Micronutrients
Micronutrients
Vitamins, minerals, and trace minerals are known as micronutrients. They are essential for many bodily processes which are important for health and performance.
Micronutrients aid growth and development and are also essential for certain metabolic reactions. Although some vitamins and minerals are involved in energy pathways, they are not direct providers of energy. Most micronutrients must be obtained through the diet because the body cannot produce them in large amounts, except for vitamin D which can be obtained from sunlight and dietary sources.
Athletes have higher energy intake demands in comparison to the general population, likely increasing their dietary micronutrient intake. However, in comparison to males, female athletes may be at an increased risk of micronutrient deficiencies due to potentially having a lower absolute energy intake, which could result in lower micronutrient intake.
Female athletes also have an increased risk of low energy availability which can negatively affect micronutrient regulation and absorption.
In general, if an athlete is consuming a well-balanced and varied diet, which meets the demands of their training load, it is likely that they are consuming enough vitamins and minerals through dietary sources. Exceptions may be if athletes are following a restricted or low-calorie diet, or a vegan/vegetarian diet. In some circumstances, dietary supplementation of vitamins and/or minerals may be required. However, athletes should always base their nutrition around a ‘food first’ approach, with micronutrient supplementation used only when deemed necessary by a sports dietitian (or other qualified professional) to support health and/or performance. The following information will focus on four key micronutrients: iron, calcium, vitamin D, and folate. For an overview of the roles of common vitamins and minerals, see below.
The main functions of micronutrients and common dietary sources
Vitamin A
Helps with vision and fighting infection
Dietary sources: Seafood, orange and green vegetables (i.e., carrot and spinach)
Vitamin C
Important for bones, skin, muscle growth and immune function
Dietary sources: Oranges, orange juice, peppers, kiwis, strawberries
Vitamin D
Maintains bone strength
and is important for the nervous system
Dietary sources: Fish (i.e., salmon and mackerel), egg yolk. Also obtained
through sunlight
Vitamin E
An antioxidant
important in protecting against cell damage
Dietary sources: Sunflower seeds, nuts, vegetable oil, broccoli
Calcium
For healthy bones and teeth, and muscle contraction
Dietary sources: Milk, yogurt, tofu, cheese
Phosphorus
Important for maintaining bone and teeth health
Dietary sources: Meat, fish, dairy products
Magnesium
Helps to form protein and is important for immune function
Dietary sources: Nuts, seeds, green vegetables
Sodium
Essential for fluid balance, muscle contraction and the nervous system
Dietary sources: Table salt
Chloride
Helps with fluid balance
Dietary sources: Table salt
Potassium
Maintains fluid balance, muscle contraction and the nervous system
Dietary sources: Meat, milk, fresh fruit
Zinc
Important for optimal immune function
Dietary sources: Lean meat, shellfish, legumes,
nuts and seeds
Iron
Helps to store
and utilise oxygen
Dietary sources: Red meat, nuts, beans
Iron
Iron is a mineral that is involved in various bodily functions, including the transport and delivery of oxygen in the blood. Iron plays an important role in other processes in the body, including:
Energy
production
Cognitive
function
Immune
function
Growth and
development
Guidance
Below you can see the Recommended Dietary Allowance (RDA) for iron, for non-vegetarians. The RDAs for vegetarians/vegans are 1.8 times higher due to the lower bioavailability of iron from plant-based foods. It should be noted that these guidelines are for the general population, as currently no athlete-specific guidelines exist. However, it is estimated that athletes may require an additional 1-2 mg of iron per day to replenish exercise-related iron losses.
Children
9-13 years
8mg
Teenage girls
14-18 years
15mg
Adult women
19-50 years
18mg
Pregnant
teenagers
27mg
Pregnant
women
27mg
Breastfeeding teenagers
10mg
Breastfeeding
women
9mg
RDA for iron, for females of different life stages (NIH, 2023)
Optimizing dietary iron intake
To prevent an iron deficiency, athletes should use a ‘food first’ approach to ensure that they are consuming sufficient amounts of iron in their diet. A sports dietitian can carry out a dietary assessment to advise on this. Iron absoprtion can be improved by combining iron intake (via iron-rich foods, or supplements) with other foods. Foods containing vitamin C can enhance the absorption of iron. Conversely, caffeine and calcium containing foods (or beverages) can inhibit iron absorption, and should not be consumed in close proximity to iron-rich foods/supplements (if the athlete’s aim is to optimize iron absorption).
Dietary sources of iron, or iron supplements, can be consumed strategically around exercise. To maximize absorption, iron-rich foods/supplements should be consumed within 30 minutes prior to exercise, or 30 minutes after the completion of exercise. In addition, it has been shown that iron absorption is greater in the morning in comparison to the afternoon.
Foods containing vitamin C
- E.g., oranges, orange juice,
peppers, kiwis, strawberries
Consuming iron-rich
foods/supplements:
- In the morning
- Within 30 minutes prior to exercise
- Within 30 minutes after exercise
- (if consumed in close proximity to iron-rich foods/supplements)
Foods containing calcium
- E.g., milk, yogurt,
cheese, tofu
Caffeine
Iron deficiency
Identification of an iron deficiency
Athletes should be aware of the signs and symptoms of an iron deficiency (see below). If an athlete is experiencing some of these signs and symptoms, they should book an appointment with a Medical Doctor to have their iron status assessed.
Heart palpitations
Tiredness, lethargy, fatigue
Poor
recovery
Reduction in performance
Paleness
Shortness of breath
Lack of energy
Iron deficiencies in athletes
If an athlete has an iron deficiency, this can lead to detrimental impacts to health and/or performance. Around 35% of female athletes have an iron deficiency, versus around 5% of the general population. In addition, female athletes are at a greater risk of being iron deficient in comparison to male athletes, with research showing that an iron deficiency is ~3 times more common in female athletes. There are several factors which can increase the likelihood of an iron deficiency (see below).
Insufficient
energy intake
- A lower energy intake can increase the likelihood of dietary iron intake being insufficient.
Menstruation
- Regular or heavy menstrual bleeding increases blood losses. Females may also have a lower overall ability to absorb iron in comparison to males, due to the fluctuation in estrogen throughout the menstrual cycle.
Plant-based
diets
- Iron from plant-based foods (i.e., non-heme iron) is not absorbed as well by the body in comparison to iron from animal-based foods
(i.e., heme iron).
Exercise-induced
iron loss
- The mechanisms which contribute to this are sweating, gastrointestinal bleeding, haematuria (blood in urine), and footstrike hemolysis.
Categorization of an iron deficiency
Various hematological variables can be used to assess an athlete’s iron status and classify the stage of an iron deficiency. However, it is suggested that the following three biomarkers should be included as a minimum for the clinical assessment of iron deficiency: serum ferritin (sFer), hemoglobin concentration (Hb), and transferrin saturation (TSAT).
Click below to see the female-specific cut-off values for the three stages of an iron deficiency.
1
Iron depletion (ID)
Iron reserves
are depleted
Serum ferritin (sFer)
<35 μg/L
Hemoglobin
concentration (Hb)
>120 g/L
Transferrin
saturation (TSAT)
>16%
Iron deficiency
non-anemia (IDNA)
2
Red blood cell production and iron supply is diminished
Serum ferritin (sFer)
<20 μg/L
Hemoglobin
concentration (Hb)
>120 g/L
Transferrin
saturation (TSAT)
<16%
3
Iron deficient
anemia (IDA)
Hb levels drop
and anemia manifests
Serum ferritin (sFer)
<12 μg/L
Hemoglobin
concentration (Hb)
<120 g/L
Transferrin
saturation (TSAT)
<16%
Current research suggests that athlete performance is only impaired when the athlete reaches stage 3 (IDA), when oxygen transport is substantially diminished. However, the combined negative effects of stage 2 (IDNA), such as impaired function of oxidation enzymes, respiratory proteins, immune function, and perceptions of fatigue, may also decrease performance.
Screening for
iron deficiency
Peeling et al. (2023)
Annually
No history of iron deficiency
No history of irregular/excessive menses (or amenorrhea)
No reports of fatigue after extended rest
Strength/power-based sports with minimal endurance component
No iron related dietary restrictions (e.g., non-vegetarian, non-vegan)
No evidence of low energy availability
No intention to undertake hypoxic (altitude) training in the next 12 months
No underlying pathology (e.g., coeliac or Crohn’s disease)
Factors to consider
Every 6 months
Factors to consider
Female
History of iron deficiency (Stage 1) in the last 2 years
History (>24 months) of irregular/excessive menses
Intend to undertake high training loads in endurance or team sports
No reports of prolonged fatigue after extended rest
No iron related dietary restrictions (e.g., non-vegetarian, non-vegan)
No evidence of low energy availability
Intention to undertake hypoxic (altitude) training in the next 12 months
Every 3 months
Factors to consider
Recent history of iron deficiency (stage 1, 2 or 3) in the last two years
Evidence of irregular/excessive menses or amenorrhea
High training loads in endurance or team sports
Prolonged fatigue even after extended rest
Reduced work capacity during training; unexplained poor performance
Restricted diet (e.g., vegetarian, vegan)
Evidence of low energy intake and availability
Intention to undertake hypoxic (altitude) training in the next 6 months
To improve the accuracy of assessing an athlete’s iron status, blood collection should occur: (1) in the morning, after a day of rest before training resumes
(2) with the athlete in a healthy and hydrated state after an overnight fast
Treatment of iron depletion/deficiency
1
Increasing dietary iron intake
This should be the first approach taken to improve iron stores. Dietary assessments can be used to explore an athlete’s overall energy intake, consumption of key micronutrients, and the timing of nutrient consumption. Assessing these factors may highlight areas of concern in relation to iron intake and absorption. For example, athletes with high training volumes or who participate in weight-sensitive sports tend to be at higher risk of low energy availability, which can result in reduced dietary iron intake. Additionally, heme iron (found in animal-based foods) has greater efficiency of absorption compared to non-heme iron (found in plant-based foods), causing subsequent reduced iron absorption in vegetarian/vegan athletes.
2
Oral iron supplements
After dietary analyses, oral iron supplementation can be considered to support increases in dietary consumption. There are many forms of oral iron supplements ranging in dose, formulation, and chemical state, however ferrous sulfate tablets are the most common. Typically, oral iron supplements are consumed daily, with lower dosages used for athletes with low gastrointestinal tolerance, and higher dosages used for those with stage 3 IDA. A daily dose of ~100 mg in combination with a vitamin C source should increase sFer levels by 30-80% over a 6-8 week period.
3
Intravenous (IV) iron infusions
The process of oral iron supplementation is relatively slow and, if the athlete has extremely low sFer stores, may have a relatively small impact. Therefore, in severe cases of IDA and when the athlete is unresponsive to oral iron supplementation, IV iron infusions may be considered. This is a rapid and effective method which bypasses the gut and delivers iron directly to the blood circulation. Research suggests IV iron treatment is only effective in increasing Hb in individuals with severe and persistent IDA. Note that there are complexities surrounding the World Anti-Doping Agency (WADA) rules that must be considered.
Calcium
Calcium is involved in many important processes in the body, for example muscle contraction and nerve function. Of relevance to female athletes, calcium is very important for bone health. During prolonged intense exercise, small amounts of calcium are lost through sweat. If this is combined with insufficient calcium intake through an athlete’s diet, circulating calcium levels can be lowered.
This can cause the breakdown of bone in the body, which is harmful to bone health and may ultimately contribute to stress fractures and osteoporosis. Therefore, it is important for athletes to include calcium-rich foods in their diet on a daily basis. It should be noted that there is emerging evidence that pre-exercise calcium intake can potentially offset loss of calcium in sweat, which may have a positive impact on bone health.
As estrogen promotes calcium absorption and retention in the bone, female athletes with low levels of estrogen (e.g., amenorrheic and post-menopausal athletes) may require additional calcium intake to maintain bone health. Research has found that amenorrheic athletes are up to 4x more likely to sustain a stress fracture compared to regularly menstruating athletes.
Guidance
Below you can see the RDAs for calcium. It should be noted that these guidelines are for the general population, as currently no athlete-specific recommendations exist. Athletes should not consume more than ~500-600 mg of calcium at one time in order to maximize absorption.
If an athlete does not currently meet the RDA, then they should be encouraged to incorporate more calcium-rich foods in their diet. Some easy ways to increase dietary calcium include drinking a glass of milk alongside breakfast, having some yogurt after meals, or including extra portions of tofu or green leafy vegetables in the diet (see below). Athletes should ensure that they have sufficient vitamin D levels (through dietary sources and sunlight exposure) because vitamin D aids the absorption of calcium from foods. Athletes should only supplement their diet with calcium after a review of their current dietary intake by a sports dietitian.
Children
9-13 years
1300mg
Teenage girls
14-18 years
1300mg
Adult women
19+ years
1000mg
Pregnant
teenagers
1300mg
Pregnant
women
1000mg
Breastfeeding teenagers
1300mg
Breastfeeding
women
1000mg
Recommended Dietary Allowances (RDAs) for calcium, for females of different life stages (NIH, 2024)
Dietary sources
Whole or skimmed cow’s milk
Serving size:
10.5 fl oz / 300 ml
Calcium: ~360 mg
Calcium enriched soy milk
Serving size:
10.5 fl oz / 300 ml
Calcium: ~360 mg
Kale (boiled)
Serving size: 3.5 oz / 100 g
Calcium: ~150 mg
Orange
Serving size: 1 medium size
Calcium: ~40 mg
Hard cheese
Serving size: 1 oz / 30 g
Calcium: ~220 mg
Tofu
Serving size:
3.5 oz / 100 g
Calcium: ~350-400 mg
Rice pudding
Serving size: 7 oz / 200 g
Calcium: ~200 mg
Dried figs
Serving size: 2.1 oz / 60 g
Calcium: ~150 mg
Greek yogurt
Serving size: 5.3 oz / 150 g
Calcium: ~150 mg
White bread
Serving size: 2 slices
Calcium: ~100 mg
Wholegrain bread
Serving size: 2 slices
Calcium: ~100 mg
Sardines in
oil (canned)
Serving size: 2.1 oz / 60 g
Calcium: ~240 mg
Vitamin D
Vitamin D is important for maintaining many aspects of health which are important for an athlete’s performance, including:
Bone
health
Cardiac structure
and function
Muscle
function
Immune
health
Estrogen
production
Vitamin D deficiency
Vitamin D deficiencies are common in both athletes and the general population, and supplemental intake of vitamin D may be warranted after consulting with a qualified professional. The main way that vitamin D is obtained is through sunlight exposure (~80-90%), with only ~10-20% obtained through the diet. It should be noted that multiple factors can impact the ability of the body to synthesize vitamin D.
It is estimated that 33-42% of female athletes present with vitamin D insufficiency. Specific to females, vitamin D plays a key role in estrogen production. Therefore, the effects of vitamin D deficiency may also influence menstrual status and fertility, as well as bone health.
UV intensity
- Latitudes 37° north or south of the equator will not be strong enough to stimulate the synthesis of vitamin D during the cooler months.
Clothing
- Covering a large portion of the skin with clothing limits exposure to UV rays.
Environment
- Individuals with darker skin tones need longer exposure to UV from the sun to synthesize similar amounts of vitamin D.
Skin colour
Guidance
- Athletes who train indoors for long periods of time, or live in hot countries and avoid going outside in summer months, may have limited vitamin D exposure.
Summer recommendations
Sunlight exposure
Time: 10am-3pm
Duration: ~15 min
Frequency: 6 days per week
Attire: T-shirt and shorts
Do not burn, apply sunscreen after ~15 min exposure
Winter recommendations
Supplementation
Supplement 1000-2000 IU of vitamin D3 daily, as regular daily doses appear to be more effective than monthly megadoses
Difficult to obtain this amount of vitamin D3 through foods
Toxicity is rare, but may occur when ingested doses exceed >10,000 IU/day
Folate
Folate is a B vitamin which plays an important role in the production of new red blood cells. A folate deficiency may result in iron deficiency anemia, and performance may decline as a consequence.
In females, a folate deficiency is especially common during pregnancy, due to the high folate demands associated with fetal growth and development. If a female athlete continues to train throughout their pregnancy, the high exercise demands may augment the likelihood of a folate deficiency. Oral contraceptive use is also associated with reduced plasma folate and red blood cell folate concentrations, therefore athletes using oral contraception may require an increase in folate consumption. Individual screening for folate status is recommended, especially throughout pregnancy.
Guidance
RDAs for folate can be seen below. It should be noted that these values have been produced for the general population, as folate requirements for athletes have not yet been established. The RDAs are listed as mcg of dietary folate equivalents (DFEs), which reflect the higher bioavailability of folic acid (~85%) compared to that of folate from foods (~50%). As such, it should be noted that:
• 1 mcg DFE = 1 mcg folate from food
• 1 mcg DFE = 0.6 mcg folic acid from fortified foods or dietary supplements, consumed alongside foods.
Children
9-13 years
300
mcg DFE
Teenage girls
14-18 years
400
mcg DFE
Adult women
19+ years
400
mcg DFE
Pregnant
teenagers
600
mcg DFE
Pregnant
women
600
mcg DFE
Breastfeeding teenagers
500
mcg DFE
Breastfeeding
women
500
mcg DFE
DFE = Dietary Folate Equivalent, mcg = microgram
Recommended Dietary Allowances (RDAs) for folate (mcg DFE)
for females of different life stages (NIH, 2023)
Dietary sources
In many countries, common ingredients such as wheat flour and breakfast cereals are fortified with synthetic folate (folic acid). Foods naturally high in folate can be seen below:
Legumes
(e.g., chickpeas)
Dark leafy greens
(e.g., broccoli,
bok choy, kale)
Oranges
Eggs
