's probably eaten more than enough Nitrogen and Phosphorus for its needs. But I'm just guessing and I can't find any data that would back up that guess.

Answer 1

Phosphorus

Your suggestion that if we are meeting our calorific requirement we will be getting enough is true for phosphorus. Most foods contain lots of phosphorus. The maximum dietary requirement occurs during adolescent growth, estimated at 1250 mg per day. Assuming a calorie intake of 2500 kcal we can calculate a 2500 kcal equivalent phosphorus content for various foods:

skimmed milk contains 7,400 mg phosphorus per 2500 kcal

roasted chicken breast contains 7,500 mg phosphorus per 2500 kcal

cooked white rice contains 3840 mg per 2500 kcal

(Calculations are based upon values obtained via this site.)

Nitrogen

Our requirement for nitrogen is met by our protein intake: inadequate protein intake manifests as kwashiorkor which is essentially due to a dietary deficiency of essential amino acids. In other words, the only way to achieve a nitrogen-deficient diet is to not eat protein, and this would not be alleviated by any inorganic source of nitrogen, even if we could consume enough of such a N source.

Answer 2

As you already mentioned N and P are major components of important biomolecules. In Sterner & Elser, ecological Stoichiometry, 2002, fig 2.2 demonstrates the percentage of N and P in biomolecules. As you can imagine these biomolecules are important and require forms of P and N to be consumed for an organism to function.

Animals are limited by nutrients such as Nitrogen (N) and Phosphorus (P). From an ecological perspective, many aspects of an organism’s life and subsequently their evolution are affected by nutrient limitation. For example, the brood mass of an aquatic snail can be limited by a low amount of dietary P (Tibbets et al., Phosphorus-mediated changes in life history traits of the invasive New Zealand mudsnail (Potamopyrgus antipodarum) 2010). Another example is a riparian crustacean growth limited by a low P treatment (Danger et al., Phosphorus content in detritus controls life-history traits of a detritivore, 2013).

There is even a ratio of the required elements for animals to survive/grow in an environment, threshold elemental ratio. The threshold elemental ratio is often a carbon to nutrient (i.e. N or P) ratio; different clades/species have differing ratios (see figure 2 in Frost et al., Threshold elemental ratios of carbon and phosphorus in aquatic consumers, 2006). Thinking about animals getting their energy from food is too simple.

Consider this easy example; we, humans, require protein to properly function; a 150 lb adult – who requires about 55 grams of protein a day (these are estimates, I am not an nutritionist). That person can eat about one chicken breast and fulfill that daily requirement or they can eat 25 ounces of potato chips to achieve that. If they only have potato chips available then they have an N limiting diet. Of course, plants and animal have different requirements for energy, but animals are limited by nutrients just as plants are.

https://www.jstor.org/stable/40864578 https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2435.12079?__cf_chl_jschl_tk__=pmd_j7iq6cCQINSFmxVZ.Cvds9jOQmS2OHgpvYjw1t.gTU8-1635260689-0-gqNtZGzNAjujcnBszQvR https://onlinelibrary.wiley.com/doi/full/10.1111/j.1461-0248.2006.00919.x?__cf_chl_jschl_tk__=pmd_g7eFplMHGUViCzJfN9vFQvHm4vmZabdJLJLo180WrCc-1635260692-0-gqNtZGzNAlCjcnBszQyl

Answer 3

Off the top of my head:

• Plants are photosynthetic so have unlimited access to energy. If an animal had unlimited access to energy it would likely have Nitrogen or Phosphorous or something else as the limiting nutrient.
• Animals eat plants and/or each other. Whatever animals eat will have to a greater or lesser degree sufficient protein/fat/nucleic acids etc to satisfy minimum nutritional requirements. As noted in Chris' comment there can be some species-specific limitations like availability of salt in a herbivore diet, or the human requirement of an external source of vitamin C.