'Just eat a balanced diet' is the most common nutritional advice given. It is also increasingly insufficient, not because whole foods aren't important (they are), but because the whole foods available today are measurably less nutritious than they were fifty years ago, and because active women have nutritional demands that exceed what blanket guidelines assume.
The soil problem
A landmark 2004 study comparing USDA nutritional data from 1950 to 1999 found reliable declines in the nutrient content of 43 garden crops. Key nutrients declined across the board, with some falling by as much as 38%. (Davis DR, et al., Journal of the American College of Nutrition, 2004)
The cause is well-documented: modern intensive farming prioritises yield and cosmetic quality over nutritional density. Crops are bred to grow faster and larger on depleted soil. A carrot today is not nutritionally equivalent to a carrot in 1970. This is not scaremongering, it is established in peer-reviewed agricultural science.
Women who train have higher demands
Standard recommended daily intakes (RDIs) are calculated for the average sedentary adult. They are not calibrated for women who train regularly.
Active women have elevated requirements for: iron (lost through menstruation and exercise-induced haemolysis); magnesium (depleted by both exercise and progesterone in the luteal phase); vitamin D (involved in muscle function, immune regulation, and mood, chronically low in northern climates and in women who train indoors); omega-3 fatty acids (anti-inflammatory, relevant for recovery and joint health); and zinc (lost in sweat, involved in testosterone metabolism and immune function).
Meeting these demands through food alone requires a dietary precision that most people, even those eating 'well', do not achieve consistently.
The case for targeted supplementation
This is not a case for replacing food with supplements. Whole foods contain thousands of bioactive compounds that supplements cannot replicate, fibre, phytonutrients, cofactors that affect how nutrients are absorbed and used. The goal is supplementation in addition to, not instead of, a nutrient-dense diet.
What the evidence actually supports for active women:
Vitamin D3 + K2: Vitamin D deficiency is widespread and significantly affects muscle function, immune health, mood, and bone density. K2 directs calcium to bones rather than arterial walls. 2,000–4,000 IU D3 with K2 is a well-tolerated baseline; test your levels and adjust accordingly.
Magnesium glycinate: Most people do not meet magnesium requirements through diet, and training increases depletion. Glycinate is better absorbed and better tolerated than oxide or sulphate forms. 200–400mg at night.
Omega-3 (EPA/DHA): 1–3g combined EPA/DHA daily from a quality fish oil or algae-based supplement. Anti-inflammatory, supports brain function, and reduces menstrual pain severity.
Iron: Only supplement if you have confirmed low ferritin, excess iron is harmful. Test first; supplement if needed; retest.
On quality
Supplement quality varies significantly. Third-party tested products (NSF, Informed Sport, USP certification) are worth seeking out, particularly for athletes. Cheap supplements often contain inferior forms of nutrients with poor bioavailability, you may be buying a product that passes through you without meaningful benefit.
Form matters: magnesium oxide (cheap) absorbs at approximately 4%. Magnesium glycinate (more expensive) absorbs at 40–50%. The difference between a cheap and a premium supplement is often this kind of trade-off.
The bottom line
Eat whole foods. Prioritise nutrient density. Cook from scratch as much as possible. And then, because you train hard, because the food supply is less nutritious than it was, and because your demands as a woman are higher than the averages, supplement the gaps strategically. Not everything. The things with evidence. The things your specific situation calls for. Start with vitamin D, magnesium, and omega-3. Test your iron. Build from there.