How to “measure” if a food is good source of nutrients?

Most of us live our lives striking a balance between eating what we like and trying to be healthy. A little bit of birthday cake? Yes please! Brussel sprouts… well, sometimes you need to just swallow it down quickly before you taste it. Living this way and trying to eat a balanced diet gives us all the nutrients we need, so there isn’t often a need to worry about individual vitamins and minerals. But if someone is looking to increase their intake of a certain nutrient they will generally turn to foods that are a good source of that specific nutrient. But what does it really mean to say that a food is a good source of something? For example, spinach, soy, and sesame are all good sources of calcium. Which one is better though? What information do we need to figure this out? Let’s dive pumpkin-first into this conundrum!


Concentration of nutrients (mg / 100 g)

One way of evaluating food sources is looking at nutrient concentration. In typing the word “spinach” into google, we can obtain information saying that it contains 99 mg of calcium per 100 g of spinach. This information is clear, scientifically accurate, and easily accessible. But is the term “a good source of ___” the same as “contains a high concentration of ___”? That, unfortunately, is a great big NOPE. Though bioavailability (how effectively the body can absorb and use a nutrient; normally dependent on other contents of the food, for example, iron bioavailability is dependent on vitamin C in a meal) is a factor that plays in here, that’s not even what we mean! Points to you if you thought that, but we are thinking of a less obvious reason why high nutrient concentration does not necessarily make something a good source or foods with low concentrations a bad source.

Orange juice is a good source of vitamin C (50 mg / 100 ml). Drinking 200 ml of orange juice will give you 100 mg of vitamin C, more than our 80 mg per day requirement. That being said, in summer when you get thirsty maybe you’re going to water down that juice to make it go further. If you add 200 ml of water to your 200 ml of juice and then drink the 400 ml of the diluted beverage, then the total amount of vitamin C remains the same, but the concentration is now half of the original. Does this make it a worse source of vitamin C? No! You’re still ingesting the same amount of vitamin C after all.

Let’s look at beans as another example. Their protein content is 21 g / 100 g. This information refers to raw beans. But let’s think for a second: do you chew on raw beans? We hope not (and so does your dentist… ouch). When you cook the beans, a lot of water enters them. As a result, the weight of the food increases 3-6 fold. The total amount of protein doesn’t change, but the concentration drops by the same factor. So even though 100 g of beans contains 21 g of protein, when you cook them up you need up to 600 g of beans to get this same amount! This is where you can run into problems as you likely won’t eat that much of the beans but you may still be thinking of them as a good source of protein. So this method of classifying foods as being a good source of something isn’t that great in the end… let’s try another.


Amount of nutrients in a portion (mg / portion)

To know the nutrient content of a food, you need to know the serving (portion) size. In the case of our orange juice, we know how much we drank. But when companies, scientists, and blogs (like us!) try to work these things out, it isn’t done for every individual. Instead, a general recommendation is used. The US Food and Drug Administration (FDA) determines “Reference Amounts Customarily Consumed” (RACC). According to this regulation, the portion size for all beverages is 360 ml. This applies to carbonated and non-carbonated beverages, water, coffee, tea, flavoured or sweetened beverages, and even wine coolers! If we go back to our orange juice, this makes our diluted orange juice look like a worse source of vitamin C. 360 ml of it WILL have less vitamin C than 360 ml of the more concentrated orange juice. But we know from previous paragraph, that due to different consumed volumes, they are equally good sources! Intake in real life is also usually much smaller or larger than what portion sizes define (let’s be honest, who eats 1 cookie?). Intake also depends on if the food is consumed by itself (for example, a bean salad) or as part of a dish (for example, beans in a vegetable soup). As a result, expressing the nutrient content per serving is useful only when the serving size is correctly determined, such as in prepackaged food. This information thus is not appropriate for the general public in terms of telling us which foods are good sources of nutrients. How else can we express the quality of nutrient sources?


Nutrient density (mg / 100 kcal)

A third option is to identify a good source as one that has a lot of a nutrient relative to its energy content (calories). This solves both our dilution and our unknown portion size problem. The amount of vitamin C per calorie in orange juice is the same whether it is diluted or not and no matter what size of glass you use. More professionally, this parameter is called nutrient density and is defined as the nutrient content expressed in terms of the energy content, which is represented in kJ or kcal. You can calculate it and see that diluted and undiluted orange juice both contain 1.14 mg of vitamin C per calorie (=114 mg / kcal), showing that they are both equally good sources of this vitamin. This is because diluting the juice dilutes not only the vitamin content, but also the calorie content. If you were to add some sugar to your juice though it would increase the calorie content but not the vitamin C. As a result of this, we would have more calories to the same amount of vitamin C and thus the nutritional density would be decreased. If we agree that sweetened juice is a worse source of vitamins than less sweetened ones then we can agree that nutritional density is a good parameter for measuring the quality of a vitamin source. The calculation works just as well for the beans and it shows that the nutrient density of proteins in beans is 6 g per 100 kcal regardless of the method of cooking and amount of water used (100 g of beans contain 21 g of proteins and 347 kcal). All this being said, this parameter does have one slight disadvantage: it is difficult to know what amount of food we are talking about. Many of us have an approximate idea of what 100 g looks like, but how many of us know how many calories are in a single bean?


What are our daily needs?

Thus far we were comparing the amount of individual nutrients in different foods (vitamin C in diluted and undiluted orange juice, proteins in green beans and kidney beans). Another challenge is comparing different nutrients. Asparagus, for example, contains 11 mg of iron per 100 kcal (=2.1 mg per 100 g) and is a very good source of iron. It contains WAY more calcium (120 mg per 100 kcal, = 24 mg / 100 g), but it is still not that good source of calcium. We aren’t messing with you, promise! This is actually because the recommended daily intake (RDA) for calcium is 1000 mg while our daily need for iron is only 5 mg for men and 8 mg for women. The same amount of asparagus that provides more than our daily need for iron and gives us only 12% of our calcium requirement. For a fairer presentation of nutrient content, it may be expressed as a percentage of our daily need rather than milligrams. Again, these percentages can relate to 100 g of food, one serving, or to a quantity of food containing a given amount of energy (1 kcal or 100 kcal usually), with the last option offering us the greatest advantage. Since we are dealing with average daily nutrient requirements, it is logical to also use an average daily energy requirement of about 2000 kcal. The parameter can be abbreviated as “% RDA / 2000 kcal.”

Asparagus contains 240% of your daily requirement for calcium per 2000 kcal, while iron content is 2200% (move over, Popeye. Yikes). With such presentation we can conclude that asparagus is reasonably good source of calcium and an excellent source of iron. Some say that this presentation is misleading since you would need to eat about 10 kg of asparagus to reach these levels (please, do not try this at home…), but we all know (hopefully) not to consume only a single type of food in a day, and our daily 2000 kcal should come from a range of things. This method has the advantage of taking our caloric needs into mind too. A snack of asparagus (let’s say 100 g of it, much more reasonable than the 10 kg…) can more than cover your calcium needs for that caloric intake. On the other hand if we eat a snack of any amount of chickpeas with a calcium content of 49 mg/100 g which is 2x as much as asparagus, we will get less calcium (60%) than required for a snack of so many calories. This is because 100 g of chickpea contains 8x more calories than 100 g of asparagus.

To further prevent implication that huge amounts of single foodstuff have to be consumed, the same number (%RDA/2000 kcal) can be viewed from another perspective. By using recommended daily intake (RDA), we can calculate the recommended nutrient density (RND). This is the average of all the nutrient densities for each food consumed during a day, which, at the end of the day after consuming our 2000 kcal of various foods, would provide us with the recommended amount for a given nutrient. The recommended nutrient density for calcium is 0.5 mg / kcal = 50 mg / 100 kcal (=1000 mg / 2000 kcal). Foods that meet this threshold are considered to have 100% of the recommended nutrient density (% RND). The calcium content of asparagus is 120 mg per 100 kcal, which is 2.4 times higher than the recommended nutrient density for this substance, so it can be said that the calcium content of asparagus is 240% of the recommended nutrient density (240% RND). This is the same as saying 240% RDA / 2000 kcal, but is much nicer to read though the meaning is unchanged. The iron content of asparagus is 2200% RND. Asparagus contains that much of calcium and iron in any portion size: in 1 spoon, in 1 bowl, in 100 g or in 10 kg.

The RND system thus allows us to compare different nutrients and shows us that asparagus is a better source of iron than it is of calcium, and it is also unaffected by the quantity of food or by dilution. For daily use when we want to know what foods to choose to enrich our intake of a particular nutrient, this parameter is very practical. Basically, you want to choose foods whose RND average out to 100%. That means some can be higher and some lower, but at the end of the day that 100% mark or above is where you want to be.


What parameters are used by researchers and in regulations?

As you can imagine, the target audience of research articles and regulatory documents differ than that for things geared towards the average person. For these sorts of texts, a basic parameter like g of nutrient per 100 g of food is usually used. However, sometimes more complex parameters are used. European legislation on nutritional claims on food (click) sometimes uses nutrient density (nutrient content per 100 kcal). For example, the claim that a food has a high fibre content may be made if the product contains at least 3 g of fibre per 100 kcal (or 6 g of fibre per 100 g). To claim high protein content, protein must provide 20% of the energy in a food. This requirement is mathematically equivalent to the nutritional density of protein in the food being 5 g per 100 kcal. In some regulatory documents, recommended nutrient density is also used. Nordic Nutrition Recommendations (click) sets the recommended nutrient density at 1 MJ or 10 MJ. 10 MJ equals 2388 kcal, which is close to the above mentioned 2000 kcal that represents the average daily energy requirement for most people. The actual nutrient density of different foods is then expressed in a percentage of this recommended nutrient density in some literature.


The bottom line is that many methods exist to express the nutrient content of a food. Their usefulness depends on who will be using them. For the general public, a percentage of the recommended nutrient density (% RND) is the most appropriate. This parameter allows comparison between different nutrients and is independent of portion size, concentration, and cooking method. So next time you’re looking for nutritional information, this is the way to go!

The Hungry Pumpkin team

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