Heart scan curiosities 1

Heart scans often reveal more than coronary plaque. From time to time, I'll show some curious findings that people have displayed during routine heart scans.

This 65-year old man had a relatively low heart scan score of 73, but showed an impressive quantity of calcification of his pericardium, the usually soft-tissue sack that encases the heart. The calcified pericardium is the white arcs that surround the heart in the center of the image.



Thankfully, because he's without any symptoms of breathlessness, excessive fatigue, or leg swelling, he won't need to have it surgically corrected. When the pericardium becomes rigid and encircles the heart, it can literally squeeze the heart, a condition called "constrictive pericarditis". The surgery is pretty awful.

This man's calcified pericardium likely resulted from one or more viral infections over his lifetime.

Annual physical

A judge who lives in my neighborhood was found dead in his bed this week from a heart attack. He was 49 years old. His teenage kids found him and performed CPR, but he was cold and long-gone by then.

A close friend of the judge told me that he'd passed an annual physical just weeks before.

This sort of tragedy shouldn't happen. It is easily--easily--preventable. Had this man undergone a heart scan, a score of at least 400 if not >1000 would have been uncovered, and appropriate preventive action could have been taken. The conversation could have centered around the strategies to correct the patterns that triggered his plaque and how he could reduce his score.

Of course, hospitals make use of stories like this to fuel fear that brings hordes to their wards for procedures. Would the judge have required a procedure to save his life, had his heart disease been diagnosed at his annual physical? Not necessarily. Hospitals and cardiologists would try to persuade you that procedures have an impact on mortality. This is simply not true. In fact, the mortality benefits of procedures are questionable except in the midst of acute illness (e.g., unstable chest pain symptoms or heart attack).

Don't be falsely reassured by passing a physical. A physical does nothing to screen you for heart disease. An EKG and stress test, if included, is a lame excuse for heart disease screening. Remember that a stress test is a test of coronary blood flow, not for the presence of coronary plaque. The unfortunate judge most likely had a 30% "blockage" that did not block flow, but ruptured and closed an artery off sometime in the night when he died. A stress test even on the day of his death would not have predicted this.

A CT heart scan would have uncovered it easily, unequivocally, safely.

A curious case of regression

Randi came to me at age 43. Before I'd met her, she'd undergone two heart scans about one year apart. The initial score was 57--not terribly high, but very high for a 41-year old, pre-menopausal female. Recall that rarely do women have any heart scan score above zero before age 50. Randi's 2nd scan had yielded a score of 72, a 27% increase.

Randi even had her lipoproteins assessed and she had the dreaded Lp(a). So when I met her, we discussed the possible choices in Lp(a) treatment: niacin and estrogens as primary treatment, along with LDL reduction to rock-bottom numbers, along with adjunctive DHEA, almonds, ground flaxseed, and fish oil. Sandi was okay with the adjunctive treatments and was already slender and active (BMI <25), and did not show Lp(a)'s evil partner, small LDL. But Randi had no interest in estrogens, even bio-identical preparations, because of the usual uncertainties associated with estrogen replacement. She also proved to be one of the people truly intolerant to anything but the most minute dose of niacin, experiencing prolonged flushing and abdominal cramps with any dose >250 mg.

Randi even attempted a trial of the Mathias Rath concoction of high-dose vitamin C, lysine, and proline as treatment for Lp(a), but we saw no effect on Lp(a).

Unfortunately, this left Randi's Lp(a) essentially uncorrected. Another scan one year later: 90, another 25% increase. 18 months after that, another scan: 120, a 30% increase.

Now 47-years old, Randi had resigned herself to not being able to control her plaque. We'd run out of options. At that point, I'd started to have everyone's vitamin D blood level assessed and then replaced with vitamin D. I did this with Randi, too.

A year after her last scan, she underwent another. The score: 92, a 23% reduction--substantial reversal following a course of unrelenting progression.

Randi and I, of course, both rejoiced with this unexpected success. But it raised some interesting questions: How important is Lp(a) when vitamin D is normalized and small LDL is not a part of the picture? How consistent with regression be with this strategy over time? Would normalization of vitamin D have stopped plaque from becoming established in the first place?

I hope these issues will clarify over time. For now, I'm thrilled with Randi's success. She remains on her present, "incomplete", though successful program.

Note: I would not ordinarily advise a young woman to undergo serial heart scanning with this frequency. Randi had unusual access to a scan center through a relationship with the staff. I am nonetheless grateful for the lessons her experience have taught us.

Fortune teller

Whenever your doctor uses your cholesterol values--total, LDL, HDL, triglycerides--to judge your heart disease risk, he/she is trying to act as your fortune teller.

In some states, fortune telling is illegal, a misdemeanor. The New York State lawbooks say:

A person is guilty of fortune telling when, for a fee or compensation which he directly or indirectly solicits or receives, he claims or pretends to tell fortunes, or holds himself out as being able, by claimed or pretended use of occult powers, to answer questions or give advice on personal matters or to exorcise, influence or affect evil spirits or curses; except that this section does not apply to a person who engages in the aforedescribed conduct as part of a show or exhibition solely for the purpose of entertainment or amusement.
(Source : Wikipedia)

Rather than occult powers, your physician claims to use "medical judgement" to tell your fortune. Except for that distinction, it might be construed as a misdemeanor.


Let's take three typical examples:

58-year old Laura has a high LDL of 195 mg/dl. Her HDL is 52 mg/dl, triglycerides 197 mg/dl. Does she have heart disease?

51-year old Jonathan has an LDL of 174 mg/dl, HDL 34 mg/dl, triglycerides 156 mg/dl. Does Jonathan have heart disease?

71-year old Marian has an LDL cholesterol of 135 mg/dl, HDL 84 mg/dl, triglycerides of 67 mg/dl.

None of the three have symptoms. They all feel well. Nobody is taking a statin cholesterol drug or other agent that would modify the numbers. Jonathan is around 30 lbs overweight. Nobody has an impressive family history of heart disease.

Can you tell who has heart disease and who doesn't? If you can, you're smarter than I am, because I certainly can't tell. But your doctor tries to divine your future by looking at these numbers.

Do they know something that we don't know? No. It's a crude odds game, a guessing game. A guessing game that frequently comes up on the losing end.

These are three real people. Laura, despite her high LDL, has no identifiable coronary heart disease. Jonathan has advanced coronary disease. These were his numbers just prior to his stent. Marian has a moderate quantity revealed by a CT heart scan score of 419.

Don't even try predicting your future from your cholesterol numbers--it simply can't be done. Every day, I see patients and physicians beating their heads over this dilemma. Telling your fortune using pretended occult powers is illegal. Telling your fortune using cholesterol numbers should be, too.

If you want to know if you have coronary plaque, that's the role of the CT heart scan. Plain and simple.

Heart scan score drops like a stone

Matt was dumbfounded when he found out about his heart scan score of 317 in the summer of 2005.

Earlier that year he'd unintentionally lost 20 lbs. in the space of two months and was feeling awful. He was diagnosed with diabetes and put on several medications. He told me that the heart scan score was just adding insult to injury.

As you'd expect in someone with diabetes, Matt had a low HDL, increased triglycerides, and small LDL. Blood pressure and inflammation (C-reactive protein) were issues as well.

Matt's primary care physician had put him on a statin cholesterol drug as soon as he heard about Matt's heart scan score, so we kept this going. What Matt's primary care physician didn't know was that his "true" LDL had been much higher than the conventional calculated LDL had suggested, so the statin agent was a reasonable solution. (Matt was also not terribly motivated to make dramatic changes in lifestyle or food choices. The statin drug was a compromise.)

We added fish oil and vitamin D to his regimen. Though recent data have cast doubt on the value of treating homocysteine levels of around 12.5, Matt's much higher value of 28 was treated with vitamins B6, B12, and folic acid, with a resultant homocysteine of 7.6.

17 months into the Track Your Plaque approach, and Matt's repeat heart scan score: 244, a 23% reduction.

How's that for an early Christmas gift?

"You don't have a uterus. You don't need progesterone"

I was talking with a hospital nurse recently who told me about her lack of energy, blue moods, and other assorted complaints. At age 49, she was exasperated. So I suggested that she ask her gynecologist about progesterone cream.

The gynecologist advised her, "You don't have a uterus. You don't need progesterone." He went on to explain that the only reason to take progesterone was to prevent uterine cancer caused by estrogen.

Then what about progesterone's weight loss benefits? It's effects on increased energy, improved mood, deeper sleep? These benefits, of course, have nothing to do with the uterus.

I've witnessed these benefits in women many times, both in the peri-menopausal period (which starts around your late 30's) and menopause.

Why talk about progesterone when our focus is heart disease and reduction of heart scan scores? Because if progesterone in a woman helps her feel better, more upbeat, and accelerates weight loss, she's more likely to succeed in her plaque-control program.

For additional comments on progesterone, read the Track Your Plaque interview with women's hormone expert, Dr. Nisha Jackson, Females, hormones, and weight control:
An interview with Dr. Nisha Jackson
found at http://www.cureality.com/library/fl_04-008njacksonhormones.asp. Dr. Jackson also has a book available called "The Hormone Survival Guide to Perimenopause".







Or, read Dr. John Lee's pioneering books, What Your Doctor May Not Tell You About Menopause: The Breakthrough Book on Natural Hormone Balance and What Your Doctor May Not Tell You About Premenopause: Balance Your Hormones and Your Life from Thirty to Fifty . (An edition that combines the two books is available, also.)

Take a niacin "vacation"

I've been seeing a curious niacin phenomenon that has not, to my knowledge, been reported anywhere in the medical literature.

People with lipoprotein(a), or Lp(a), are best treated with niacin, particularly given the relative lack of other effective therapies. I now have seen approximately 10 people with great initial responses to niacin, only to observe Lp(a) levels slowly drift back up to the starting level over a period of 2-3 years.

In other words, if starting Lp(a) is 200 nmol/l (approximately 80 mg/dl), drops to 70 nmol/l on niacin. Then, over 2-3 years of treatment, it drifts back to 200 nmol/l. Very frustrating.

Somehow, your body's Lp(a) manufacturing mechanism circumvents the niacin, sort of like antibiotic resistance (without the bacteria, of course).

My response to this, though untested, is to have people take an occasional "niacin vacation". I don't mean take a trip to the Bahamas while on niacin. I mean take 2 weeks off from niacin every three months or so. My hope is that the occasional vacation from niacin will allow the body to continue to respond and suppress "resistance". When resuming niacin, you may have to escalate the dose gradually to avoid re-provoking the "flush".

The same "resistance" seems to develop to testosterone in males: an initial drop followed by a gradual increase. Curiously, I've not seen this in females with estrogens, which seems to generate a durable Lp(a) suppressing effect. For this reason, an occasional testosterone "vacation" might also be considered.

So far, I've advised several people to try this. The long-term success or failure, however, is uncertain. I know of no other solutions, however.

If you have Lp(a) and are on long-term niacin, you should consider talking about this issue with your physician. Like many aspects of Lp(a), while fascinating in its complexity, much remains uncertain. Stay tuned.

When LDL is more than meets the eye

Jerry wanted to know what to do with his LDL cholesterol of 112 mg/dl. "My doctor said that it's not high but it could be better."

So I asked him what the other numbers on his lipid panel showed. He pulled out the results:

LDL cholesterol 112 mg/dl

HDL 32 mg/dl

Triglycerides 159 mg/dl


I pointed out to Jerry that, given the low HDL and high triglycerides, his calculated LDL of 112 was likely inaccurate. In fact, if measured, LDL was probably more like 140-180 mg/dl. LDL particles were also virtually guaranteed to be small, since low HDL and small LDL usually go hand-in-hand (though small LDL can still occur with a good HDL).

So Jerry's LDL is really much higher than it appears. To prove it, Jerry will require an additional test, preferably one in which LDL is measured, such as LDL particle number (NMR), apoprotein B, or "direct" LDL.

It's really quite simple. Jerry likely has a high number of LDL particles that are too small. This pattern confers a three- to six-fold increased risk for heart disease.

Treatment requires more than just reducing LDL. Small LDL--an important component of this pattern, responds, for instance, to a reduction in processed carbohydrates like wheat products (breads, breakfast cereals, pretzels, etc.), NOT to a low-fat diet. Weight loss to ideal weight, especially loss of abdominal fat, will yield huge improvements in these numbers. Niacin may be a necessary component of Jerry's treatment program, since it increases LDL size and raises HDL.

For more discussion on measures superior to LDL cholesterol, see my upcoming editorial, Let Dr. Friedewald Lie in Peace (an expansion of a previous Heart Scan Blog). It will be posted on the Cardiologist on Call column on the Track Your Plaque website within the next week.)

Oil-based vitamin D


As time passes, I gain greater and greater respect for the power of restoring vitamin D blood levels to normal, i.e. 50-70 ng/ml. Just yesterday, I saw several people with blood levels of <10 ng/ml--severe deficiency.

Vitamin D deficiency this severe poses long-term risk for osteoporosis, arthritis, colon cancer, prostate cancer, inflammatory diseases, diabetes, and heart disease. Vitamin D appears to make coronary plaque reversal--reduction of your heart scan score--easier and faster.

But it is important that you take the right kind of vitamin D. Several of the people I saw yesterday with vitamin D levels of somebody living in total darkness were taking vitamin D, but they were taking tablets. Tablets are the wrong form. Powder-based tablets, in my experience, yield little or no rise in blood levels. Some preparations generate a small rise but the dose required is huge.

If you're going to take vitamin D, take a preparation that yields genuine and substantial rises in blood levels. This requires an oil-based capsule. I commonly see blood levels of 25-OH-vitamin D3 rise from, say, 10 ng/dl to 60 ng/ml when oil-based capsules are taken.

The most common dose I prescribe to patients is 2000 units per day to females, 3000-4000 units per day to males in non-sun exposed months. Ideally, your dose is adjusted to blood levels.

The Vitamin Shoppe preparation pictured here is one I've used successfully and generates bona fide rises in blood levels. And it costs around $5. Just be sure the preparation you buy is oil-based.

For rapid success, try the "fast" track

Have you tried fasting?

Before your eyes glaze over, let me tell you what I mean. I don't mean a water-only fast for two weeks while you drool over all the temptations around you and you feel sorry for yourself.

I also don't mean the juice fasts that some people use that turn into fruit juice fasts of pure sugar.

Here's another way to do it. Usually, 48 hours of doing this will yield several benefits:

--Weight loss of 1 lb. You will likely experience an even greater weight loss of 2-4 lbs, but much of this will be water loss.

--If you're like me and share a heightened sensitivity to sugars and carbohydrates (like wheat), you may find out just how awful you feel when you eat certain foods. Many people tell me they feel absolutely wonderful when they fast--clearer thinking, increased energy, improved mood. Not the constant gnawing urge to eat they expected.

--After your fast is over, you look back and realize just what large portions of food you were eating. You'll be content with smaller quantities--and enjoy it more.


The "fast" I've used successfully includes two foods:

1) Vegetable juices--that you either juice yourself or purchase. V8 or its equivalent works pretty well. Though purchased V8 is not the best, it's better than nothing and does work reasonably well. If you juice your own vegetable juices, watch out for the diarrhea if you're unaccustomed to vegetable juices. Four 8 oz glasses per day works well.

2) Soy milk--for a source of protein and modest quantity of sugar and fat. I like the Light Silk Soymilk (Vanilla) which contains 80 calories, 2 g fat (0.5 g monounsaturated), 7 g sugar, 6 g protein per 8 oz glass. Four 8 oz glasses of soymilk also work well. In my neighborhood, 8th Continent is another good choice.


Sip both of these throughout the day. Of course, drink water in unrestricted amounts.

What can you expect in your coronary plaque control/heart scan score reversal program? When the fast is over, a rise in HDL, reduction in small LDL, reduction in triglycerides, reduction in blood sugar and insulin, and a smaller tummy. This strategy can be useful to kick-start weight loss efforts or as a periodic way to maintain control over weight and lipid/lipoprotein patterns.


Nutritional Composition Silk Soymilk--Vanilla

Nutrition Facts
Serving Size 1 cup (240mL)
Servings per container 8 H/G OR 4 QT

Amount per Serving

Calories 70
Calories from Fat 20

% Daily Value
Total Fat 2g 3%
Saturated Fat 0g 0%
Trans Fat 0g
Polyunsaturated Fat 1g
Monounsaturated Fat 0.5g

Cholesterol 0mg 0%
Sodium 120mg 5%
Potassium 300mg 8%
Total Carbohydrates 8g 3%
Dietary Fiber 1g 4%
Sugars 6g
Protein 6g
Vitamin A 10%
Vitamin C 0%
Calcium 30%
Iron 6%
Vitamin D 30%
Riboflavin 30%
Folate 6%
Vitamin B12 50%
Magnesium 10%
Zinc 4%
Selenium 8%
Does fish oil cause blood thinning?

Does fish oil cause blood thinning?

Omega-3 fatty acids from fish oil have the capacity to "thin the blood." In reality, omega-3s exert a mild platelet-blocking effect (platelet activation and "clumping" are part of clot formation), while also inhibiting arachidonic acid formation and thromboxane.

But can fish oil cause excessive bleeding?

This question comes up frequently in the office, particularly when my colleagues see the doses of fish oil we use for cardiovascular protection. "Why so much fish oil? That's too much blood thinning!"

The most recent addition to the conversation comes from a Philadelphia experience reported in the American Journal of Cardiology:

Comparison of bleeding complications with omega-3 fatty acids + aspirin + clopidogrel--versus--aspirin + clopidogrel in patients with cardiovascular disease.(Watson et al; Am J Cardiol 2009 Oct 15;104(8):1052-4).

All 364 subjects in the study took aspirin and Plavix (a platelet-inhibiting drug), mostly for coronary disease. Mean dose aspirin = 161 mg/day; mean dose Plavix = 75 mg/day. 182 of the subjects were also taking fish oil, mean dose 3000 mg with unspecified omega-3 content.

During nearly 3 years of observation, there was no excess of bleeding events in the group taking fish oil. (In fact, the group not taking fish oil had more bleeding events, though the difference fell short of achieving statistical significance.) Thus, 3000 mg per day of fish oil appeared to exert no observable increase in risk for bleeding. This is consistent with several other studies, including that including Coumadin (warfarin), with no increased bleeding risk when fish oil is added.

Rather than causing blood thinning, I prefer to think that omega-3 fatty acids from fish oil restore protection from abnormal clotting. Taking omega-3 fatty acids from fish oil simply restores a normal level of omega-3 fatty acids in the blood sufficient to strike a healthy balance between blood "thinning" and healthy blood clotting.

Comments (20) -

  • Marc

    10/26/2009 9:46:32 PM |

    Long time reader, first comment.
    Thank you for so freely sharing all the information.

    Marc

  • Daniel

    10/26/2009 11:02:46 PM |

    Thank you for this!  I have had this question for a long time given the number of things I take that "thin the blood."

  • Kevin

    10/26/2009 11:44:45 PM |

    As a veterinarian I've dispensed fish oil capsules for several years.  Some owners give so many that the dogs smell 'fishy' when seen for routine care.  The owner doesn't smell it since they're with the dog a lot.  The coats are gorgeous, something that doesn't often happen in Wyoming at 7000ft altitude.

  • Dr. William Davis

    10/26/2009 11:47:45 PM |

    Hi, Kevin--

    My two Boston terriers jump for their fish oil capsules, two every day!

    I'm glad to hear from a veterinarian that the coat sheen is indeed from the fish oil.

  • Rich

    10/27/2009 1:27:09 AM |

    Due to an afib episode a couple of years ago, I was taking 20 mg of warfarin per day, plus around 5000 mg of EPA+DHA, and never had bleeding issues.  

    My INR was always a stable 2.0.

    As I've not had an afib reoccurrence, I've replaced the 20mg coumadin with 325mg aspirin daily, and still take around 5000 mg EPA+DHA.  No bleeding issues with that combo either.

  • Catherine

    10/27/2009 3:55:32 AM |

    Glad this topic came up.
    Over the last 5 years, I've had to periodically eliminate my fish oil intake as I would start to bruise badly. My internist said she has seen this occasionally with fish oil and called it "capillary fragility." I bruise easily anyway, but it would really get bad with fish oil. So there must be some quality in fish oil that influences this.

    Then about 6 months ago I started a strong supplement change to help with my low bone density--already taking magnesium and calcium but added:
    Boron, K2, silica,pomegrantate juice, and BIG increase in vitamin D.
    I also increased omegas to 3,000 a day which I was not able to tolerate before.

    It has been over 4 months since I have had ANY bruise---which is just unheard of for me. I usually have 3-4 different bruises on arms/legs. So something in these supplements  strengthened my capillaries I guess, and I can now take high fish oil doses!
    Anyone else had a bruising problem with fish oil?

  • Dr. William Davis

    10/27/2009 11:04:59 AM |

    Hi, Catherine--

    Fascinating observation!

    I'll bet it has something to do with the vitamin D, more than anything else. Vitamin D seems to strengthen structural tissues in bones, muscle, heart valves, and perhaps capillaries and other small blood vessels.

  • trix

    10/27/2009 11:59:37 AM |

    Several years ago I bruised easily for a while and attributed it to taking garlic supplements daily.  I started taking Vit C and the bruising stopped.  I don't think it had to do with fish oil (in my case); I don't think I was taking fish oil at the time.

  • Daniel

    10/27/2009 9:37:33 PM |

    I too achieve rapid blood thinning when taking 2400mg of EPA/DHA per day. That's only 4 pharmaceutical grade capsules. Even after my vitamin d levels were normalized I still got bruising.

    I now take Vitamin K2 (MK-7 natto extract) twice a week and it's allowed me to bump my EPA/DHA up to 3600mg with no ill effects or bruising.

    It was either supplement or eat a lot of aged cheese, they both seemed to do the trick in my particular case.

  • Healthy Oil Guy

    10/27/2009 9:53:51 PM |

    Thank you for sharing this study with us.  It helps clarify whether there is a risk for blood thinning from taking fish oils.  This information may help individuals who are taking blood thinning medications and considering adding fish oils to their daily diet.

  • Dave

    10/28/2009 2:22:01 AM |

    Catherine,

    Without a doubt, your cessation of bruising was due to vitamin k2. I routinely take nattokinase, large doses of fish oil, curcumin, and other blood thinning agents, and if I don't take vitamin K2, I will begin bruising. (I also take high doses of Vitamin D). When I take K2, I have absolutely no bruising.

    Vitamin K2 has many clinical trials showing that it helps endothelium  integrity and elasticity.

    Also, grapeseed extract and pine bark extract (specifically oligomeric proanthcyanins) has the same beneficial effect.

  • Catherine

    10/28/2009 4:41:41 PM |

    Daniel,

    That's really interesting! There is a lot of research on K2's effect on strengthening weak bones. Bone fractures go down considerably when high doses of K2 are used (Japan is using K2 as osteoporosis treatment) BUT studies show it needs to be in conjunction with adequate calcium and Vitamin D---they work synergistically for bone strength.  So it makes sense that K2 and D could do the same with strengthening fragile capillaries. I am also taking the M7 natto form.

  • Catherine

    10/29/2009 12:01:36 AM |

    Dave,

    Thanks for sharing your experience with this, you've really confirmed it now for me.  I can't believe I have suffered with this for most of my life with no answers (tried high dose Vit C, grape seed, etc) and now within months on K2, there's no bruising and I can tolerate fish oil. Hope my bones are responding this well!
    This blog is so helpful....

  • Mina

    10/29/2009 12:21:31 PM |

    Thanks for posting this. The question recently came up in our office. I like your assertion that omega-3s restore the blood to normal and remove abnormal clotting. And to comment on a post above, our dog has a beautifully shiny coat and takes 2 pure EPA capsules each day!

  • Term papers

    1/26/2010 3:40:08 PM |

    I have enjoyed reading That During nearly 3 years of observation, there was no excess of bleeding events in the group taking fish oil. (In fact, the group not taking fish oil had more bleeding events, though the difference fell short of achieving statistical significance.

  • Viagra Online

    8/23/2010 6:41:39 PM |

    I've been drinking fish oil for many year and I don't have any chance in my body people use to said me that but I think it is just a rumor.

  • buy jeans

    11/3/2010 10:19:55 PM |

    I'm also especially gratified that a woman now holds our record. I'm uncertain why, but the ladies have been shy and the men remain the dominant and vocal participants in our program. Speak up, ladies!

  • moseley2010

    12/7/2010 2:37:16 AM |

    I haven't heard of this problem
    fish oil supplements. But now we know what to tell them when this sort of concern comes up. Fish oil or Omega-3 is really beneficial to health. It's just important that it comes from clean waters.

  • Jack

    3/12/2013 7:03:38 PM |

    What is an appropriate dose of fish oil for someone taking coumadin?

  • dorange

    6/15/2014 3:53:03 PM |

    Dr. Davis, when  person is taking Tamoxifen...
    (1) is it safe to take vitamin k2 or K1?
    (2) will fish oil have a role in preventing blood clots?

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