There is a distinct lack of evidence for either, i.e. if anyone is looking to do a health study, research, "How long a child can be exclusively on breastmilk, and when to introduce what nutrients for optimum nutrition" would be a very good choice!
Check out this page:http://www.kellymom.com/bf/bfextended/ebf-benefits.html
She has a lot of links, there might be a few more, I don't remember, I have a list of all the studies I've found, relating mainly to iron, but also other nutritional needs, from there, and from other studies:Iron
K.G. Dewey, et al., Iron supplementation affects growth and morbidity of breast-fed infants: results of a randomized trial in Sweden and Honduras
Journal of Nutrition 132, no. 2 (Nov 2002): 3249-55.http://jn.nutrition.org/cgi/content/full/132/11/3249
Contents: 4-6 month old infants on breastmilk but allowed tastes of vegetables, control group supplemented with iron, result: extra iron un-necessary.
A. Pisacane et al., Iron status in breast-fed infants, Journal of Pediatrics 127, no. 3 (Sep 1995): 429-31http://www.ncbi.nlm.nih.gov/pubmed/7658275
Contents: small study of ca 20 infants, partially or exclsively breastfed with no supplementing (given solids possibly, but no formula or vitamins/iron). Found no risk of iron feficiency.
Pastel RA, Howanitz PJ, and Osk, FA. Iron sufficiency with prolonged exclusive breast-feeding in Peruvian infants, Clin Pediatr 20:625-26, 1981.http://www.ncbi.nlm.nih.gov/pubmed/7273572
Contents: 6 exclusively breastfed infants, some over 12 months old, no iron deficiency.
Siimes MA, Salmenpera L, Perheentupa J. Exclusive breast-feeding for 9 months: risk of iron deficiency. J Pediatr 1984 Feb;104(2):196-9.http://www.ncbi.nlm.nih.gov/pubmed/6694011
Contents: The study behind WHO's recommendations. 30 breastfed infants compared with 30 on formula. None of the breastfed babies were iron deficient by 7 months. By 9 months 2 or 3 were slightly deficient.Optimal duration of exclusive breastfeedinghttp://www.ncbi.nlm.nih.gov/pubmed/11869667
Contents: A study of studies. Basically says exclusive breastfeeding for at least 6 months totally safe from nutritional perspective.
Risk of Infant Anemia Is Associated with Exclusive Breast-Feeding and Maternal Anemia in a Mexican Cohorthttp://jn.nutrition.org/cgi/content/full/136/2/452
Contents: Larger study, but only 5 infants exclusively breastfed past 6 months. 3 of them iron deficient. They do not state that the mothers of the deficient babies were deficient, but they do say that anemic mothers were more likely to have iron deficient babies, and more often the longer the babies breastfed.
Fat and Energy Contents of Expressed Human Breast Milk in Prolonged Lactation
Dror Mandel, MD, MHA, Ronit Lubetzky, MD, Shaul Dollberg, MD, FACN, Shimon Barak, MD, Francis B. Mimouni, MD, FAAP, FACNhttp://pediatrics.aappublications.or...ull/116/3/e432
Contents: The milk from 34 mothers whose babies were over a year old was compared to milk form 27 mothers of less than 6-month olds. “Human milk expressed by mothers who have been lactating for >1 year has significantly increased fat and energy contents, compared with milk expressed by women who have been lactating for shorter periods. During prolonged lactation, the fat energy contribution of breast milk to the infant diet might be significant.”
Nutritional composition of human milk for full-term infantshttp://www.uptodate.com/patients/con...iL7ncr_TSU_9Ui
Article, with references.
Breast milk volume and composition during late lactation (7-20 months) (Dewey)http://www.ncbi.nlm.nih.gov/pubmed/6502372
Contents: “Breast milk composition of 119 samples collected by 46 women during months 7-20 of lactation was compared with composition of 101 samples collected at 4-6 months. Breast milk intake of 10 infants was determined by test-weighing for 1 or more months during months 7-16 of lactation. Longitudinal decreases in milk concentrations of zinc, copper, and potassium, previously documented for the first 6 months, continued into the second 6 months, while protein, iron, and sodium concentrations showed no further decline. Lactose, fat, calcium, and magnesium concentrations were similar to those in earlier stages of lactation. Weaning was associated with significant changes in milk composition: When milk volume fell below 300 ml/day, there was an increase in protein and sodium and a decrease in lactose, calcium, and zinc. Breast milk intake of infants not supplemented with cow's milk or formula averaged 875 ml/day (93% of total energy intake) at 7 months and 550 ml/day (50% of total energy intake) at 11-16 months. Total energy intake increased from 610 to 735 kcal/day, but energy intake per kilogram remained constant at a relatively low 70-79 kcal/kg/day. Our results suggest the need for further studies of nutrient intake and requirements of breast-fed infants during late lactation.”
Complementary Feeding and Breastfeedinghttp://pediatrics.aappublications.or.../106/5/S2/1301
Contents: Another study by Dewey, this one about how solid intake influences intake of breastmilk. The more solids that go in, the less breastmilk.
Dewey K. G., Cohen R. J., Brown K. H., Rivera L. L. Age of introduction of complementary foods of low birthweight breastfed infants: a randomized intervention study in Honduras. Am. J. Clin. Nutr. 1999;69:679-686http://www.ajcn.org/cgi/content/abst...e2=tf_ipsecsha
Conclusion: 'There was no growth advantage of complementary feeding of small-for-gestational-age, breast-fed infants between 4 and 6 mo of age.” Which is interesting, since the doctors/nurses seem to believe the opposite!
Dewey KG. Nutrition, Growth, and Complementary Feeding of the Breastfed Infant. Pediatric Clinics of North American. February 2001;48(1).http://www.ncbi.nlm.nih.gov/pubmed/11236735
Contents: “Although additional research is needed [...] the following conclusions are well substantiated by the evidence available to date: Breast milk alone can meet nutrient needs during the first 6 months, with the possible exception of vitamin D in certain populations and iron in infants of relatively low birth weight. Complementary foods offered before 6 months of age tend to displace breast milk and do not confer any growth advantage over exclusive breastfeeding. Breast milk continues to provide substantial amounts of key nutrients well beyond the first year of life, especially protein, fat, and most vitamins. Breastfed infants tend to gain less weight and usually are leaner than are formula-fed infants in the second half of infancy. This difference does not seem to be the result of nutritional deficits but rather infant self-regulation of energy intake. [...]The nutrients most likely to be limiting in the diets of breastfed infants are minerals, such as iron, zinc, and calcium. Using the following guidelines can help to ensure that the nutrient needs of the breastfed child are met: Continue to breastfeed as often as the infant desires. Aim for a variety of complementary foods, with fruits, vegetables, and animal products (e.g., meat, fish, poultry, or egg) offered daily. […] Be alert to any signs that the child's appetite, growth, or development is impaired. When in doubt, a balanced vitamin-mineral supplement is advisable. Make mealtimes enjoyable.”
“In the second year (12-23 months), 448 mL of breastmilk provides:
29% of energy requirements
43% of protein requirements
36% of calcium requirements
75% of vitamin A requirements
76% of folate requirements
94% of vitamin B12 requirements
60% of vitamin C requirements”
-- Dewey 2001
Although the studies done are rather small and too few, the WHO made the call EBF for 7 month is totally fine, no risk. And as it is known that the baby has stores of iron for at least the first half year, probably longer (I need to check this research next, I guess), it is probably likely to be so. However, there are hardly any studies that include any babies exclusively breastfed past 6 months, only one with any babies exclusively breastfed past 12 months (The Peruvian study with only six infants, the ages of all infants are uncertain).
In all honesty, the idea that babies breastfed exclusively for longer than say 6 or 7 months are at any greater risk than other babies of iron deficiency, or any other deficiency, must be a myth, as there is NO EVIDENCE at all.
But as most American babies are tested for iron deficiency (I thought?), wouldn't it be quite an easy research for someone? All you'd need to do would be collect the data from doctors (or parents), and question parents on baby's food: Breastfeeding, formula, combination, when introduced formula, when introduced solids.
Personally, I would breastfeed exclusively until baby wants something else (past 6 months if possible, but much longer if needed). And if my 8+ month old was still exclusively breastfeeding, I'd check iron with a blood test.