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In vivo effects of polyunsaturated, monounsaturated, and saturated fatty acids on hepatic and peripheral insulin sensitivity

Objective: Free fatty acids (FFAs) cause insulin resistance and are often elevated in obesity. Chronic ingestion of diets rich in saturated fat induces more insulin resistance than diets rich in unsaturated fat, however, it remains unclear whether different FFAs cause distinct levels of insulin resistance in the short-term, which is relevant to the feeding and fasting cycle. Protein kinase C (PKC)-δ is implicated in hepatic insulin resistance. Therefore, we investigated the effects of short-term elevation of fatty acids with different degrees of unsaturation on hepatic insulin action and liver PKC-δ membrane translocation, a marker of activation.

Materials/Methods: Triglyceride emulsions of Soybean Oil + Heparin (polyunsaturated (POLY)), Olive Oil + Heparin (monounsaturated (MONO)), Lard Oil + Heparin (saturated (SATU)), or saline (SAL) were infused intravenously for 7 h to elevate plasma FFA concentrations ~ 3–4 fold in rats. During the last 2 h of infusion, a hyperinsulinemic–euglycemic clamp with tritiated glucose methodology was performed to examine hepatic and peripheral insulin sensitivity.

Results: Surprisingly, SATU, MONO, and POLY impaired peripheral insulin sensitivity (glucose utilization divided by insulin) to a similar extent. Furthermore, all lipids induced a similar degree of hepatic insulin resistance compared to SAL. Although there were changes in hepatic content of lipid metabolites, there were no significant differences in liver PKC-δ membrane translocation across fat groups.

Conclusions: In summary, in the short-term, FFAs with different degrees of unsaturation impair peripheral insulin sensitivity and induce hepatic insulin resistance as well as hepatic PKC-δ translocation to the same extent.


Alex’s Notes: In humans (i.e. us), most but not all studies show an association between insulin resistance and composition of dietary fat intake, with the general trend being that saturated fatty acids are more detrimental than monounsaturated and polyunsaturated fatty acids. These effects appear to be more pronounced in women than in men, and they may be mediated in part by increased inflammatory gene expression. Interestingly, the benefits on insulin sensitivity disappear when total fat intake exceeds 37% of total energy intake, at which point both saturated and monounsaturated are equally detrimental to glucose tolerance. Together, these studies suggest that the quantity and quality of dietary fat plays a role in insulin sensitivity, and that unsaturated fats are less detrimental than saturated fats until total fat intake becomes the primary dietary energy source.

However, high-fat diets are not ideal, per se, to study the effects of fatty acids on insulin sensitivity. There are too many confounding variables. As such, intravenous (IV) administration has become a popular method for examining the effects of select fatty acids on glucose metabolism. The current study compared IV infusions of lard, soy, and olive oil emulsions on insulin sensitivity using the gold-standard hyperinsulinemic-euglycemic clamp technique with glucose tracers.

Side note – what is a hyperinsulinemic-euglycemic clamp?

The two key words are hyperinsulinemic and euglycemic. The former indicates that insulin will be raised, while the latter indicated that blood glucose will be maintained at fasting levels. Plasma insulin concentrations are raised to about 100 muU/mL (fasting is <25) by continuous infusion of insulin into the bloodstream. Additionally, enough glucose is infused to maintain fasting glucose concentrations and prevent hypoglycemia. Under these steady-state conditions of euglycemia, the glucose infusion rate equals the glucose uptake by all the tissues in the body, and thus the glucose infusion rate is a determinant of insulin sensitivity whereby higher levels indicate greater sensitivity (because the tissues are taking up glucose so rapidly that more must be infused to maintain fasting glucose levels).

Back to the study

The researchers took a group of rats that were raised under standard rodent conditions (meaning they were metabolically normal), and after an overnight fast, intravenously supplied them with one of the three oil emulsions (lard, soybean, or olive) or a saline control. The rough composition of the oils appears below.





Lard (SAT)




Soybean (POLY)




Olive (MONO)




The infusion rate wasn’t huge, and was designed to elevate plasma fatty acid concentrations to levels seen after meals or during fasting (~3-fold increase). This helps make the results more physiologically relevant. The infusion went on for seven hours, and the euglycemic clamp with tracer isotopes of glucose began at the five hour mark. Blood samples were collected during the last 30 minutes of infusion.

What was found?

As per the study design, free fatty acid concentrations were elevated three- to four-fold above the saline control in all the fat groups with no significant differences between one another. That said, the concentration was lower (3-fold increase) in the SAT group relative to the MONO or POLY groups (4-fold increase).

As mentioned in the paragraph about the glycemic clamp, insulin was infused at a constant rate in all the groups. All three fat groups demonstrated greater plasma insulin levels than the saline control, but the differences were only significant for the SAT and POLY fat groups. At physiological levels of insulin, the liver is the primary site of clearance (i.e. insulin degradation). In order for insulin to be cleared, it must bind to insulin receptors on the liver. By having significantly greater plasma insulin levels in the current study, it demonstrates a reduction in insulin clearance, suggesting that the SAT and POLY interfere with insulin binding on the liver. Moreover, it has been shown that there is an association between levels of liver fat and insulin clearance in persons with and without type-2 diabetes, and that impaired insulin clearance is a factor in obesity and predictor of type-2 diabetes even after adjustment for demographics, lifestyle factors, HDL cholesterol, indexes of obesity and adiposity, and insulin secretion.

The above is supported by the rate of endogenous glucose production, since the liver produces glucose constantly and requires insulin binding to tell it to stop. Thus, greater levels of endogenous glucose production indicate insulin resistance of the liver. In this regard, glucose production was significantly greater in all fat groups relative to the control, and there were no significant differences between the fat groups.

Given the link between insulin resistance of the liver and PKC-δ, the researchers also measured activation of this protein. It was found that all three fat groups significantly reduced cytosolic PKC-δ levels compared to the control, although the membrane concentration was only significantly elevated in the MONO group and this strongly correlated with levels of diacyl-glycerol (DAG). DAG is a precursor for triglyceride synthesis and requirement in the activation of Protein kinase C (PKC), which acts to promote the creation and release of glucose from the liver. Its elevation thus indicates some degree of insulin resistance as insulin binding to the liver deactivates of PKC. Additionally, only the POLY group significantly increased liver ceramide content, which is a molecule involved in apoptosis.

Moving back to the paragraph about the glycemic clamp, the glucose infusion rate was significantly reduced in all fat groups relative to the saline control, indicating reduced whole-body insulin sensitivity. We know that this was in part mediated by the reduced insulin sensitivity of the liver. This reduction in insulin infusion rate was about 54% in the MONO and POLY groups, with no significant difference between the two. However, the reduction was 74% in the SAT group, and although not statistically significantly different from the MONO or POLY groups, the p-values were much lower (p=~0.29) compared to the p-value between the MONO and POLY group (p=0.9981), suggesting that perhaps a larger sample size or longer infusion duration (to better mimic long-term changes in plasma fatty acid concentrations with diet or obesity) may yield different results. Similarly, the fatty acid concentrations of the SAT group were lower, which may have underestimated the size of the effects.

For peripheral insulin sensitivity (i.e. glucose uptake into muscle and adipose tissue), only POLY and SAT fat groups demonstrated significant reductions, whereas the MONO fat group showed a trend for reduction (p=0.0705). Similar to above, SAT fat showed the greatest detriment compared to MONO and POLY although the difference was not significant. The longer infusion duration and/or greater number of subjects would likely make the MONO trend become significant and may also have a more noticeable difference between the groups.

Bringing it all together

Overall, physiologically elevated levels of plasma fatty acids (such as after a meal) reduce whole-body insulin sensitivity and glucose uptake independent of the type of fatty acid. However, the effects appear to be more pronounced with saturated fatty acids, which may have become apparent if the SAT group had equally elevated blood fatty acid concentrations from the beginning (remember it was only about 75% the level of the MONO and POLY groups), if there was a larger sample size, or if the infusion duration were longer.

This is supported by previous human studies. In obese persons, oral ingestion of fat emulsions containing predominantly monounsaturated, polyunsaturated (ω6), or saturated fat resulted in insulin resistance only in the saturated condition. Similarly, I have previously written about how adding butter (mostly saturated fat) to a potato leads to a more pronounced insulin response to the meal, suggestive of peripheral insulin resistance. I have also written about how low-fat diets are more beneficial for reducing liver fat than high-fat diets, which we know is associated with insulin resistance of the liver, and that even a single high-fat meal can increase liver fat accumulation for at least five hours afterwards. Finally, we looked at a brief review of fat quality that concluded saturated fats to be the most detrimental in terms of liver fat accumulation during weight-stable conditions and over-eating.

Bottom line

Thus far, the evidence supports the notion that both high-fat diets and high-fat meals lead to impaired glucose tolerance and reduced whole-body insulin sensitivity. Regarding diet, seeing how all types of fat are detrimental when intake is greater than 37% of total energy intake, it appears prudent to not exceed this amount to maintain insulin sensitivity. Regarding single-meals, it appears prudent to not consume meals composed of both fats and carbohydrates, as this leads to what has been termed the Randle Cycle.

An addendum

The day after this study published I received some great input on Facebook and a couple questions that I thought were worth including in this article. A friend asked the questions, "this only applies to the meal with carbohydrates correct? So you can still eat a meal thats really high in fat early on in the morning and includes most of your calories, and then a meal later on at evening with carbs but little fat and not be insulin resistant?"

My response was basically that this is where things start to get more complicated (and interesting). The fatty acid levels in this study mimic a post-prandial state, but they also represent a level seen during times of fasting (such as during sleep) if a person is obese because plasma fatty acid levels correlate with the amount of body fat you carry. This may partially explain why excessive adipose tissue causes insulin resistance (also the release of inflammatory cytokines, but that is another discussion).

So the answer to his question really depends. For someone who is relatively lean, the answer is likely yes. For an overweight and/or obese person, the answer is probably no. It is also of note that a single high-fat meal can elevate liver fat levels for at least five hours afterwards in healthy young lean men & women. To quote myself from the hyperlinked article above, "this got me thinking. Many people backload their carbohydrates and eat most their fat in the morning meals. If liver fat remains elevated for five hours after a fatty meal, and liver fat is central to insulin resistance, would it be so off-base to speculate that the carbohydrate meal later in the day may run into an acute insulin resistance situation brought about by the morning feeding? Perhaps after five hours the levels normalize, but given that they were hardly changed from the 3-hour mark, they may stay elevated for some time. We don't know. We also don't know how great an impact this slight elevation would have on insulin signaling, or even if this hepatic accumulation of lipids would occur in the absence of carbohydrates. The meals contained an astonishing 92g of carbohydrates, and overall the meals made up 50% your daily calories. So, food for thought."


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