Types of fats can be a major point of confusion for consumers who choose to look at the nutrition label. The topic of the five primary types of fats (saturated fat, trans fat, monounsaturated fat, Omega-6 polyunsaturated fat, and Omega-3 polyunsaturated fat) is a complex topic.
Trans fats are very well recognized in the scientific community as the worst types of fats for you.
A 2006 animal study of trans fat consumption found the same caloric intake of trans fat increased weight gain to the same am. Similar studies would not be able to be conducted on humans for ethical reasons .
A 2014 meta-analysis of 4 studies concluded several but not all studies show an association between trans fat consumption and indreased risk of cognitive decline .
A 2014 study on rats found trans fats enhance anxiety from stressful situations .
A 2011 cohort study of 12,000 college students found trans fat intake associated with increased incidence of depression .
In a cohort study of 4,000+ breast cancer survivors, the highest quartile in consumption of saturated and trans fats had a significantly higher risk of dying from any cause .
They’ve been associated with increased risk of cardiovascular disease and diabetes .
A study of dietary fat intake in the first Nurse's Health Study of 80,000 women found "CHD risk roughly doubled for each 2% increase in trans fat calories consumed instead of carbohydrate calories" , .
A 2006 systematic review of trans-fat related studies concluded "On a per-calorie basis, trans fats appear to increase the risk of CHD more than any other micronutrient" , .
A 2009 meta-analysis of 130,000+ participants concluded the replacement of trans fat partially hydrogenated vegetable oils with alternative fats and oils would substantially lower coronary heart disease .
A 2016 meta-analysis of ~900,000+ participants found high consumption of total, saturated and trans-fats increase ovarian cancer risk .
Products high in trans fats are, along with sodas, among the worst FDA-approved foods you can possibly put in your body which is why the FDA has revoked their Generally Recognized as Safe status and has required PHVOs be removed from all food products by June 2018 . Americans get the most Trans Fat on average from cakes, cookies, crackers, pies and breads followed by animal products. Make sure to steer clear of these foods that continue to contain trans-fat despite an FDA notice that partially hydrogenated vegetable oils will be permanently banned in 2018.
Saturated Fat vs. Unsaturated Fat
Saturated fats are found in highest quantities in animal products. Nutritionally, saturated fats occupy a little bit more of a gray space than trans fats based on evidence. Elementary school curriculum has long taught that saturated fats are the bad fats and unsaturated fats are good and, until recently, advocated low-fat diets. However, the treatment of fats in dietary recommendations has been significantly challenged in the last decade and a number of the largest health institutions such as Harvard School of Public Health have classified low fat diets as “continuing to lose credibility” . Still, all major medical institutions including Harvard Chan School of Public Health, USDA, AHA, ADA, ACS, NHS, WHO etc. as explained on our linked institutional guidelines page continue to assess that minimizing consumption of saturated fat is a component of a healthy diet. As a result, they place an emphasis on fat quality and choosing poly and monounsaturated fats like nuts and olive oil that have been linked to healthier outcomes.
As institutions were incorrect in initially recommending low fat diets for a number of decades, many low-carb diet related sites online continue to challenge the recommendations that saturated fat be primarily avoided. So, let's look at the evidence. Low fat diets can, in fact, be a successful way to lose weight as proven in a few meta analyses cited earlier , . Institutions like Harvard School of Public Health acknowledge this but raise long term safety questions around the Atkins Diet being too high in saturated fat and protein, which can be hard on the heart, kidney, and bones, as well as concerns over the lack of carb-rich fruits and vegetables .
The current state of evidence favoring the argument that saturated fat is more harmful than unsaturated fat is supported by the following points:
All major medical organizations including Harvard Chan School of Public Health, USDA, Healthcare.gov, AHA, ADA, ACS, NHS, WHO continue to assess that avoiding saturated fat is a component of a healthy diet. Increasingly, these organize advocate against prior recommendations for very low fat diets and instead in favor of a focus on consuming good unsaturated fats. See more on institutional recommendations here.
A major 2016 cohort study of 115,000+ participants concluded high dietary intakes of saturated fat are associated with an increased risk of coronary heart disease .
The 2014 study questioning the link between saturated fat and heart disease that is often cited by low carb diet advocates was funded by the dairy industry and classified by HSPH as “seriously misleading .”
Pooled analyses and randomized control trials have mostly found when saturated fatty acid intake is replaced with polyunsaturated fat intake, risk of heart disease and diabetes is reduced , , , , 
Epidemiologic studies have concluded the Mediterranean diet (high in monounsaturated fat, low in saturated fat) is beneficial for health relative to standard diets ,  .
A study of ~4,000 participants found accelerated cognitive decline leading to the onset of Alzheimer’s for those who consume a high intake of copper in conjunction with a high intake of saturated and trans fats. For those whose diets were not high in saturated and trans fats, copper intake was not associated with any cognitive changes .
Higher consumption of saturated fat relative to unsaturated fat may hamper blood sugar control which may elevate risk factors for obesity and other diseases , , .
Quantitative meta-analyses of metaboloic ward studies, in which doctors have complete control over a patient’s diet, have found replacing saturated fat with unsaturated fat lowers LDL bad cholesterol .
The arguments some low carb diet advocates including the Atkins Diet, Ketogenic diet, Paleo, Bulletproof diet, AuthorityNutrition.com and Gary Taubes make around saturated fats being a "middle of the road" food (better than sugar and refined grains, worse than fruits and vegetables) and not that bad to consume involve the following studies:
The 2015 De Souza Meta Analysis of 12 cohort studies conclude "Saturated fats are not associated with all cause mortality, CVD, CHD, ischemic stroke, or type 2 diabetes, but the evidence is heterogeneous with methodological limitations. Trans fats are associated with all cause mortality, total CHD, and CHD mortality, probably because of higher levels of intake of industrial trans fats than ruminant trans fats." . The study has been criticized for misrepresenting results in conclusion:
The new study, published in BMJ, was a meta-analysis of 41 previous reports. The statistical analysis was done in two ways, because certain statistical adjustments can influence results. For example, saturated fat increases cholesterol levels which, in turn, can increase cardiovascular risk. If the data are adjusted for cholesterol levels, the link between saturated fat and cardiovascular risk can be made to disappear. Using unadjusted data, the study found that people whose diets were heaviest in saturated fat had a 12 percent higher risk of developing heart disease and a 20 percent higher risk of dying of it, compared with those whose diets were lowest in saturated fat. Saturated fat was also associated with risk of ischemic stroke. These risks were statistically significant—that is, they were unlikely to be due to chance.
The 2014 Chowdhury Meta Analysis of 72 cohort and randomized control trial studies concluded evidence did not clearly support cardiovascular guidelinse encouraging high consumption of polyunsaturated fat and low consumption of saturated fat. The study was funded by grants from the British Heart Foundation; Medical Research Council; Cambridge NIHR Biomedical Research Centre; and Gates Cambridge, UK . The study attracted serious media attention and was on the cover of Time magazine. The study contained major research errors and was called on by some to be retracted . The study has been strongly criticized by Harvard School of Public Health  and NutritionFacts.org:
The 2010 Siri-Tarino Meta Analysis of 21 studies concluded "A meta-analysis of prospective epidemiologic studies showed that there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD" . The study was done by doctors funded by the National Dairy Council as acknowledged in the disclosure and has been criticized for similar reasons as the Chowdhury meta-analysis for being designed to fail from the start.
CQ Opinion: Eating healthier is all about what categories of food replace current calories. To our team at CQ, based on the current evidence, the institutional recommendations to replace saturated fat with poly and monounsaturated fat seem justified. However, the low carb diet advocates' advice that replacing saturated fat with refined grains and sugar is a poor choice also seem reasonable.
Omega 6 and Omega 3
Omega 6 polyunsaturated fats are primarily found in vegetable oils while Omega 3 polyunsaturated fats are primarily found in fish (DHA) and chia / flax seeds (ALA). Omega 6 and Omega 3 fatty acids are both essential for health and the body cannot make them on its own. The average American diet contains 14 to 25 times as much Omega-6 as Omega 3 because of the prevalence of high Omega-6 vegetable oils . The importance of the ratio of Omega 6 to Omega 3 fatty acids is unclear based on current evidence but medical organizations recommend Americans try to consume more Omega 3 fat. Below are some studies highlighting the protective effects of Omega 3 fatty acid consumption:
A 2013 meta-analysis of 26 studies of 880,000+ participants found higher consumption of dietary marine Omega 3s is associated with a lower risk of breast cancer .
A 2012 meta-analysis of 8 cohort studies concluded while Omega-3 and Omega-6 consumption alone were not associated with any prostate cancer outcomes high dietary intake of ALA may significantly reduce prostate cancer risk .
A 2007 discussion of the current state of Omega 6 and Omega 3 research and results of the Nurse's Health Study concluded "Adequate intakes of both Omega-6 and Omega-3 fatty acids are essential for good health and low rates of cardiovascular disease and type 2 diabetes, but the ratio of these fatty acids is not useful" .
A 2015 meta-analysis of 20 randomized control trials concluded early intervention with Omega 3 fatty acids may bring out more beneficial clinical outcomes in type 2 diabetic patients .
A 2016 meta-analysis of 13 studies concluded " present meta-analysis suggested a beneficial overall effect of omega-3 PUFA supplementation in Major Depressive Disorder patients .
A 2011 study of 40,000+ participants concluded "consumption of nonmarine sources (ALA) of omega-3 FAs is associated with a decreased risk of type 2 diabetes" .
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