Nutrient Requirements of Women in Sport
Female and male athletes respond to training in a fairly comparable way. As volume and intensity of training increases, so does aerobic capacity and hence performance. Body composition tends to change, whether male or female, indicating that physiologically, we are all actually quite similar.
Nutritionally speaking, fuelling of training is similar too. Regardless of the sport in question, energy intake must match energy output in order to fuel training and recovery. For endurance athletes, carbohydrate intake needs to equate to approximately 7-10g per kg/bwt (or 4g per lb/bwt). If it doesn't performance tends to suffer, and fatigue creeps in.
It is important for any athlete, regardless of gender, to train and compete with optimum fuel reserves, and, of course be well hydrated.
Despite seemingly parallel training responses and "fuel" requirements between males and females, women engaged in regular exercise, and especially those with demanding training and competition schedules have quite unique nutritional needs.
These special needs often mirror a particular time in a female's sexual development, or during one of the many hormonal changes, which govern a women's life. Dramatic hormonal shifts initiate quite unique metabolic and chemical changes within the body that demand specific nutrients. Needs change as a female enters her pubertal years (onset of menarche), during her reproductive years and during pregnancy, and then at the stage that marks the end of reproduction (menopause). Disruption in a female's normal menstrual functioning (e.g. amenorrhoea) may create increased requirements in macro and micronutrients (e.g. calcium, magnesium, vitamin K, protein and essential fatty acids). The BNF's briefing paper, Nutrition and Sport, reports increased calcium requirements in amenorrhoeic women, and advises all female athletes to pay attention to energy, calcium and iron intakes (1). Vitamin K supplementation has been shown to improve markers of bone metabolism in a small group of amenorrhoeic female elite athletes (2). Vitamin K functions in the synthesis of calcium-binding proteins.
Iron and calcium requirements of the female athlete
The two main nutrients that require most attention are the minerals iron and calcium.
Levels of iron in the body are particularly important given iron's role in many enzyme functions and it's fundamental role in the formation of haemoglobin (75% of total body iron is in this form) and as a constituent of myoglobin (the O2 carrying material that functions inside the cells).
Iron performs the overwhelming activity of transporting oxygen from the lungs to the mitochondria within muscle cells - vital for the athlete.
Females have a higher rate of iron loss than men mainly via blood loss through menstruation, as well as during pregnancy and childbirth. This creates a higher iron requirement in women generally.
An athlete's iron status (measured by levels of blood haemoglobin, haematocrit concentration and plasma ferritin levels) may further be compromised due to a number of factors directly related to training. These have been identified as bleeding within the digestive system, inadequate diet and poor iron absorption, loss of iron through heavy sweating, red blood cell breakdown due to trauma created by certain high-impact activities (e.g. long-distance running), and even over-frequent blood donation.
Iron-deficiency anaemia (haemoglobin levels below 12g/dl) has a major impact on performance and immune status. It decreases aerobic capacity and endurance, induces fatigue, and lowers resistance to infection.
It has not yet been clearly established whether iron depletion (low ferritin concentrations and reduced bone marrow iron) negatively affects performance, but certainly low ferritin is not something to be ignored. Many however, suggest changes in plasma ferritin concentration are due to either heavy training, or as a response to inflammation, and low blood haemoglobin in some athletes is simply due to plasma volume expansion.
Assessment of iron status in athletes is clearly not straightforward. Taking into account measured indices of iron status, individual dietary habits, digestive function, menstruating patterns and other significant factors should help determine the impact iron status may be having on a particular individual's performance. It is fair to say that in some cases, borderline measurements or those at the lower end of "normal" are often clinically significant, and iron supplementation produces noticeable improvements in iron status and performance (3).
The use of iron supplements at this point may also prevent the development of full blown iron-deficiency anaemia in some female athletes, which is often when "re-pletion" is most difficult, especially via diet alone.
Inorganic forms of iron (e.g. ferrous sulphate, ferrous gluconate) are notoriously poorly absorbed, and often cause gastrointestinal problems such as constipation. More importantly, they often fail to raise Hb levels. Where iron supplementation is deemed appropriate (i.e. anaemia), serious consideration should be given to using new "food-form" iron supplements. Food-form iron is a version of iron that has been grown into yeast cells, and the absorbability of yeast-based iron is much closer to haem-iron. It also produces little or no uncomfortable side effects.
National surveys have consistently reported low calcium intake is young and adult females (4, 5, 6), as well as female athletes (2, 7).
This is normally due to low energy intakes, fad diets, or poorly planned vegetarian and vegan diets. Inadequate calcium intake and consequently poor calcium status is compounded by diets that contain high phosphorous, high salt and high caffeine food and drink. These have a negative impact of calcium balance, due to an increase in urinary calcium excretion (8).
Calcium and bone health
About 60% of adult bone is laid down during adolescence (9), when calcium deposition is at it's highest (10). This is due to increases in the hormones oestrogen, growth hormone and calcitriol. Mechanisms are put to work that lead to an overall stimulation of bone cell production and maturation. Bone resorption is out-weighed by bone deposition, leading to an increase in overall bone mineralisation. There seems to be a critical 4-year period during teenage years, from the ages of about 11-15 years, during which time most of the total gain in bone mineral density (BMD) and content (BMC) is accumulated (9).
Peak bone mass is a major determinant of osteoporosis in later life, so building the largest bone mass possible is one of the most important strategies to protect against osteoporosis in later life (11).
Females in the UK, aged 19-50 years, are thought to need at least 700mg calcium daily in order to meet the demands for calcium deposition in bone. Recommendations are lower than in most other industrialised countries and it has been suggested that 11-18 year olds require 1200-1500 mg/day to optimise peak bone mass (12).
Numerous well-controlled longitudinal studies have produced consistent positive effects of calcium supplementation on BMD in adolescent females (13, 14, 15), which suggests that our UK reference values are sub-optimal.
Female athletes are a different sub-class all together with regard to calcium needs. Up to 400mg of calcium has been shown to be lost (in males) via sweat alone, from a 2-hr training session (17), and although Ca losses in females are unlikely to be that high, any female athlete such as marathoners or triathletes training twice a day? could be at risk of not getting enough calcium in the diet to achieve a positive Ca balance.
Dr Michael Colgan, renowned New Zealand research scientist believes athletes (both male and female, and especially females with amenorrhoea) need to supplement between 1000-2000mg Ca daily.
Supplementation needs should always be assessed in relation to what is actually being obtained from the diet. Dietary intake should therefore always be assessed, along with identifying factors that could potentially increase calcium excretion - e.g. high sodium and phosphorous diets, high protein diets, and an overall high "acidic" load. Knowledge should also be sought as to the types of calcium available and their rates of absorption.
The female athlete triad
A major focus in recent years within nutrition and sport for women has been with respect to the "female athlete triad". Components of the triad are disordered eating, amenorrhoea (absence of periods), and osteopenia (as opposed to osteoporosis).
A review paper on BMD data in athletes found osteopenia (as defined as BMD scores between 1 and 2.5 SD below the mean of young adults) to be significantly prevalent in those at risk of the female athlete triad. Interestingly, osteoporosis (BMD above 2.5 SD below the mean) was relatively uncommon, even in this selected "athletic" population (16). This by no means relegates the problem as any less significant. A diagnosed case of osteopenia in a young female athlete may actually be a worse scenario in terms of long-term bone health, when compared to a diagnosed osteoporotic in her 60's. An athlete with osteopenia is at greater risk of developing osteoporosis than is an athlete who has normal bone mass.
There is indeed much concern amongst sports dieticians and nutritionists, who are commonly faced with various subclinical eating disorders, or "disordered eating" (a significant risk factor for female athlete triad).
Disordered eating disrupts menstrual function, and together with intense training schedules, often leads to amenorrhoea, or cessation of periods. A lack of oestrogenic stimulation of bone cells leads to decreased calcium uptake, and over time, loss of bone mass.
Cases such as these do tend to be sport-specific, being confined to sports that either require a low body mass (martial arts, rowing), where a low body weight is thought to improve performance (long-distance running, triathlon) and in those sports that requests athletes to be aesthetically pleasing to the eye (ballet, figure skating, diving).
Of course, any female, athlete or non-athlete, under stress, or with low self-esteem, a tendency toward perfectionism, or family problems is at risk for "disordered" eating, and a down-regulation of sex hormone production, in favour of stress-hormone production.
Decreasing training intensity and optimising energy and nutrient intake must be the key strategies to dealing with any component of the female athlete triad.
Although calcium intake in the diet cannot make up for a lack of oestrogen due to menstrual irregularities, it should be optimised in the diet and by supplementation if necessary, especially if a contributory cause of osteopenia is lack of dietary calcium.
Practical suggestions to increase intake of calcium and iron
· Eat low-fat dairy foods such as skimmed milk and natural yogurt daily
· Add 100g of tofu and sunflower seeds to stir-frys and salads
· Add almonds, dried figs and seeds to breakfast cereals
· Add blanched spinach to scrambled or poached eggs
· Use Tahini (sesame seed spread) on bread and crackers or add a tsp to natural yogurt
· Eat plenty of dark green leaves and leafy vegetables such as kale, broccoli, watercress and spinach- always steam or lightly cook brocolli, kale, cabbage and spinach
· Try soft-bony fish (tinned salmon, sardines, pilchards) as a topping on baked potatoes or wholegrain toast
· Eat vitamin-C rich foods to enhance the absorption of iron (i.e. plenty of fresh fruit and colourful vegetables)
· Be aware of substances that interfere with iron absorption (e.g. phytates found in bran, and tannin in tea).
Try NOT to drink tea and coffee with food
1) Briefing Paper (2001) Nutrition and Sport. British Nutrition Foundation.
2) Craciun AM, Wolf J, Knapen MHJ, Brouns F, Vermeer C (1998) Improved bone metabolism in female elite athletes after vitamin K supplementation. International Journal of Sports Medicine 19, 479-484.
3) Matter M, Stiffal T, Graves J et al. (1987) The effect of iron and folate therapy on maximal exercise performance in female marathon runners with iron and folate deficiency. Clinical Science 72, 415-422.
4) Department of Health (1991) Dietary Reference Values for Food, Energy and Nutrients. Report on Health and Social Subjects 41. London: HMSO
5) MAFF, Ministry of Agriculture, Fisheries and Food (1994) The Diet and Nutritional Survey of British Adults-further analysis. London: HMSO
6) HEA, Health Education Authority (1995) Diet and Health in School-age Children. London: HEA
7) Van Erp-Baart AMJ, Saris WHM, Binkhorst RA, Vos JA, Elvers JWH (1989) Nationwide survey on nutritional habits in elite athletes Part 2. Mineral and vitamin intake. International Journal of Sports Medicine 10, 11-16.
8) Matkovic V, Ilich JZ, Andon MB et al. (1995) Urinary calcium, sodium and bone mass of young females. American Journal of Clinical Nutrition 62, 417-425.
9) Bonjour J, Theintz G, Bertrand B, Slosman D, Rizzoli R (1991). Critical years and stages of puberty for spinal and femoral bone mass accumulation during adolescence. Journal of Clinical Endocrinology and Metabolism 73, 555-563.
10) Weaver CM, Martin BR, Plawecki KL, Peacock M, Wood OB, Smith DL, Wastney ME (1995) Differences in calcium metabolism between adolescent and adult females. American Journal of Clinical Nutrition 61, 577-581
11) Christiansen C (1991) Consensus Development Conference on Osteoporosis. American Journal of Medicine 5B, 1S-68S.
12) National Institutes of Health Consensus Development Panel on Optimal Calcium intake (1994) Optimal Calcium intake. JAMA 272, 1942-1948.
13) Johnston CC, Miller JZ, Slemenda CW, Reister TK, Hui S, Christian JC, Peacock M (1992) Calcium supplementation and increases in bone mineral density in children. New England Journal of Medicine 327, 82-87.
14) Matkovic V, Fontana D, Tominac C, Goel P, Chestnut CH. Factors which influence peak bone mass formation: a study of calcium balance and the inheritance of bone mass in adolescent females (1990) American Journal of Clinical Nutrition 52, 878-888.
15) Lee WTK, Leung SSF, Wang S, Xu Y, Zeng W, Lau J, Oppenheimer SJ et al. (1994) Double-blind, controlled supplementation and bone mineral accretion in children accustomed to a low-calcium diet. American Journal of Clinical Nutrition 60, 744-750.
16) Khan KM, Lui-Ambrose T, Sran MM, et al. (2002) New Criteria for female athlete triad syndrome? British Journal of Sports Medicine 36,10-13.
17) Kiesges, RC, et al. (1996) Changes in bone mineral content in male athletes. J Amer Med Assoc 276:226-230,
Lucy-Ann Prideaux has an MSc degree in Human Nutrition and Metabolism, and a BSc (Hons) degree in Sports Science. She is a registered Nutritionist with The Nutrition Society.
Aside from her own private practise and consultancy work, she is the resident Nutritionist at the Sussex Centre for Sport and Exercise Medicine with Dr Nick Webborn.