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Zinc and Forgetfulness
Zinc is an essential micronutrient in the body and it is a component of many complex structures in the body including enzymes and proteins. The link between zinc and memory is well studied especially because zinc deficiency can cause memory loss. Read on to find out how zinc affects memory and how to enhance memory with zinc supplementation.
Zinc is an essential trace mineral with important roles in the body. It makes up the cores of some important enzymes in the body including the enzyme responsible for breaking down alcohol.
In fact, zinc is the only metal to appear in all classes of enzymes.
The human body usually contains 2 – 4 g of zinc at any time. This quantity is widely distributed and mostly found in the brain, liver, kidney, muscle and bone but highly concentrated in the prostate and the eyes. The semen is another rich site of zinc and the mineral is known to contribute to the growth of reproductive organs.
Zinc is often found in other complex structures in the body especially in proteins. It has an affinity for the amino acids, aspartic acid, glutamic acid, histidine and cysteine. By the last estimate, zinc can be found in 10% of all the proteins naturally occurring in humans.
In the brain, zinc is held in the glutamatergic pathway and its chief function is to control excitability. It also contributes to learning.
However, high levels of zinc in the brain can be neurotoxic. Therefore, the central nervous system closely regulates the amount of zinc it holds.
Once absorbed into the blood, zinc is bound to the proteins, albumin and transferrin. Since transferrin is also responsible for binding iron, zinc can reduce iron absorption and vice versa.
Furthermore, some of the absorbed zinc is usually held in reserves in the intestine or liver by a complex known as metallothionein. However, this complex is also responsible for holding copper in reserve. Therefore, excess zinc may reduce the absorption of copper.
The recommended daily intake value of zinc is 8 mg/day for women and 11 mg/day for men.
Rich dietary sources of zinc include red meats (beef and lamb, especially), liver and shell fish such as lobsters and oysters.
The zinc contents of plants depend on the amount of zinc in the soils in which they grown. Where the soil is rich in zinc, the best plant dietary sources of zinc include the seeds of alfalfa, sesame, sunflower, poppy, pumpkin, celery and mustard; wheat germ and bran; as well as beans, nuts, whole grains, blackcurrant and almonds.
Other sources of zinc include supplements and fortified foods.
Zinc supplements are made using various salts of zinc. The over-the-counter oral, dietary zinc supplements available are usually zinc oxide, zinc acetate and zinc gluconate.
These oral zinc supplements are taken for their antioxidant, anti-aging and immune-boosting properties.
Zinc supplements have also been proven to speed up healing after an injury, stimulate the sense of smell and promote the activities of over 100 enzymes in the body.
Topical zinc preparations are used to protect against sunburn, windburn and diaper rash. Because zinc ions have antimicrobial effects, zinc lactate is used in toothpastes and zinc pyrithione is an ingredient of shampoos.
Although zinc deficiency is largely uncommon in the developed world, it affects about 2 billion people worldwide.
Zinc deficiency can be caused by insufficient dietary intake of the mineral and also by some underlying conditions including malabsorption syndrome, liver and renal disease, diabetes, sickle cell disease and other chronic illnesses.
Signs of zinc deficiency include impaired growth, impaired immune system, delayed sexual maturation, loss of appetite, diarrhea, impotence, hair loss, soft tissue lesions and cognitive decline.
Over the years, it has been shown that some people with memory impairment do have zinc deficiency.
In a study done on mice by researchers from Duke University Medical Center and Massachusetts Institute of Technology, the role of zinc in the brain was highlighted.
In that study, the researchers injected a chemical that binds to zinc into the brains of the test mice. By binding to zinc, the injected chemical removed the mineral and made it unavailable for nerve cells in the brain. The result was a dramatic reduction in signal communication between the neurons of the brain.
This effect was even more pronounced in the hippocampus known to be one of the parts of the brain in control of memory and learning.
This study goes a long way to explain what zinc does in the brain. Normally, zinc is stored in the vesicles in the synaptic junctions between nerves.
Most of the stored zinc in the brain is found coupled to proteins in the glutamatergic neurons.
This means that zinc is stored and released in the brain from nerve cells that are also responsible for releasing the neurotransmitter, glutamate.
The released zinc can act on glutamate receptors and other voltage-gated ion channels in the brain. It can also modulate synapses (a property known as synaptic plasticity). Both of these effects contribute significantly to learning and memory.
By inhibiting the transporter responsible for storing zinc in the vesicles of synapses and releasing it from nerve cells, researchers were able to demonstrate that zinc deficiency (in this case, unavailability) do affect some forms of learned memory (it caused impaired fear memory and spatial memory in the mice).
However, there is definitely more to how zinc effects memory that this study could answer.
In a study published in the journal, PLoS One, in 2012, Harvard researchers demonstrated that zinc deficiency was linked to memory loss in patients suffering from Alzheimer’s disease.
On close examination, the brains of patients suffering from Alzheimer’s disease have two types of lesions: beta-amyloid plaques that are found outside nerve cells and neurofibrillary tangles that are found within these nerve cells.
The researchers showed that zinc was the link between these two forms of Alzheimer lesions.
The neurofibrillary tangles inside nerve cells are caused by the “tau” proteins sticking to the microtubules inside these cells. However, zinc is known to stabilize protein complexes such as the tubulin making up these microtubules.
Other protein complexes stabilized by zinc are known to be important for regulating synapses and the biochemical consolidation of memory in the nerve cells.
Of the two lesions seen in the brains of Alzheimer patients, it is the inner entanglement of “tau” proteins to microtubules that causes most of the symptoms. The Harvard researchers were able to show that this is caused when the beta-amyloid plaques outside the cells lower the zinc levels inside the cells.
Therefore, zinc can prevent memory loss in Alzheimer patients because it stabilizes the microtubules of neurons and prevent them from tangling with “tau” proteins.
Most of studies done on the effect of zinc on memory have used animal models especially rats and mice.
There are a number of reasons for this including the fact that knocking off genes responding for coding proteins that store, transport and use zinc is easily done in these animals. In addition, the results usually bears close resemblance to what would have happened in humans.
In a 1986 study published in the journal, Physiology & Behavior, the effect of zinc deficiency was studied on a population of rats. The study determined the extent of zinc deficiency on memory and if supplementation helped.
The result showed that zinc supplementation can correct zinc deficiency and improve short-term memory.
A 2001 study published in the same journal investigated the effect of zinc deficiency on memory as correlated to age. In this study, rats aged between 43 days and 18 months were used. These rats were divided into different age groups and in each group some rats were placed on zinc-deficient diets while others are placed on zinc-adequate diets.
The results of the study showed that only rats younger than 62 days and placed on zinc-restricted diets experienced significant short-term memory loss.
While this study is not a conclusive indication of the effect of zinc deficiency on memory impairment and age in humans, it is a demonstration that short-term memory may well developed in older adults to an extent that zinc deficiency does not significantly impact it.
A 2000 study published in the Journal of Nutrition also involved rats and studied the effect of zinc deficiency and supplementation of memory. The result showed that rats fed with diets low in zinc performed lower in short-term memory tests than those who got zinc-rich diets.
However, zinc supplementation was seen to reverse the memory deficits in these rats.
In addition, adding L-histidine supplements improved the results and time taken for the zinc deficient rats to improve their short-term memory performance.
Although there is more to be discovered about the way zinc affects memory, the available body of research work done in this area has shown that:
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