Alzheimer’s Disease as Glucose Dysfunction of the Brain

I’d heard of Alzheimer’s disease (AD) being referred to by some clinicians as “Type 3 Diabetes” but until yesterday the link between AD and abnormalities in glucose metabolism in the brain was an academic interest. Now it’s personal. You see, yesterday I found out that my dad (90 years old) was diagnosed with Alzheimer’s disease. His once-sharp mind is no longer capable of recalling what happened yesterday or in fact what just happened. It is as though he has “partial amnesia”.

The majority of those that are diagnosed with AD (95%) have the same form of the disease that my dad has called sporadic Alzheimer’s disease. Only 5% are diagnosed with a genetically-linked form inherited from the maternal side of the family [2].

In sporadic Alzheimer’s disease the first part of memory that is affected is the person’s unique memory of specific events (called episodic memory).

I remember three or four years ago asking my dad to recount the details of our family history from his side of the family that he often told yearly at holiday dinners – yet he couldn’t even remember their existence! I tried prompting him with parts of the story to try and trigger the memory but was met with “I’m sorry dear, I don’t recall”.  I was dumbfounded because he told this same exact story over and over again for years, and then suddenly it was gone. From his perspective, it didn’t exist. His behaviour and other forms of memory were completely normal, so I discounted his forgetfulness to “aging” but I now know this was the first noticeable indication that something was not working as it should.

As Healthy People Age

As healthy people age, brain cells waste away (atrophy) and are not replaced such that brain volume decreases at a rate of about 1.6% / decade after the age of 30 years old [1]. So, at 40 years old, a person is expected to have a 1.6% decrease in brain volume, at 50 years old a 3.2% decrease, and so on. At 90 years old, a healthy person would be expected to have lost a little less than 10% of their brain volume.

My maternal grandmother was healthy and lived until well over 100 years old and I can see in retrospect that the kinds of memory changes my dad was showing a few years ago (at age ~85) were not simply part of normal aging.

Changes in Alzheimer’s Disease

Over the last 30 years, there has been a lot of progress in terms of understanding changes in brain energy metabolism during AD as compared with what occurs in normal, healthy aging. Until recently, many thought that lower brain function of those developing AD led to less use of glucose by the brain, but now it is thought that it is actually the other way around; that decreased glucose uptake into the cells of the brain leads to decrease metabolism in the brain. This decrease glucose uptake into the cells of the brain are believed to be a critical part of the early development of AD and that significantly lower brain glucose metabolism may be present long before the onset of any clinically measurable mental decline in AD [2].

It is now widely believed that there is decreased glucose metabolism in the brain in those with AD. When compared with healthy aged-matched people, those with AD have ~25% more brain atrophy than would be expected for their age. This is, after correction for age-associated brain atrophy, the majority of PET scan studies conducted from 1981- until the present, show that glucose utilization by the brain is decreased by as much as ~25% in AD [2].

While healthy, normal aging is associated with some slow brain atrophy, it is not thought to be associated with decreased glucose metabolism. 

Normal Brain Glucose Use

The brain, heart, liver and kidneys together use ~ 60% of the body’s resting metabolic energy needs and while the heart and kidneys are metabolically more active than the brain, the brain is larger. As a result, the brain uses about ¼ of the body’s total energy needs [2]. This energy is used for blood flow in the brain, use of oxygen by the brain and for glucose metabolism – but most of the glucose used by the brain is used to maintain a glucose gradient (difference) between glutamate neurons which enable communication along this neurotransmitter system.

Glucose transporters (GLUTs) bring glucose into the brain in a three-step process: (i) Transport across the blood-brain barrier (ii) transport into the brain cells (astrocytes) and (iii) transfer of the glucose into the neurons of the glutamate neurotransmitters.

When the brain is active, the Adenosine Triphosphate molecule (ATP) which is the “currency” of energy transfer inside cells decreases, and the brain needs more glucose, so glucose uptake is stimulated by the cell. It is unknow at what point partial reduction in glucose transport begins to limit brain function in AD.

Brain Glucose Use in Alzheimer’s Disease

Alzheimer’s Disease is a neurodegenerative disease that results in progressive worsening of memory and cognitive function, as well as behavior changes and disorientation.

Even though normal healthy aging is not associated with AD. Aging itself is the main risk factor for sporadic AD, with rates ~doubling every five years after 65 years of age [3] and affecting more than 60% of people over the age of 95 years of age [4].

The brain of those with AD is marked by an accumulation of βeta-amyloid plaques between brain cells and by neurological “tangles” within brain cells.  As mentioned earlier, there are two types of AD – familial / early onset AD and sporadic / late onset AD. The early onset type is much rarer (~5% of all AD) and is inherited from the maternal (mother’s) side of the family. Except for a different age of onset, what is seen clinically, and the progression of decreased cognitive function is not significantly different between the two types of AD. The βeta –amyloid plaques occur slowly before any change in memory or understanding become apparent. The progressive brain atrophy speeds up later in the disease process, bringing the cognitive decline frequently associated with AD.

There are also other forms of dementia besides Alzheimer’s disease, including fronto-temporal dementia and vascular dementia but these are very different from either of the two sub types of AD.

PET scan studies point to lower brain glucose metabolism in AD, with difference between normal aging subjects and those with AD being as much as ~20–25% lower in AD with most of the atrophy occurring in the region of the brain called the hippocampus, which is involved in memory processing.

Mild Cognitive Impairment (MCI)

There is intermediate stage between normal healthy aging and AD, called Mild Cognitive Impairment (MCI) which includes some decreased thinking ability (cognitive decline). When these thought process changes and memory loss are present in the elderly, but don’t significantly affect daily life or interactions it is considered to be MCI. There are a few studies of glucose metabolism in MCI which show that it is lower than in healthy aged-match controls but less than in moderate to severe AD.

As MCI progresses to AD, glucose usage decreases in additional regions of the brain (cingulate, inferior parietal lobes, temporal lobes) [2].

Nutritional Factor that Affects Glucose Metabolism

The omega-3 fatty acid found mainly in fatty fish known as Docosahexaenoic acid (DHA; 22:6ω3) is known to have an important role in normal brain development. In animal studies,  supplementation with DHA was found to increase expression of the glucose transporters (GLUTs) that bring glucose into the brain and in primate studies, brain DHA concentration was found to be directly proportional to brain glucose uptake in the same region of the brain [2]. Insufficient intake of DHA and/or low levels in the hippocampus (the region of the brain initially impacted by AD) may play a role in cognitive decline in older adults.

Metabolic Factors that Affect Glucose Metabolism

While the glucose transporters (GLUTs) involved in getting glucose across the blood-brain barrier and into the brain cells (GLUT1) and across glutamate neurons (GLUT3) are not sensitive to insulin, GLUT4 which is another glucose transporter involved in memory and cognition in areas of the brain including the hippocampus are insulin-sensitive [5]. It is thought that brain insulin signaling may be defective in AD [5].

Older adults and the elderly often develop glucose intolerance which often progresses to Type 2 Diabetes then to Metabolic Syndrome which is a combination of Type 2 Diabetes, high blood pressure (hypertension), increased waist circumference (visceral obesity) and abnormal cholesterol tests (dyslipidemia).

Insulin resistance, which often comes before glucose intolerance / high blood sugar tops the list of known risk factors to cognitive decline [5, 6] and younger adults that are obese are predisposed to Metabolic Syndrome which is associated with increased risk of degenerative changes in the brain [6].

Decreased skeletal muscle mass (sarcopenia) in older adults and the elderly may contribute to the increased risk of insulin resistance associated with aging, as muscle is the main site of insulin-mediated glucose utilization in the body. In older adults, adequate dietary protein intake as well as incorporating some form of resistance training of large muscle groups may play a role in decreasing cognitive decline by increasing glucose update from the blood to the muscle where it can then be transported to areas of the brain.

Ketones: the body’s preferred alternative fuel

In healthy people that haven’t eaten in while (such as after an overnight fast or a during relatively long period of time between meals) ketone bodies (ketones) are the body’s key replacement fuel which maintains brain function. The brain even has a separate transport system for ketones which is independent of glucose transport [2].

When blood sugar levels drop over a period of several hours or even days during fasting the energy requirements of the body are dependent on the availability of two ketonesacetoacetate and β-hyydroxybutyrate for normal function.  During prolonged fasting over a period of days and in starvation up to ~60% of the human brain’s energy requirements can be met by a combination of acetoacetate and β-hydroxybutyrate [7].

The brain can convert ketones to ATP, the energy “currency” of the cell by oxidizing ketones (converting β-hydroxybutyrate to acetoacetate, acetoacetate to acetoacetyl CoA, and acetoacetyl CoA to acetyl CoA which then is used to generate ATP).  While brain cells (astrocytes) can beta-oxidize fatty acids [8] to produce ketones, transport of fatty acids across the blood-brain barrier is too slow to make fatty acids as useful alternative as fuel for the brain.

Ketones cannot fully replace glucose as a brain fuel as a small quantity of glucose is essential for the brain, however this does not need to be supplied in the diet but can be manufactured by the liver (as well as to a lesser degree by the kidneys and the intestines) from fat or protein in a process known as gluconeogenesis (literally “making new glucose”).

The body can make ketones from fat stores in a process called ketogenesis but first there needs to be a lowering of blood glucose, which will result in decreased blood insulin levels. This can occur during fasting, as well as by following a low-carbohydrate diet. When insulin level decreases, free fatty acids from fat cells (adipose tissue) can be freed into the blood. These long chain fatty acids are then brought to the liver where they are broken down (β-oxidized) to acetyl CoA, which are then condensed into ketones.

Use of a Therapeutic Ketogenic Diet in Alzheimer’s Disease

It is thought that in Alzheimer’s disease the combination of brain glucose insufficiency and the inadequate supply of naturally-produced ketones (which normally would naturally be produced by the body in response to low blood glucose) puts the high energy consuming areas of the brain in mild, but constant shortage of energy.

Since the brain can’t get its main fuel source which is glucose nor its preferred back- up fuel source which are ketones (because blood glucose doesn’t drop) this forces the brain to rely on a third, but inadequate source of energy – which is making glucose from fat stores or protein (gluconeogenesis).

Over time, specific regions of the brain such as the hippocampus are thought to be put in a situation of long-term chronic fuel shortage and gradually these brain cells ‘burn out’, which leads to the brain changes seen in Alzheimer’s Disease [2].

It is thought that if brain ketone metabolism is unaffected in AD – or at least is affected less than glucose, a ketogenic diet may provide the brain with ketones it can use as an alternative fuel to glucose, enabling it to function more normally, reducing cognitive decline resulting from brain glucose insufficiency.

If you have questions about how eating a low carbohydrate diet can significantly reduce insulin resistance, a major risk factor for Alzheimer’s disease, as well as reverse symptoms of Metabolic Syndrome, please send me a note using the “Contact Me” form on this web page and I’ll be glad to reply as soon as I’m able. Remember, I provide services via Distance Consultation (via secured Skype) as well as in-person in my Coquitlam office.

To our good health!

Joy


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LEGAL NOTICE: The contents of this blog, including text, images and cited statistics as well as all other material contained here (the “content”) are for information purposes only.  The content is not intended to be a substitute for professional advice, medical diagnosis and/or treatment and is not suitable for self-administration without the knowledge of your physician and regular monitoring by your physician. Do not disregard medical advice and always consult your physician with any questions you may have regarding a medical condition or before implementing anything  you have read or heard in our content.

 

References

  1. Svennerholm, L., K. Boström, and B. Jungbjer, Changes in weight and compositions of major membrane components of human brain during the span of adult human life of Swedes. Acta Neuropathol, 1997. 94(4): p. 345-52.
  2. Cunnane, S., et al., Brain fuel metabolism, aging, and Alzheimer’s disease. Nutrition, 2011. 27(1): p. 3-20.
  3. Canadian study of health and aging: study methods and prevalence of dementia. CMAJ, 1994. 150(6): p. 899-913.
  4. Brayne, C., et al., Dementia before death in ageing societies–the promise of prevention and the reality. PLoS Med, 2006. 3(10): p. e397.
  5. Watson, G.S. and S. Craft, Modulation of memory by insulin and glucose: neuropsychological observations in Alzheimer’s disease. Eur J Pharmacol, 2004. 490(1-3): p. 97-113.
  6. Raffaitin, C., et al., Metabolic syndrome and risk for incident Alzheimer’s disease or vascular dementia: the Three-City Study. Diabetes Care, 2009. 32(1): p. 169-74.
  7. Cahill, G.F., Fuel metabolism in starvation. Annu Rev Nutr, 2006. 26: p. 1-22.
  8. Guzmán, M. and C. Blázquez, Is there an astrocyte-neuron ketone body shuttle? Trends Endocrinol Metab, 2001. 12(4): p. 169-73.

 

Author:

Joy Y Kiddie MSc., RD - I'm a Registered Dietitian in private practice in British Columbia, Canada that provides services in-person in my Coquitlam office, as well as by Distance Consultation (using Skype / telephone).