Ask: Do we need 100% of nutrients every day?

As humans, we've evolved with cycles of food availability. For example, you couldn't eat bread until the grain had been harvested and milled. If you wanted strawberries you had to wait until they ripened. Today, we hear about recommended daily allowances of vitamins and minerals constantly. We take daily vitamins to be sure we're getting 100%. But do our bodies need 100% every day? Do we, for example, need more vitamin C just when the summer fruits come in season? Do our bodies need more protein in the winter, when fresh fruits and veggies weren't available but meat could be caught? I'm wondering if our bodies need a cycle, and not a daily dose.


This is a multi-part question, and I will do my best to address all of it - some ventures outside the realm of my expertise, and some parts are simply opinion or do not have clear cut answers at this point.

Part 1: Eating 100%...of what?

In my opinion, we do not need to eat 100% of the RDA for every nutrient every day. Some vitamins are lost more easily than others, such as the water soluble vitamins (all B vitamins, folate, vitamin C). Some fat soluble vitamins, like vitamin A,  are stored in our bodies in such amounts that it could take several months of absolutely no intake to deplete! The Daily Value percentages that you see on the nutrition facts label are general guidelines. Imagine coming up with exact percentages of what will be best for millions of people with varying metabolic rates, etc. Not so easy!  There's a ton of epidemiological research behind these numbers.

The % DVs are based on a 2,000 calorie diet for healthy adults. Most people do not need to eat that many calories in a day, so looking at these exact percentages is not quite accurate. I usually tell people to use the % DVs to compare products. For example, instead of counting milligrams of sodium, you could use the 5/20 rule: 5% or below is “low” and 20% or above is “high.”

If you are interested in what the actual daily values are for you, check out this handy calculator or the tables of dietary reference intakes. There are 4 reference intakes for 22 life stages and gender groups. Since I've always found the language used very confusing, here's a quick translation of what you'll see in the tables:

• Estimated Average Requirement (EAR)
• The basis for setting the RDA. EAR is the nutrient intake level estimated to meet requirements of 50% of healthy people in a particular life stage and gender group.
• You don't really need to pay attention to this. 
• Recommended Dietary Allowance (RDA)
• Nutrient intake level sufficient to meet requirements of almost all (97-98%) healthy people in a particular life stage and gender group. 
• Pay attention to this number! 
• Adequate Intake (AI)
• Nutrient intake level assumed to be adequate but based on experimental approximations. 
• Pay attention to this too - you'll see it when no RDA can be determined (due to ethical concerns or other issues).
• Tolerable Upper Intake Level (UL)
• Highest level of nutrient intake that poses no risk of adverse health consequences for most healthy people in a particular life stage and gender group.
• As nutrient intake exceeds this amount, risk of toxic effects increases!

Part 2: A physiological needs cycle?

I love eating food in season, and I try to do so as much as possible for health and environmental reasons - and the sheer deliciousness of fresh food! I love one of the ideas presented in this question: that our bodies could be so in tune with nature that our physiological needs are expressed along with the flourishing crops. However, I believe this idea doesn't hold up since, well, the Earth is populated not in one climatic zone, but many. Growing seasons vary drastically across the globe. Does that mean that humans living near the equator have completely different basic nutrient needs than humans living elsewhere? I don't think so. I mean, I'm sure people living in warmer climates need to replenish electrolytes lost in sweat more frequently than people living in the far north. But the nutrient requirements of the body do not substantially change; it is the environment that changes what actions must be taken to meet those same nutrient requirements. So, as far as I can tell, the answer to that part is no - our physiological needs do not exist in a cycle, since not everyone lives in a climate where a cycle exists.

Using your example of vitamin C: fruits and vegetables available during the summer have plenty of vitamin C, but potatoes and winter squash provide a decent amount too. One medium white potato (2 1/4 - 3 1/4 in) eaten with skin provides 42mg of vitamin C. It doesn't sound like a lot, but that's 70% of the Daily Value of vitamin C! Since vitamin C plays a vital role in immune function, one could argue that it's consumption is even more important during the fall/winter months when viruses tend to circulate (and when fresh fruit not available in the places experiencing winter). Sure, potatoes are harvested during late summer, but luckily they are a storage crop and are available as a source of vitamin C throughout the winter.

We tend to compartmentalize and think of things in terms of days or weeks, but our bodies are constantly functioning.  I think it's much more important to think more long term and eat a healthful and varied diet. By doing so, it's actually quite easy to meet your RDAs for most nutrients!