Although exclusive breast feeding for 6 months is recommended , Apart from reduction of Obesity, I would like to know what health benefits does extended breastfeeding duration (2yr and beyond) as recommended by WHO provide to both the mother and the baby?
An important thing which most analyses of breastfeeding studies that I have read note is that women who breastfeed tend to have higher IQs and incomes than women who don't, at least in first world countries. Because of the changes in lifestyle factors which result, it is nearly impossible to sort out what impacts on women and children are from breastfeeding and what are from better access to quality foods, better access to educational resources, and so on. This is even more true of women who breastfeed for an extended period of time.
From the World Health Organization comes a paper on the long-term effects of breastfeeding - in general, with some studies only going to 3 months - dated 2013. The Skeptical OB has a summary. In the case of most health benefits that are commonly claimed to be associated with breastfeeding and even more with extended breastfeeding the WHO determined that confounding variables such as maternal intelligence, education, and income could not be ruled out. These benefits included a slight reduction in obesity, a tiny increase in IQ, and a small decrease in blood pressure. The conclusion is:
Total cholesterol. There was no effect in the overall meta-analyses. In the 2007 review, there was a significant effect among adults, which is no longer present in the updated analyses. The UK trial of preterm infants showed a small protective effect (3), while the Belarus trial did not report on this outcome. We conclude that breastfeeding does not seem to protect against total cholesterol levels.
Blood pressure. The pooled estimate from the high-quality studies indicates a small reduction of less than 1 mmHg in systolic pressure among breastfed subjects, and no significant protection in terms of diastolic pressure. Residual confounding may be an important problem. The Belarus and UK preterm trials found no effect of breastfeeding (2,4). We conclude that the protective effect of breastfeeding, if any, is too small to be of public health significance.
Diabetes. There was substantial protection in the pooled analyses, with a 34% reduction, but few studies are available and their results were considerably heterogeneous. Only two high-quality studies were identified, with conflicting results (one showing an increase and another a reduction among breastfed subjects). The randomized trials did not present any results on these outcomes. Our conclusion is that further studies are needed on this outcome.
Overweight-obesity. In the pooled analyses of all studies, breastfeeding was associated with a 24% reduction in overweight and/or obesity, but the reduction was only 12% in the high-quality studies. Residual confounding may be still affecting these results, because protection is not evident in studies from low and middle-income countries where the social patterning of breastfeeding is not clear cut. The Belarus trial did not find an association (4). We conclude that breastfeeding may provide some protection against overweight or obesity, but residual confounding cannot be ruled out.
Intelligence tests. Breastfeeding was associated with an increase in 3.5 points in normalized test scores in the pooled analyses of all studies, and 2.2 points when only the high-quality studies are included. The two randomized trials also found significant effects (1,5). We conclude that there is strong evidence of a causal effect of breastfeeding on IQ, although the magnitude of this effect seems to be modest.
The most recent Harvard study referenced in the other answer also has serious flaws as reported by the Skeptical OB. As with other breastfeeding studies, the major flaw is that this study did not take out the confounding variables.
After doing some research I have found two studies which attempted to separate out socio-economic status and educational status in the mother from breastfeeding status. Unfortunately neither measured breastfeeding duration over 2 years and both are based on the same group of children. The studies looked at a less than six month duration and a 12-18 month duration of breastfeeding. Both studies  and  did show the minor IQ bump, as mentioned above in the quote from the WHO paper.
I am not convinced that all the benefits of breastmilk have been discovered yet, as it seems that something that is newly discovered and amazing about breastmilk is being discovered every year. However so far the scientific evidence does not support a significant public health benefit over the long term to extended breastfeeding.
The initial challenges to breastfeeding tend to be in the early months - getting the milk supply established, getting past cluster feeding and growth spurts, dealing with pain for the mother, the transition to going back to work, and so on. In my opinion overstating the small benefits of breastfeeding in general and extended breastfeeding in particular will not help families who desire to breastfeed be any more motivated to overcome these challenges.
No matter what the studies say, for some families there will be benefits to breastfeeding up to and past age two. There are all sorts of intangibles, many mentioned here that are backed up by experience, not science. Breastfeeding provides cuddle time with notoriously on-the-go toddlers. It may help with hydration during illness. It can be a tool for calm in the emotionally stormy times of toddlerhood. If extended breastfeeding is working, that's great. There's no need to exagerate the health benefits to defend it.
Breastfeeding up to two years of age or beyond is based on the fact that breast milk is a key organic source of DHA (Omega fatty acids), vitamin A, calcium and proteins and provides protection against infectious agents.
In 2008 the American Academy of Family Physicians said this in their position paper:
It has been estimated that a natural weaning age for humans is between two and seven years. Family physicians should be knowledgeable regarding the ongoing benefits to the child of extended breastfeeding, including continued immune protection, better social adjustment and having a sustainable food source in times of emergency. The longer women breastfeed, the greater the decrease in their risk of breast cancer. There is no evidence that extended breastfeeding is harmful to mother or child.
Many scientific studies prove the point that longer breastfeeding enhances the neurodevelopment of infants and their intellectual and scholastic ability in later life. from one of these studies increase in the IQ by breastfeeding is directly related to the duration of breastfeeding. These benefits are strongest for the infants of low birth weight on breastmilk feeding. It is found that Poor who are more likely to enter into child malnutrition are more likely to benefit from enhanced optimal breastfeeding practices.
longer breastfeeding promotes suppression of following Diseases and benefits to the infant :
- Enhanced stereoscopic vision
- Neuromotor development
- Lower chances of obesity in Adult life
- Increase in height
- suppression of gastrointestinal and respiratory infectious morbidity
- suppression of atopic eczema
- suppression of asthma
- Weight loss
- lower chances of breast cancer
- lower chances of premenopausal ovarian cancer
- suppression of hypertension
- suppression of type 2 diabetes
- suppression of myocardial infarction (heart attack).
WHO recommends that breastfeeding should continue for two years or more without accurately establishing a maximum duration for breastfeeding.In addition, there is insufficient evidence on the advantages and disadvantages of continuing breastfeeding after two years.
It is unfortunate that about many mothers are unable to breast feed even upto 1 year. This may be because We’ve sexualized the breast so much that people have forgotten breasts are for breast-feeding. There is A Harvard Study which looks at both health AND cost outcomes for U.S. mothers if they were to breastfeed for 1 year.The study found that if 90% of mothers breastfed, U.S. women would significantly lower their risk of heart attack, breast cancer, and hypertension. On a macro level, further investment in lactation support for moms could prevent $17.4 billion in maternal health costs.
Many mothers have shared positive Experiences of extended breastfeeding in this article.
Note:I am updating as an answer what I found on the internet.
There is a significant lack of research on breastfeeding past age 2. It will be difficult to locate any studies.
Benefits that we can be certain of are the same as before that age, such as protective benefits of the actual milk & the fact that it has properties that aim to fight off illness. There is no scientific doubt that it contains antibodies & that those will help a child to be sick less often & when they are to a lesser degree. It also contains oggliosaccharides that are sticky sugars that attract foreign substances & then attach to them so they are flushed through the system without a chance to infect.
The nutrient balance of course is correct for human children, unlike cow's milk, which is correct for calves. I am not anti-cow milk. We have it in my home, but it is for certain an inferior milk for a human child compared to human milk. All species makes species specific milk intended to optimize the growth & development of that specific species' requirements.
I would love it someone would study the thymus of breastfed toddlers. It has been shown in studies in infancy that they thymic index of breastfed children is significantly impacted by how much breastmilk they intake. Children who receive none have a thymus half the size, children partially breastfed show a range between & for the breastfed infant, it is then, of course, the size it should be, since from a biological perspective, breastmilk would be the default "normal". The oldest I have seen this measured I believe is 10 months old. It would stand to reason that if this is true at 4 months & even more pronounced by 10 months, that a child that is much older likely still would show a larger thymic index than his/her weaned counterparts. I have looked for years to see if anyone has ever done this, but have yet to see it. The reason this could be significant is that the thymus is what we name T cells after. It is what trains our body to try to fight off infections as well as attack & destroy cancerous cells.
Here is one if the thymic studies in abstract. https://www.ncbi.nlm.nih.gov/pubmed/10592070