Sunday, May 27, 2012

The Role of Infections and The Hygiene Hypothesis

The Hygiene Hypothesis

The hygiene hypothesis (HH) suggests that a lack of exposure to infections, parasites and symbiotic microorganisms (gut bacteria) particularly in early life predisposes one to allergies and autoimmune disease.  There are a number of observations that support the hygiene hypothesis

  • In western countries rates of infectious disease have decreased while rates of allergies and autoimmune disease have increased [1]
  • Countries with high rates of infectious disease tend to have low rates of allergic and autoimmune disease [2]
  • People who live in rural areas are less likely to get asthma, autoimmune disease, allergies [2]
  • Most people were infected with the hepatitis A virus before 1975 and has declined rapidly [3]
  • Some parasitic worms (whipworm, hookworm) improve or prevent autoimmune disease [4] 

The original proposed mechanism was that infants have more T helper 2 cells (Th2), which is the part of the immune system associated with allergies, and that bacterial and viral infections are needed to promote a T helper 1 cell (Th1) response.  However, there are many problems with the mechanism [3] and also rates of autoimmune disease have increased alongside of allergies, and autoimmune diseases are associated with the Th1 cells [5] 

Another proposed mechanism is that we need infections/parasites to keep our immune system busy and if it’s no longer challenged it will attack innocent bystanders (self-antigens).  A counter to this is that Human populations have historically been too small to sustain endemic infections (such as influenza, measles, mumps, smallpox) and therefore it’s unlikely that we need them, also common childhood infections don’t protect against allergy [3] 

A more likely explanation is that parasites (such as whipworm and hookworm) and some viruses (such as hepatitis A virus) suppress the immune system to improve their survival [3] [4] [5].  Parasitic worms need to be tolerated because clearing them would result in a lot of tissue damage, and so the host deliberately suppresses the immune system once infected with them [3] 

Infections and Autoimmune Disease 

Infections aren’t always helpful in regards to autoimmune disease or other immune related diseases.  Some infections seem to trigger autoimmune diseases and most will tend to accelerate them due to increased immune activity and inflammation [6].  Some infections promote eczema and animal models of eczema are symptom-free under specific pathogen-free conditions [7]


In T1D the infection is probably enteroviruses (especially coxsackievirus B).  Enteroviruses preferentially infect the thymus and pancreatic β-cells and trigger an inflammatory response.  T1Ds tend to have enteroviruses and their components in the pancreas and enterovirus RNA and anti-enterovirus antibodies in serum.  Exposure to enteroviruses in early life and of the mothers during gestation increases the risk of developing T1D and the peak in enterovirus infections tends to occur before the peak in onset of T1D [8] 

Many other viral* and bacterial** infections are associated with autoimmune diseases.  The diversity of antigens from infections may help explain why there are so many different autoimmune diseases. 

* EBV with multiple sclerosis [9] and lupus [10], HSV with stromal keratitis (herpetic keratoconjunctivitis) [9], hepatitis C with myasthenia gravis, autoimmune hepatitis, cryoglobulinemic vasculitis, and rheumatoid diseases, coxsackievirus strains B3 and B4 with autoimmune myocarditis and Sjorgen’s syndrome,, HSV-1 with autoimmune keratitis and rotavirus, coxsackie B viruses and rubella virus with T1D [6] 

** H. pylori with autoimmune gastritis, A Streptococcus with rheumatic heart disease, S. pyogenes induces rheumatic fever through molecular mimicry, N. aromaticivorans with primary biliary cirrhosis, B. burgdorferi with many different autoimmune diseases, T. cruzi with Chagas’ cardiomyopathy and a helminth worm (schistosomas) is thought to promote autoimmunity [6] 

Molecular Mimicry and the Bystander Effect 

Infections can induce autoimmune disease by either the process of molecular mimicry or the bystander effect [9]. 

Molecular mimicry is where the immune system develops antibodies to a particular amino acid sequence expressed by a pathogen, which happens to be structurally similar enough for the antibodies to cross react with self-antigens, our own tissues. 

The bystander effect is where an infection leads to inflammation and tissue damage.  Then the infected and damaged tissue is considered foreign by the immune system and antibodies are made against it, but these antibodies are cross-reactive with nearby undamaged tissue. 

Molecular mimicry and the bystander effect are observed in animal models of autoimmune disease [9].  But these mechanisms are incomplete for two reasons: (1) why does the immune system continue to attack the body after the pathogen has been destroyed; and (2) why has the incidence of autoimmune disease increased while infectious disease has decreased?

Sunday, May 20, 2012

Osteoporosis

Summary

Bones have a natural turnover rate – osteoclasts break down damaged bone cells (bone resorption) and osteoblasts form fresh bone cells (bone formation).  This process is necessary to adapt to exercise, recover from damage, remove old cells and maintain calcium homeostasis.  Osteoporosis seems to be the result of too much bone resorption and/or too little bone formation.

Osteoporosis is thought to be due to a negative calcium balance, where the body compensates by taking calcium from the bones to maintain calcium levels.  Under this paradigm dairy foods are essential, low intakes of calcium cause osteoporosis and calcium supplements are therapeutic.

Calcium supplementation only reduces the rate of bone loss by ~50% (it doesn’t stop it or reverse it) and in trials of high compliance calcium supplementation reduces fractures by ~20% (the low doses of vitamin D had very little effect, but did reduce the incidence of falls by 19%).  Calcium supplementation has been found to have has other effects in the body: it increased heart attacks by about 24-31% and total cardiovascular disease by about 15-18%, but decreased cancer by 14%.  These results suggest a low calcium intake can be part of the problem but not the problem we are facing.  Something else is signalling the excessive bone resorption.

The dietary acid-base theory of osteoporosis suggests that an acidic diet (high in protein, meat, dairy, grains and soft drinks and low in fruit and vegetables) leeches calcium from the bones to balance acidity in the kidneys.  But there’s no good evidence to support an alkaline diet for osteoporosis, our body regulates pH but doesn’t use calcium to do so, and in fact protein increases calcium absorption growth factors like IGF-1 that promote bone growth

Inflammation promotes bone resorption.  Inflammatory markers (IL-6, CRP), sources of inflammation (LPS, homocysteine, oxidative stress) and inflammatory diseases (IBD, CVD) are strongly associated with osteoporosis.  The beneficial effects that estrogen has on bone health seem to be largely mediated through anti-inflammatory and antioxidant activity.

Exercise should be effective as it increases bone mineral density, bone strength and prevents falls.  Although the trials find only very moderate effects (~10% reduction in fracture)

Vitamin K2 activates osteocalcin (a mineral binding protein).  People with osteoporosis have low K2 and inactivated osteocalcin is a strong risk factor for fractures (OR: 3.1-5.9).  A meta-analysis of clinical trials found that vitamin K2 supplementation in mega-doses is very effective for osteoporosis, reducing fractures by 60%.

Finally, osteoporosis/osteopenia is strongly associated with arterial calcification and cardiovascular disease [1] [2] [3].  This suggests that there usually isn’t a calcium deficit, but rather that each disease can promote the other, there’s a source of chronic inflammation underlying both conditions, a problem with calcium handling (vitamin K2 deficiency) or another common mechanism such as vitamin D deficiency or hyperparathyroidism [2]

“In general, postmenopausal women are advised to take calcium supplements to prevent or treat osteoporosis, implying that bone loss is due to insufficient dietary calcium. Yet, in many patients with osteoporosis, loss of bone tissue from the skeleton occurs at the same time as formation of bone in the artery wall.” [3]


Strategies for Osteoporosis

This is for informational purposes only and is not meant to diagnose or treat any medical condition.

Reduce Chronic Inflammation

In the absence of a low vitamin K2 intake or calcium deficiency, it seems to me that inflammation may be the main cause of osteoporosis.  See Causes of Inflammation.

Being overweight is thought to be protective due to putting the bones under greater stress, and it may be, but obesity seems to decrease bone mineral density [4] [5] [6] and cause fat infiltration in bone [7], which may be mediated by the pro-inflammatory environment usually present in obesity.  Also, T1D and T2D are associated with an increased risk of osteoporosis [8]

Adequate Nutrition

I’ve mentioned calcium and vitamin K2, but there are other nutrients needed for bone health and deficiencies in them can result in osteoporosis [9] (this this topic seems to be poorly researched and low intakes of nutrients could simply reflect a poor diet).

Vitamin D is often mentioned but doesn’t seem to have much effect in clinical trials [10], which may be due to the low doses used in the trials, and may be probably more important for elderly (who have poor vitamin D synthesis) and people with vitamin D deficiency.  Interestingly, vitamin D supplementation reduces the risk of falls [11] [12], which might be related to vitamin D deficiency causing myopathy and vitamin D supplementation having a slight effect on muscle recovery in healthy people [13]

Even though vitamin K2 is about 3x more effective than calcium I don’t consider it ‘better’ than calcium.  Rather I think the reason why vitamin K2 is more effective is because most people get enough calcium due to consumption of dairy foods, whereas vitamin K2 intake is very low because we have been told to avoid animal fats (the only source besides fermented soy).  Vitamin K1 (found mostly in plants) is not an adequate substitute for K2 [14].  The cardiovascular risks of calcium supplementation may not be an issue if one is supplementing or has a good intake of vitamin K2 because vitamin K2 (but not K1) is associated with a reduced risk of coronary heart disease* [15] and can reverse arterial calcification in animal models [16].  However, vitamins A, D and K2 work synergistically so if you take K2 you may need to increase vitamin A intake and get more sun to compensate

Exercise Regularly

Exercise increases BMD and bone strength, but also would likely reduce the risk of falls (and improve the landing of falls).  Resistance training and power/impact exercise are preferred (and obviously weight bearing exercises) [17] [18].  Bone also desensitises to the anabolic effects of mechanical loading and responds better to intermittent loading and recovery periods [18].

Sunday, May 13, 2012

Mechanisms of Osteoporosis: Part 2

Exercise 

Exercise is often recommended to prevent osteoporosis and build strong bones.  Bed rest causes rapid bone loss as bones become resistant to IGF-1 and sclerostin increases (although this effect plateaus after several months).  Exercise increases bone mineral density [1] and weight bearing exercise is more effective as swimmers only have slightly more bone mineral density than sedentary people, while other athletes like gymnasts have much more [2].  Exercise may only reduce femoral neck fractures by 11% lumbar spine fractures by 10% [3], though this doesn't include hip fractures and would depend a lot on the kind of exercise.  There are many mechanisms by which exercise promotes bone health: 

  • Exercise inhibits sclerostin through mechanical stress* [4]
  • Mechanical stress** from exercise increases IGF-1, IGF-2, PGE2, and nitric oxide, which has the effect of increasing osteoblasts and their activity*** [5]
  • Bones can convert mechanical/kinetic energy to electrical energy (piezoelectricity) as a signal and energy source for protein synthesis in bone formation.  Electrical stimulation of muscle and electromagnetic fields can also protect against bone loss [6].
  • Increased muscle mass improves exercise induced bone strength [7]
  • Between 28-35% of >65 year olds fall each year, which increases with age, and 5-10% of falls result in a fracture [8].  Exercise improves a number of physical competencies that reduce falls such as strength, power, balance, flexibility and coordination. 

That being said, overtraining may lead to osteoporosis because of elevated cortisol, a reduction in sex hormones, nutrient depletion and elevated pro-inflammatory cytokines such as IL-6 (which increases during exercise) [9

* Sclerostin seems to act like a ‘mechanosensor’ or ‘mechanostat’ to communicate to the body how much bone is needed to endure anticipated mechanical stressors 

** This mechanism is like 'a response to injury’.  Bone desensitises to mechanical stress after a few repetitions and is most sensitive when there is rest every few reps, when exercise is spread throughout the day and when there are week long rest periods 

*** Inhibiting PGE2 (NSAIDs) or nitric oxide synthesis suppresses mechanically induced bone formation

Vitamin K2 

People with osteoporosis and those who have had fractures have lower levels of vitamin K1 and K2 [10].  Vitamin K2 activates osteocalcin (the mineral binding protein), stimulates protein synthesis in osteoblasts and inhibits osteoclast activity [11].  This process is quite important as high levels uncarboxylated osteocalcin are associated with a much higher risk of fracture compared to normal levels (OR: 3.1-5.9) [12].  Vitamin K2 supplementation has some very successful outcomes:

  • Vitamin K2 protects against bone loss from age-related decline, testosterone deficiency (males), estrogen deficiency (females), glucocorticoids, stress and calcium deficiency in rats [11]
  • Trials of vitamin K2 supplementation  (often megadoses of 45 mg) increase [13] or maintain [14] bone mineral density and have been found to massively reduce fracture rates.  A meta-analysis found vitamin K2 supplementation reduces fracture rates by 60% [15]

The vitamin K2 supplements were often megadoses of 45 mg.  1.5 mg supplements also result in much lower uncarboxylated osteocalcin [16], but even this is much more than what is possible from the diet.  You would be doing well to get 50 µg from the diet [17], which is 30 times lower than 1.5 mg and 900 times lower than 45 mg.  Grass-fed animal products may have much more vitamin K2, but I doubt they have 30-900 times more. 

It might be necessary to use mega-doses because vitamin K2 is a fat soluble vitamin and these trials are essentially treating vitamin K2 deficiency (after 12 months the vitamin K2 levels in the treatment group were 65.2 ng/ml compared with 0.3 ng/ml in the control group) 

* Vitamin K1 has a similar effect on osteocalcin, but to a lesser degree [18

** Maintaining bone mineral density is an interesting finding because it seems that calcium just slows down the loss of bone mineral density (drugs and hormone replacement increase bone mineral density).  The possible implication is that with adequate K2 levels one could maintain their peak bone mineral density throughout life fairly well 

*** Vitamin K2 supplements of 1.5 mg decreased 25-hydroxyvitamin D levels over 4 weeks [16].  Vitamins A, D and K2 work together so taking one increases the need for the others. 

Further Reading:
(1) On the Trail of the Elusive X-Factor: A Sixty-Two-Year-Old Mystery Finally Solved

Sunday, May 6, 2012

Mechanisms of Osteoporosis: Part 1

Inflammation 

The signals involved in the inflammatory response promote bone loss.  The receptor activator for NF-κB ligand (RANKL) is essential for osteoclastogenesis and NF-κB is one of the key pro-inflammatory cytokines.  Various other pro-inflammatory cytokines (such as IL-1, IL-6, TNF-α, M-CSF) and prostaglandin E2 (PGE2) increase osteoclast synthesis and activity and induce osteoblast apoptosis, leading to a higher rate of bone resorption and a lower rate of bone formation [1] [2]. 

Evidence to support the role of inflammation in osteoporosis: 
  • Markers of oxidative stress [5] and inflammation (IL-6 [3] and CRP [4]) are strongly associated with lower bone mineral density
  • Periodontitis is associated with osteoporosis and LPS increases bone resorption and bone mineral density loss [6]
  • Osteoporosis is associated with bacterial infections and markers of chronic infection, such as CRP [7]
  • Osteoporosis is associated with many inflammatory diseases such as liver cirrhosis [8], rheumatoid arthritis, coeliac disease, lupus, inflammatory bowel disease [9] and cardiovascular disease [10]
  • Two common risk factors for osteoporosis are smoking (RR 1.25 or 4.9 if low vitamin C and E) [11]) and drinking (> 2 standard drinks RR 1.23) [12].  Both increase oxidative stress and inflammation [13] [14].
  • Bone mineral density declines with age and inflammation increases with age [15]
  • Conjugated linoleic acid and long chain omega 3s (EPA+DHA) seem to increase bone mineral density by decreasing pro-inflammatory cytokines and PGE2 [16] [17] [18] 

Overall, estrogen* and testosterone inhibit osteoblast apoptosis promote osteoclast apoptosis and reduces osteoclast activity.  Estrogen or testosterone deficiency results in rapid bone loss.  Estrogen also has anti-inflammatory and antioxidant effects.  It reduces the pro-inflammatory cytokines mentioned above [19], increases glutathione peroxidase (the antioxidant enzyme for hydrogen peroxide) [20] and increases osteoprotegerin, which inhibits the effects of NF-κB by acting as a decoy receptor for RANKL [19]. 

Menopause causes a reduction in estrogen, which leads to elevated hydrogen peroxide and pro-inflammatory cytokines, which may explains the rapid bone loss during menopause [1] [21].  After menopause bone loss is not quite so rapid, but leaves postmenopausal women at greater risk compared to men. 

Homocystinuria is a rare genetic disease with high levels of homocysteine and early-onset osteoporosis.  Homocysteine is an inflammatory substance that is associated with lower bone mineral density, higher rates of bone loss and higher fracture risk (RR 1.4 [22] and 2.0-2.2 [23]).  Vitamin B6, B9 (folate), B12 and choline are needed to metabolise homocysteine and low vitamin B12 and B6 are associated with low bone mineral density and low bone strength respectively (after adjusting for other micronutrient intake) [11].  Mega doses of folate and vitamin B12 reduced fracture rates by 50% following a stroke** [24] 

Some inflammatory diseases are treated with glucocorticoids* (such as cortisone) to reduce inflammation.  This further increases the risk of osteoporosis as glucocorticoids increase osteocyte apoptosis and fracture rates by 55-418% depending on the dose ** [25] [26] 

* Estrogen also inhibits sclerostin [27] 

** There was no significant difference in bone mineral density as homocysteine acts on the collagen crosslinks and bone strength 

*** The effects of glucocorticoids might suggest a possible link between stress/anxiety and osteoporosis.  Depression is associated with an increased risk of osteoporosis [28], but depression is also an inflammatory disease, there are other confounders with depression.

**** People taking glucocorticoids who have fractures tend to have higher bone mineral density than people with age-related osteoporosis, which suggests glucocorticoids act on osteocytes rather than bone formation/resorption.