As a doctor (or foodie), it is best to avoid being dogmatic. Partly, because we need to respect the views of our patients (or friends) but largely because the "dogma" tends to change as new science emerges.
Take saturated fat as an example.
My grandmother was a great cook and used lard, butter, and cream when she could access them and when she could afford them. Her food was totally amazing.
My mother and her generation bought into the "diet‐heart" hypothesis which says that high intake of saturated fats increases the level of serum cholesterol which leads to heart disease. She only used low-fat, skimmed, or no fat. She, too, was a great cook but the food was possibly not quite as amazing as my grandmothers.
I may not be a good cook, but I do love to cook. I also love to watch the cookery channel. I could sit for hours watching The Barefoot Contessa. It does amaze me that every Barefoot recipe involves "a stick of butter" and/or "a pint of cream." The Contessa reminds me so much of my grandmother.
Emerging data (including two key studies just published this month) are challenging the ‘diet-heart’ hypothesis which makes me very happy indeed. What do the studies say?
What is Saturated Fat?
Fat is a macronutrient that plays a key role in our enjoyment of food. There is a relationship between fat and flavor as many natural flavors are fat-soluble. Additionally, fat gives our food a pleasurable texture. No wonder my grandmother's food tasted so good.
There are three main types of fat: saturated, unsaturated and trans fats.
Saturated fat is solid at room temperature.
From a biochemical perspective, saturated fats contain no double bonds.
Biochemically, saturated fats would look like this:
There is a single bond between the two carbon molecules which means that there is no space for new atoms to be added which is why they are called saturated fats.
While unsaturated fats would look more like this:
The double-bond between the two carbon atoms can be broken down and new atoms can be added which is why they are called unsaturated fats.
Saturated fats are sub-classified as long, medium or short-chain fatty acids depending on the number of carbon atoms they contain. Short-chain fatty acids are considered to have 6 or fewer carbon atoms, medium-chain fatty acids have 8–10 carbons, and long-chain fatty acids have 12 or more carbon chains.
Sources of saturated fat include: butter, cheese, red meat and animal-based foods. Plant-based oils including palm oil, palm kernel oil and coconut oil contain mostly saturated fats, but do not contain cholesterol.
As an aside, I often consult the United States Department of Agriculture Food Composition Database for detailed analysis of foods but see that it now carries a disclaimer that says "Due to a lapse in federal funding, this USDA website will not be actively updated. Once funding has been reestablished, online operations will continue"(1). What a shame. I think it is best not to consult a service that is not being updated.
Medium-chain triglycerides (MCT) are very trendy right now and deserve a special mention. These are processed saturated fats made from a mixture of coconut and palm kernel oils. Many celebrity health coaches recommend MCT for weight loss, exercise performance and brain health (2).
Is There any Research?
There are 500,000 studies and 30,000 clinical trials on saturated fats.
To put this into context, there are 26,000 publications and 2000 clinical trials on unsaturated fats. Olive oil has 10,000 publications and 1,000 clinical trials. MCT have almost 3000 publications and over 350 clinical trials.
Does Saturated Fat Boost Brain Health?
Researchers from the George Washington University School of Medicine reviewed available data on saturated fat and brain health (3). The study showed very divergent results. Saturated fat intake was positively associated with the risk of Alzheimer's Disease in 3 of 4 studies.
Positively associated means that people who ate saturated fats were more likely to have Alzheimer's. The fourth suggested the opposite effect. Saturated fat intake was also positively associated with dementia in only 1 of 2 studies.
Saturated fat was also associated with mild cognitive decline in just 1 of 4 studies, and with cognitive decline in 2 of 4 studies. It is pretty hard to draw any real conclusions from this.
MCT deserve a special mention here. MCT are metabolized into ketone bodies and ketone bodies are being considered as agents for promoting brain health. A very recent study looked at the effect of an MCT-based ketogenic formula on cognitive function in 20 Japanese patients with mild to moderate Alzheimer's disease (4).
The study compared an MCT formula with a placebo in a single dose study and in a 12 weeks study. The single dose study showed no benefits of the MCT formula on cognitive function. At 8 weeks patients receiving MCT showed significant improvements in their immediate and delayed logical memory tests compared to their baseline scores.
At 12 weeks the MCT arm showed significant improvements in a digit-symbol coding test and immediate logical memory test compared to baseline. The authors concluded that MCT can help with verbal skills and speed processing.
The data on saturated fats and brain health is inconclusive at this time. There is some emerging and interesting data on MCT saturated fats but this is very much preliminary data for now.
Does Saturated Fat Cause Cancer?
This is a very controversial area. It was previously thought that saturated fat can lead to cancer. Let's look in detail at two important forms of cancer.
A comprehensive review published this month from researchers in Korea looked at the association between dietary fats and colorectal cancer (5). Drawing on data from 18 studies, their analysis showed no correlation between dietary fat intake and the risk of this common cancer.
A 2001 pooled analysis of dietary fat and breast cancer only found a very weak positive association between replacing 5% of total energy intake carbohydrates with saturated fat (6). In 2014, Iranian investigators re-visited this topic and looked at data from 68 studies (7).
They made some interesting observations including the fact that saturated fat from meat sources cannot be directly compared to fat from other sources as cooked meats may contain other carcinogens.
They concluded that "we are not at a stage where we can justifiably advise women to reduce their fat intake to decrease the risk of developing breast cancer" but added that "it seems the current guidelines to lower total fat consumption and .... reduction of saturated fatty acids (meat and dairy products) intake .... is also useful for breast cancer risk."
They finished off with the recommendation that "it does not seem that efforts for severe reduction of total dietary fat intake in relation to breast cancer are useful now."
A 2017 meta-analysis of saturated fat intake on breast cancer mortality did find that saturated fat intake negatively impacted on breast cancer survival (8).
The data on saturated fat and cancer is far from conclusive at this stage.
Does Saturated Fat Help Form Foundation of Cell Membranes?
The membranes that surround our cells are made up of a double layer of lipid. These lipids are derived from fats including saturated fats (9). There are no specific studies looking at what this might/might not mean in clinical practice.
Saturated fats do help form cell membranes but I can't comment on whether or not this translates into any kind of a clinical take-home message.
Does Saturated Fat Help Increase Beneficial HDL Cholesterol?
For quite some time, we have been told that it is important to limit our dietary intake of saturated fat to keep our cholesterol low and reduce our risk of heart disease (10).
This simplistic and linear approach to health overlooks three important concepts.
Firstly, we are not really interested in having perfect blood-work. We are interested in living long and healthy lives. This means that we are more interested in whether saturated fat kills us as opposed to whether it affects good or bad cholesterol.
Secondly, we usually eat food and not just individual components of food (11). We have seen before that it sometimes does not make sense to just look at an individual component of food. Sometimes nature conveniently co-produces protective factors which can limit the impact of individual components of foods. Not all sources of fat are created equally as we will see later.
Thirdly, what is the knock-on effect of reducing saturated fats? Chances are that we replace fat with something else. What food swop do we choose? That has to affect our health - for good or for bad.
What do the studies and particularly meta-analyses have to say about saturated fat and heart health (12)?
A systematic review and meta-analysis done in 2014 looked at 12 studies and over 7000 participants and found that recommending higher intakes of polyunsaturated fatty acids in replacement of saturated fatty acids was not associated with risk reduction from heart disease (13).
A Cochrane review from the UK looked at data from 15 trials and almost 60,000 study participants to address the question of whether reducing saturated fat intake and replacing it with carbohydrate, polyunsaturated or monounsaturated fat and/or protein helps with mortality and cardiovascular morbidity (14).
The final verdict on this rich dataset was that there was a small but potentially important reduction in cardiovascular risk on reduction of saturated fat intake. The review recommended replacing saturated fat with polyunsaturated fat rather than carbohydrates.
A key paper in this debate was published in 2016 (15). The study was entitled ‘Re-evaluation of the traditional diet-heart hypothesis’ which pretty much says it all.
The study found that replacement of saturated fat in the diet with linoleic acid lowered serum cholesterol but ‘does not support the hypothesis that this translates to a lower risk of death from coronary heart disease or all causes’. Sounds like a dogma has just been overturned to me.
A 2017 review really makes us re-think the current guidelines on fat and heart disease (16). The study looked at data from 125287 participants from 18 countries in North America, South America, Europe, Africa, and Asia in the Prospective Urban Rural Epidemiology (PURE) study. The data told a very interesting story.
The data tells us that reducing saturated fatty acid intake and replacing it with carbohydrate has an adverse effect on blood lipids. To get into the details, the research found that people who ate the most fat (an average of 35.3 percent of calories) were 23 percent less likely to die than those whose fat intake was lowest (an average of 10.6 percent of total calories).
And here is the kicker. It did not matter whether the fat was saturated or unsaturated. The authors concluded that ‘focusing on a single lipid marker such as LDL cholesterol alone does not capture the net clinical effects of nutrients on cardiovascular risk’.
A 2010 meta-analysis of prospective studies which involved almost 350,000 study participants showed that there was no significant evidence for concluding that dietary saturated fat is associated with an increased risk of heart disease (17).
A study published just this week looked at 40 studies with 12,619 vegans and 179,630 omnivores (18). Food questionnaires indicated that vegans compared less energy and less saturated fat as compared to their omnivore counterparts. In most countries (except Taiwan for some reason), the vegan lifestyle was associated with a more favorable cardio-metabolic profile as compared to an omnivorous diet.
A low saturated fat diet may reduce bad cholesterol but has not been shown to benefit health. The focus should be on which saturated fats to eat as part of a balanced healthy diet.
Does Saturated Fat Reduce Stroke Risk?
Two studies that we looked at in the previous section included data on stroke risk.
The 2010 meta-analysis by Siri-Tarino and colleagues found no convincing evidence that saturated fat was associated with an increased risk of stroke (16).
In fact, more recent data brings the link between saturated fat and stroke to a whole new level.
The 2017 Mente study mentioned above also found that higher fat diets were associated with lower risks of strokes (15).
Chinese investigators carried out a meta-analysis of 15 prospective studies which included 476,569 individuals and 11,074 strokes (19). This study showed that a higher saturated fat intake was associated with reduced overall stroke risk. They cautioned that specific food types (specifically meat versus animal sources) needed to be elucidated in future studies.
Finally, a study published this week looked at saturated fats and the risk of stroke (20). The study was a review paper with two interesting design features. Firstly, the study looked at two different sub-types of stroke (bleeding and clot). Secondly, they also looked at ethnic origin.
The study found that a diet high in saturated fat is associated with a low risk of either type of stroke but that this was only seen in people of Japanese origin.
There is evidence to suggest that higher-fat diets (regardless of the type of fat) are associated with lower risks of stroke. There is emerging data that ethnicity may play a key but as yet undefined role.
Are Saturated Fats Safe?
Now this is the multi-billion dollar question. Previously, the answer to this question would have been a resounding "NO." Now, this is less definitive. We have looked at the current evidence for saturated fats and risks of heart disease and stroke which previously would have been the commonly quoted side-effects of saturated fats.
As saturated fats are just part of everyday life for most people, there are no individual case reports implicating saturated fats in certain health conditions. How could you deconstruct an individual's life and lay the blame for any condition squarely at the door of fats as opposed to anything else?
I think that it is safe to say that we can be a lot less dogmatic about the 'diet-heart' dogma. Saturated fats are no longer public enemy number one. That being said, we probably have more questions than answers at this stage.
What do we do in this in-between stage when the dogma looks less certain but we don't have new dogma to replace it?
Here is my approach (for what it's worth).
I am very happy to move from a low-fat/no-fat diet to enjoy (guilt-free) a small amount of saturated fat in my diet. Back to grandma's cooking. Of course, that's what I was hoping for.
However, the data are limited and I too, have limits when it comes to saturated fats. I am not quite ready to follow all of the Barefoot Contessa's recipes (just too much fat for me). I am certainly not ready to buy fat bombs (energy balls laden with saturated fats such as butter and coconut oil) to have with my daily coffee.