Dr Frances Sladek is a world leader in molecular biology research. Here she talks to us about her latest research, how soybean oil affects health and what the molecular mechanisms that underlie these outcomes may be.

What is your research background and what triggered your interest in studying the effects of diet on metabolic health?

I was trained as a biochemist, doing my undergraduate work at Princeton and graduate work at Yale University. I became fascinated by molecular mechanisms and structural aspects of proteins, especially protein-DNA interactions. At the same time, due to illness in my family and friends, I became fascinated by cancer and its root causes, which is why I did my PhD dissertation on DNA repair and mutagenesis in bacteria. At the time, in the early 1980s, we didn’t really have the tools to do mechanistic work in mammalian cells and I figured that we had to first figure out how bacteria repaired damaged DNA before we could hope to figure it out in mammalian systems. My postdoctoral work at Rockefeller University took me initially away from the cancer focus as I purified and cloned a transcription factor, HNF4a, which is now considered to be a master regulator of liver-specific gene expression. HNF4a is a member of the nuclear receptor superfamily of ligand-dependent transcription factors such as the Vitamin A and D receptors, the oestrogen, progesterone and glucocorticoid (stress hormone) receptors. Around the time I cloned HNF4a, several other oprhan nuclear receptors were being cloned by other groups. Several of those receptors, and later HNF4a, were found to have fatty acids as ligands, including fats from the diet like linoleic acid.

What effect can polyunsaturated fatty acids such as linoleic acid (LA) have on health?

As an unsaturated fatty acid, LA in the past was generally assumed to be beneficial for health. There are, for example, reports on using LA to help prevent coronary heart disease. However, there are also a growing number of studies indicating that LA can have detrimental effects, such as insulin resistance and obesity. Nonetheless, compared to saturated fats which have been very well studied, since LA was found to be an essential fatty acid in the late 1920s, it has only been in the last five to ten years that researchers have begun looking at potential negative effects of LA.

LA is the precursor to arachidonic acid (AA), which is linked to inflammation, a key player in obesity, diabetes, inflammatory bowel disease and cancer so high amounts of LA could have detrimental effects compared to the relatively low amounts that are required for its essential functions.

What were the major differences between the diets high in soybean oil versus diets high in coconut oil?

The diet enriched in soybean oil led to obesity, diabetes, insulin resistance and a fatty liver phenotype with large lipid droplets and hepatocyte ballooning, a sign of liver injury. In contrast, an isocaloric diet in coconut oil led to relatively little weight gain, considering the amount of fat in the diet, no diabetes or insulin resistance. There was lipid accumulation in the livers of the coconut oil fed mice but much less than in the soybean oil fed mice.

How can these results be applied by healthcare professionals in their practice?

My recommendation is to try to avoid soybean oil. This is not easy to do as soybean oil is ubiquitous in the American diet. It is found in many, if not most, processed foods and vegetable oil, the main cooking oil in the US, is comprised of a mix of oils, one of which is almost always soybean oil. Restaurants in the U.S. use vegetable oil so if you eat out or eat pre-packaged food you are very likely consuming soybean oil. Even Jiff peanut butter has soybean oil on its list of ingredients! We never use vegetable oil at home but I have started adding coconut oil to my diet. We also stay away from processed foods but we do eat in restaurants. I should also add that I use soymilk every day in my tea or coffee and I love tofu. Soymilk and tofu do not have a lot of soybean oil in them so I would not recommend that one has to avoid all soy products.

What impact do you think this study and similar research could have on obesity prevalence?

According to our mouse studies, everything else being equal, if we cut down on the consumption of soybean oil as a society in general then I think we will see a decrease in obesity. A study in 2011 showed that the component of the American diet that has changed the most in the past 100 years is soybean oil, more than fructose, corn oil or chicken. Even though soybeans have been cultivated for 5000 years, we are now consuming more soybean oil than we ever have in the past.

Is Obesity Down to the Oil We Use?

It is no secret that obesity is on the rise in the Western world. There has been a steep increase in obesity in the last forty years, with 36 per cent of the US population and 26 per cent of the UK currently obese. This figure is expected to rise to up to 60 per cent by 2050. Obesity is associated with a number of conditions, including high blood pressure, heart disease, diabetes and insulin resistance, which together make up a disease known as metabolic syndrome. Of course several factors can be attributed to this increase: lifestyle, genetics, activity levels and environment, just to name a few. Diet, however, remains one of the key contributors to rising obesity. Recent years have led to a dramatic increase in fructose consumption, predominantly in the form of high fructose corn syrup which is found in soda and processed foods. Fat, and particularly the type of fat, has also been extensively scrutinised.

First, let’s get back to basics. There are three main types of dietary fats: saturated, monounsaturated and polyunsaturated. These simply refer to the types of bonds within the molecule of fat. Saturated fats are solid at room temperature and include butter, margarine, coconut oil and palm oil. Monounsaturated and polyunsaturated fats are liquid at room temperature. Monounsaturated fats include olive oil, canola oil, avocados and nuts. Polyunsaturated fats include soybean oil, corn oil and sunflower oil.

After a study in the 1960s which showed an association between saturated fats and cardiovascular disease, there was a significant increase in the consumption of soybean oil, found in many vegetable and seed oils, processed foods, salad dressings, snack foods and fast food meals. However, there appears to be a correlation between this increase in soybean oil consumption and the rising rates of obesity. This may be due to the linoleic acid (LA) contained within soybean oil, which is 55 percent LA. LA is an essential fatty acid that animals cannot produce so it must be consumed. However, LA needs to be only 1 to 2 per cent of our diet and we are currently consuming closer to 8 to 12 per cent. LA binds to a nuclear receptor called HNF4a, which regulates genes involved in carbohydrate and lipid metabolism and has been linked to both diabetes and fatty liver.

On the flipside, certain saturated fats such as coconut oil and palm oil may be useful in preventing and treating metabolic syndrome because they are full of medium chain triglycerides. These mediumchain triglycerides are low in LA and are metabolised more quickly than other fatty acids and therefore contribute less to weight gain.

Metabolism at a Genetic Level

In order to examine the effects of unsaturated fats, as well as fructose, on obesity and diabetes, an experiment was designed which assigned mice to one of four diet groups. All diets contained the same amount of calories and each had 40 per cent total calories from fat, which is comparable to the average US diet today. The first high fat diet was made up of coconut oil, which was chosen because it is naturally low in LA and polyunsaturated fatty acids. The second group was a high fat diet in which ~50 per cent of the coconut oil was replaced with soybean oil to give 10 percent LA, comparable to the average US intake. The third group was a high fat diet with added fructose and the fourth was a high fat soybean oil enriched diet with added fructose. As well as the four diet groups, there was also a low fat control group in which the mice were fed a standard diet. Carbohydrate and protein levels were consistent across all diets and food intake did not differ significantly across groups of mice.

The results after 20 weeks are surprising. Overall, the diet high in soybean oil was the most detrimental to the health of the mice. This group gained almost 25 per cent more weight than the coconut oil group. This group also gained weight faster and had markedly more subcutaneous fat than the other groups. Very severe non-alcoholic fatty liver disease was also observed in this group. Researchers found very large fat droplets in the livers of these mice, as well as ballooning of the liver cells.

The soybean oil group also had significantly higher rates of diabetes, glucose intolerance and insulin resistance than the other groups, even when compared to the diets supplemented with fructose. This is particularly unexpected considering the links between high fructose diets and insulin resistance. Conversely, the coconut oil diet showed no signs of diabetes or glucose intolerance or insulin resistance at any time during the experiment.

Furthermore, while the fructose plus coconut oil diet did not cause diabetes, other health problems such as rectal prolapse and fatty liver were observed in these mice. While fructose had been linked previously by many other groups to fatty liver, the prolapsed rectum finding was new but perhaps not surprising. Dr Sladek explains: ‘The main negative effect of the high fructose diet that we did not observe with the soybean oil was a high rate of prolapsed rectums, which is part of the disease index of inflammatory bowel disease, which is on the rise. Fructosefree diets have been found to relieve gastrointestinal problems in humans.’

An analysis of the chemical processes involved in metabolism revealed an increased accumulation of polyunsaturated fatty acids and their metabolites in the livers of the soybean oil mice. Further study into the gene activity of the liver showed altered expression of a number of genes involved in metabolism, obesity, diabetes, inflammation and cancer. ‘Some genes increased in expression while other genes decreased. There were several genes associated with obesity, diabetes, inflammation, and cancer that were up-regulated while some anti-cancer genes were down-regulated’, Dr Sladek explains. ‘But the largest category of genes that were altered were those involved in xenobiotic and drug metabolism. These are cytochrome P450 genes, many of which are known to be regulated by HNF4a and other nuclear receptors and which play an important role in metabolising not just toxic compounds that we ingest in our food and the drugs found in our pharmacy but also the steroids, retinols (Vitamin A compounds) and fatty acids that act as ligands for other nuclear receptors’.

‘Fructose induced additional weight gain in the mice fed coconut oil but actually produced less weight gain than the mice fed soybean oil. This was rather surprising given the considerable attention given to high fructose corn syrup in the diet… In terms of the other tests, soybean oil, but not fructose, induced glucose intolerance and fructose actually lowered the insulin resistance induced by the soybean oil.’

Next Steps in Solving Soybean Oil

Studies like these are the only the first stage. Although the amounts of fat and fructose used in the experiment are similar to the average American’s plate, these findings must be validated in humans before they can inform policy.

Dr Sladek and her team also want to find out the specifics of why soybean oil is causing such negative health effects. As Dr Sladek states: ‘LA seems to be involved but we don’t yet know if it is LA itself or if it is a metabolite of LA. We also would like to determine whether the negative effects of soybean oil are acting through HNF4a or other nuclear receptors that are also known to bind LA. Our results that we are submitting for publication now suggest that HNF4a and LA metabolites may indeed be very important in the obesogenic effects of soybean oil.’ So what’s the takeaway? The shift towards soybean oil consumption may have improved cardiovascular health but also appears to have aggravated a host of other problems associated with metabolic syndrome. While the study doesn’t indicate that diets high in fructose and saturated fat are the healthy option, it does show that a diet enriched with soybean oil can lead to a number of issues. This is an important factor for patients and healthcare providers alike to consider when planning a diet.

CONTACT

E: frances.sladek@ucr.edu

T: (+1) 951 827 2264

W: http://sladeklab.ucr.edu/

W: http://nrdbs.ucr.edu/

W: http://nrmotif.ucr.edu/aaSNP/

W: http://stemcell.ucr.edu/

KEY COLLABORATORS

Dr Poonam Deol, Assistant Project Scientist, UC Riverside

FUNDING

NIDDK at the NIH

National Institute of Food and Agriculture at the USDA

UCR SEED Grant

United Soybean Board

T32 ETOX Training Grant, NIEHS

REFERENCES

Sladek FM, Zhong W, Lai E and Darnell JE, Jr., Liver-enriched transcription factor HNF-4 is a novel member of the steroid hormone receptor superfamily, Genes and Development, 1990, 4, 2353–2365.

Yuan X, Ta J, Lin M, Evans J, Dong Y, Bolotin E, Sherman MA, Forman BM, Sladek FM, Identification of an endogenous ligand bound to a native orphan nuclear receptor, PLoS ONE, 2009, 4, e5609.

Chellappa K, Jankova L, Schnabl JM, Pan S, Brevilter Y, L-S Fung C, Chan C, Dent OF, Clakre SJ, Robertson GR, Sladek FM, Src tyrosine kinase phosphorylation of nuclear receptor HNF4α correlates with isoformspecific loss of HNF4α in human colon cancer, Proc. Natl. Acad. Sci. USA, 2012, 109, 2302–7.

Deol P, Evans JR, Dhahbi J, Chellappa K, Han DS, Spindler S, Sladek FM, Soybean oil in more obesogenic and diabetogenic than coconut oil and fructose in mouse: potential role for the liver. PLoS One, 2015, 10, e0132672.

Chellappa K, Deol P, Evans JR, Voung LM, Chen G, Briancon N, Bolotin E, Lytle C, Nair M, Sladek FM, Opposing roles of nuclear receptor HNF4α isoforms in colitis and colitis-associated colon cancer, eLIFE, May 11, 2016, eLife 2016;10.7554/eLife.10903.

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