Calculating our nutritional needs and assessing adequacy can seem like a daunting task. On the surface it may seem like we always need professional help. On the contrary, assessing our micronutrient (vitamin and mineral) needs is actually quite simple, and in this post I am going to teach you how!
*Quick disclaimer that I am writing this post under the assumption that you are a healthy individual without nutrient deficiencies. If you exhibit symptoms of a deficiency, please seek professional medical help.
Calculating Our Needs
How much of different micronutrients to eat can seem really complex. However, if you stop to think about it for a moment, it’s actually really simple. For our body to function, we require a certain amount of micronutrient to perform various functions in our body. To keep functioning normally, we simply need to maintain how much nutrient we have in our body. In other words, the micronutrients we put in need to be equal to the micronutrient we are losing.
Nutrient In = Nutrient Out
Now before you get carried away, we need to remember one important caveat. Simply eating an amount of a nutrient does not need mean that our body will absorb it. In fact, our body only absorbs a limited amount of the nutrients we are eating. Therefore, we can make a small amendment to our equation.
Nutrient In * % Nutrient Absorbed = Nutrient Out
Armed with our new equation, we can solve for our nutritional needs of any micronutrient by figuring out how much of a nutrient we are losing, and how much of the nutrient we are absorbing. This is where our personalized nutrition process may get a little hairy as we need to conduct a bit of research. First, let’s tackle the nutrient absorbance percentage. The absorbance percentage of a nutrient should be fairly straight forward to find with a google search. Sometimes, the absorbance rate will vary for a micronutrient depending on food source. Using a bit of math, we can easily take this into account as will be examined in the example. Our other unknown value in the equation is the amount of micronutrient we are losing on a daily basis. Once again, this will require a bit of research as we need to account for the various ways in which a nutrient leaves our body, and how much is leaving. Several google searches can provide us with this information. Now all we need to do is plug in our numbers, and out pops how much of a nutrient we should be consuming. That’s enough talking, let’s get to doing an example so you can see how easy it is!
An Example: Calculating Iron Needs
So how do we actually put the ideas above into practice. Hopefully an example will help clear up any confusion. In our equation above, we realized that there are two variables we need to account for, how much of a nutrient is absorbed, and how much we are losing. Once again, let’s begin with nutrient absorbance.
In the case of iron, not all iron is created equal. There are two possible sources of iron in the diet, heme and non-heme iron. Based on studies, it has been found that approximately 17% of non-heme (vegetarian) iron is absorbed while 25% of heme iron (meat source) is absorbed (for vegans, the iron absorbance is closer to 10%). Now we could go ahead and figure out how much of the iron on a per food basis is absorbed, but it would be far simpler if we approximate our total iron absorption throughout all our meals. The standard American diet consists of approximatley 90% non-heme iron and 10% heme iron, therefore, we can perform the following calculation to get total iron absorption.
.9(17%) + .1(25%) = ~18%
To figure out your own absorbance rate, simply examine how much of your diet consists of meat and non-meat sources (keep a food diary to help you figure out your eating habits!). In my own case, I don’t eat any animal products. Therefore 100% of my iron comes from vegan sources, and my absorption rate is approximately 10%.
Next, we need to figure out our iron losses. Iron is lost through sweating, defecation and bleeding. To simplify, through an online search we can find that our basal iron loss is a product of our weight. It has been found that we lose approximately .014mg iron per kilogram of bodyweight per day. Women, I know what you’re thinking about the bleeding, and we will get there in just a moment, but first, let’s figure out our basal losses.
Suppose we are calculating the needs for a 65kg woman. This woman’s losses would be the following:
65kg * .014 mg/kg/day = .91mg/day
Now men, you can skip ahead to the final calculation of needs, but women must also take into account iron lost through their menstrual cycle. To figure this out, we need to figure out how much blood a woman is losing throughout her period (one sanitary product holds ~5mL blood). An average woman loses approximately 30mL of blood throughout her cycle. Now we just need to convert this number to iron loss per day. An average menstrual cycle is approximately 28 days. Therefore, we can do a simple division to get iron loss per day.
30 mL / 28 days = 1.07 mL blood per day
Finally, we need to convert this to amount of iron lost per day. With a bit of research, we can find that there approximately .5mg. Therefore, the total menstrual iron loss calculation will be the following:
1.07 mL/day (basal) * .5mg iron/mL (menstrual) = 1.535 mg iron/day
Now that we have both basal and menstrual iron losses for our example subject, we can finally figure out how much iron she needs to consume. Her total iron loss is the sum of her basal loss and menstrual loss.
.91mg/day + .535mg/day = 1.445mg/day.
Armed with our iron loss and absorption percentage, we can finally calculate how much iron our woman needs to consume. Assuming, she eats a standard American diet: (.18 comes from 18% iron absorption)
Iron Intake * .18 = 1.445
Iron Intake = 1.445/.18 = 8.02 mg/day
That’s it folks! The same basic principle as explained here applies to any other micronutrient. The only challenge will be figuring out how much of a nutrient is absorbed by the body, and how much of a nutrient you lose. Both will require a bit of research.