We often talk about the nutrient content of foods and the importance of those nutrients, but something that we discuss less is how much of what we eat actually makes it from the food and into our bodies?
There are many different factors that influence how much of a nutrient we absorb from the foods we eat. Often, we’re led to believe that the higher the nutrient content of a certain food, the better it must be for us. However, just because a food is high in a particular vitamin, does not automatically mean that we’ll take in a higher amount of that nutrient. Nor does it mean that our bodies will utilise all of the nutrient content of that ingredient. In this post, we’ll take a closer look at some of the key factors at play when it comes to the amount of nutrients we actually absorb from the foods we eat as well as highlighting a few key nutrients that can be most affected.
Bioavailability of Nutrients
Generally, the bioavailability of macronutrients (carbohydrates, fats, protein) is around 90% whereas for micronutrients (vitamins and minerals) it can vary greatly (EUFIC). Particularly with some plant-based sources, certain vitamins and minerals can be less “bioavailable” to us or interactions between the chemicals in our bodies mean we absorb the nutrients less efficiently than we may do from animal sources. If you’re following a plant-based diet, this just means you need might to include more sources of those nutrients to ensure you’re getting enough.
When we consume foods, the nutrients are released from the chemical composition of the food and broken down by our digestive system to be used for specific functions within the body. If there is excess of a nutrient, it will either be excreted or stored, depending on its role. Interestingly, in the case of certain minerals including Calcium, Zinc and Iron, the less of that nutrient we have in our bodies, the more efficient our bodies become at absorbing it (Fairweather-Tait & Hurrel, 1996, Collings et al. 2013, Foster & Samman, 2015). Once we have an adequate store of a mineral, we’ll absorb it less efficiently as our needs are lower. As is true for many areas of nutrition, there is no one size fits all and therefore recommendations and considerations will differ for everyone.
Some of the main factors that will affect nutrient bioavailability are:
- Body composition
- Health status
- Life stage (e.g. pregnancy / children)
- Our own nutrient store of a particular nutrient
- Cooking & processing
- The chemical form a nutrient is delivered in
- Inhibitors – factors that negatively impact the bioavailability of vitamins and minerals
- Enhancers – factors that increase absorption
Overall, considering all of the different factors that affect our absorption of nutrients, for healthy individuals, eating a balanced and varied diet with plenty of fruits, vegetables, proteins and whole-grains is the most simple and effective way to ensure you’re taking in enough vitamins and nutrients.
Key nutrients to consider
There are two forms of dietary iron; haem and non-haem. Haem iron is found primarily in animal sources and is much more easily absorbed in humans, while non-haem iron is found in plant-sources. While the absorption of haem iron depends only on our iron status (how much iron we have stored in our body), the absorption of non-haem iron is influenced by other factors, including other components of the diet. For example, inhibitors of non-haem iron bioavailability include phytates from certain vegetables and cereals, polyphenols found in tea and coffee and also high levels of dietary calcium. Haem iron from animal sources has also been shown to increase the absorption of non-haem iron, so if you don’t include meat in your diet, it’s important to be aware of how to increase your iron absorption.
While non-haem iron may be less easily absorbed, many studies have shown that this can be increased by consuming vitamin C at the same time as plant-based iron sources. Research suggests that ascorbic acid (vitamin C) may balance out the negative effects from all of the inhibitors mentioned (Gibson, 2007 pp S82), hence why it’s so important to include a source of vitamin C with plant-based meals to ensure you’re absorbing iron from the foods you’re eating. Drinking tea or coffee in-between mealtimes rather than at the same time can also limit the inhibiting effect certain compounds may have on the absorption of iron from plant-based sources. (Zipj et al 2010)
Calcium is another important mineral where absorption can be affected by various factors. Dairy, eggs and fish are calcium rich foods and calcium is more readily absorbed from milk and dairy products so if you’re following a plant-based diet it is important to include plenty of alternative sources of calcium in your diet. Once again, naturally occurring compounds including phytates found in plant foods as well as oxalates, found in spinach, rhubarb and beetroot, can inhibit the absorption of calcium as they bind to the calcium and prevent it from being absorbed into the blood. Examples of plant-based foods where calcium absorption is better include broccoli, kale, Brussel sprouts, calcium-set tofu and calcium-fortified milks. Vitamin D promotes the absorption of calcium so ensuring you are taking a Vitamin D supplement, particularly between October – April in the UK, can be a useful way to increase your calcium intake.
Table 1 below is a useful resource showing the difference in Calcium absorption from a variety of sources, in comparison to milk. Clearly, milk is a really useful source of bioavailable source of calcium so it’s really important to be aware of your calcium sources if you don’t include dairy in your diet.
Table 1 – Absorption of Calcium from foods
Adapted from Weaver et al, 1999 (https://academic.oup.com/ajcn/article/70/3/543s/4714998)
Note on fortified plant-based milks: Research is limited to compare the absorption rates of calcium from plant-based milks. The absorption rate will depend on the form as well as the amount of the fortified nutrient and so will differ between brands.
Zinc is a nutrient that is highly bioavailable from animal sources and therefore the bioavailability of Zinc from vegetarian diets may be less than that of non-vegetarian diets. Plant foods such as nuts, seeds, legumes and cereals which often make up the bulk of vegetarian diets are high in phytates, which reduce the absorption of Zinc, while animal protein may enhance Zinc absorption. Unlike Iron, dietary Zinc intake is the principle determinant of a person’s Zinc status, rather than how much of a store your body may have (Hambidge et al., 2010), meaning we should ideally be consistently consuming enough Zinc from our diet to avoid becoming deficient. Consuming fermented foods such as miso or tempeh, fortified breads and cereals or soaking dried beans before cooking them can be ways to lower the phytate content and to include great sources of Zinc in your diet. As most of us don’t have the time to soak dried beans for hours on end, try using miso paste as a base for noodle soups and make sure you’re choosing fortified breads and cereals where possible – kimchi, sauerkraut and sourdough bread are all great fermented options that can increase Zinc intake as well!
Cooking & Processing
It’s sometimes suggested that cooking foods lowers the nutrient content and while this can sometimes be the case, the opposite is often also true. For example, research has shown that absorption of beta-carotene from carrots and green leafy vegetables, including spinach is improved through cooking (Van Het Hof et al, 2000). Similarly, lycopene, a beneficial nutrient found in tomatoes is more bioavailable from cooked or processed tomatoes (canned, pureed) than fresh, uncooked tomatoes (Richell et al, 2002).
As mentioned earlier, in the case of plant sources of Zinc and Calcium, process such as milling, soaking, germination and fermentation can reduce the phytate content of those foods, therefore reducing the inhibiting effect they may have on mineral absorption.
What this shows is that while fresh fruits and vegetables are incredibly rich sources of many vitamins and minerals, including all kinds of different forms and varieties of them are the best way to increase our intake of important nutrients. And, if fresh isn’t always accessible or convenient to you, canned and frozen varieties can be just as, if not more, beneficial for your health and wellbeing!
Supplements & Fortification
A popular question is whether we should be supplementing our diets to ensure we’re getting enough of certain nutrients. From a bioavailability standpoint, with so many different supplement brands on the market, it’s impossible to know the form of the nutrient being used or the processes it may have undergone before making it to the shelf. This means it’s difficult to know whether the nutrient will actually be well absorbed by our bodies. Regulation of supplements is limited and consumers can be easily misled by confusing messaging on packaging. It’s important to note that most of us can get everything we need from a healthy, varied and balanced diet and so there shouldn’t be any need to supplement.
There are a few exceptions, although these are not generally related to bioavailability:
- Vitamin D is recommended in the UK between the months of October – April due to a lack of sunshine
- B12 is recommended for vegans
- Folic Acid is recommended for pregnant women or those considering becoming pregnant
Talk to your GP or a nutrition professional if you feel you may require supplements for other nutrients.
Fortification involves adding nutrients to foods that may have been lost due to processing (e.g. refined flour), or to substitute where nutrients may be missing (e.g. plant-based milks) or simply just to increase the nutrient content of the foods. Fortified foods can be a really important way to ensure we’re including vitamins and minerals in our diet, particularly when cutting out any food groups on vegan or vegetarian diets. When choosing dairy alternatives always ensure you’re choosing fortified options (Calcium, B Vitamins and Vitamin D).
Folic acid is another nutrient that is often discussed in relation to fortification, and this will be potentially made law in the UK. Refining grains may remove some of compounds that could inhibit absorption of other vitamins and minerals, but is also means losing out on some of the nutrients originally in the grains. This is when fortification of refined grains can be really helpful and some research suggests that folic acid from fortified foods has a higher bioavailability than natural folates in foods.
Overall, more research still needs to be done on the bioavailability of nutrients from fortified foods and they shouldn’t be relied on for your sole intake of vitamins and minerals. However, in the context of a varied, balanced diet, fortified foods are a great way to increase your intake and reduce your risk of deficiency.
Is there any difference for children?
In most cases, bioavailability and absorption of nutrients will be the same for children. For certain nutrients, children have a higher requirement as their bodies are growing and developing at a much faster rate. The BNF has some helpful information about the changing requirements for children. Overall, if you’re aiming to fit in plenty of variety in your child’s meals, especially if they’re following a plant-based diet, you should be able to get all the vitamins and minerals you need from food.
Take Home Points
- If you’re following a mostly plant-based diet, Iron, Calcium and Zinc may be less bioavailable due to the compounds found in many plant foods – ensure you’re choosing fortified options and increase your intake of these nutrients where possible
- To increase absorption of iron from a plant-based diet, always include a source of Vitamin C with meals
- Vitamin D supplements are important for absorption of calcium
- In most cases, cooking and processing does not have a significant negative effect on the bioavailability of nutrients. In some cases, it can have an enhancing effect; green leafy vegetables, carrots and tomatoes
- Food first; a balanced, varied diet is the best way to make sure you’re getting in enough nutrients
Written for SR Nutrition by Kat Thomas, Nutrition and Dietetics Student at London Metropolitan University with support of Charlotte Stirling-Reed