Heating's Influence Trans Fatty Acid in Oils

Heating's Influence Trans Fatty Acid in Oils: History

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Subjects: Chemistry, Organic

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Consumption of trans fatty acids (TFA) is associated with adverse health outcomes and places a considerable burden on morbidity and mortality globally. TFA may be generated by common cooking practices and hence contribute to daily dietary intake. Heating edible oils to common cooking temperatures (≤200 °C) has minimal effect on TFA generation whereas heating to higher temperatures can increase TFA level.

Trans Fatty Acid
cooking
baking

1. Introduction

Trans-fatty acids (TFA) are fatty acids with at least one unsaturated, non-conjugated double bond in the trans configuration. Consumption of TFA is associated with cardiovascular disease risk factors including increased low-density lipoprotein cholesterol, triglyceride, and lipoprotein (a) levels, decreased high-density lipoprotein cholesterol, and increased insulin resistance, adiposity, and endothelial dysfunction [1]. Increased TFA consumption has also been associated with greater incidence of diabetes and coronary heart disease [2,3,4]. Approximately 645,000 annual deaths globally are attributed to a diet high in TFA [5]. In view of its adverse health effects, the World Health Organisation recommends that TFA intake be minimised and contribute no more than 1% of total daily energy intake [6], in line with other food and health advisory organisations [7]. While global TFA intake appears to have gradually declined over the past three decades [8], consumption of TFA remains high in many parts of the world, accounting for up to 6.5% of total daily energy [9].

Trans fatty acids in diet originate from naturally occurring TFA from ruminants or industrially produced TFA formed by partial hydrogenation of vegetable oils. In addition, thermal treatment of cooking oils can alter their physicochemical properties and potentially generate TFA from cis-unsaturated fatty acids during cooking procedures (e.g., deep-, pan-, or stir-frying) [10,11,12].

2. Heating's Influence on Trans Fatty Acid in Oils

Heating to temperatures most commonly used in cooking ( $\leq$ 200 °C) had minimal impact on TFA levels but heating to higher temperatures (>200 °C) could increase levels of TFA. The effect of heating on formation of individual types of TFA appeared to be largely consistent, leading to gradually increasing TFA levels with increasing temperature above 200 °C. Furthermore, levels of some TFA subtypes increased further with prolonged heating, especially at temperatures above 200 °C.

The median TFA levels (shown separately for subtypes of TFA and total TFA) at different temperature intervals are depicted in Table 1. Compared to room temperature, heating oils to temperatures <200 °C (i.e., common cooking temperatures) had no significant impact on TFA levels (Figure 2). However, within this temperature range, C18:3t levels increased significantly, albeit modestly, with higher temperature (0.02% increase per 10 °C rise in temperature, 95% CI: 0.01 to 0.02%). While mean TFA levels in oils heated to between 200 and 240 °C did not differ significantly from TFA levels in unheated or less heated oils, the levels of C18:2t (0.05% increase per 10 °C rise in temperature, 95% CI: 0.02 to 0.05%), C18:3t (0.18%, 95% CI: 0.14 to 0.21%), and total TFA (0.38%, 0.20 to 0.55%) increased with rising temperature within this temperature range.

Figure 2. Change in C18:1t (A), C18:2t (B), C18:3t (C), and total TFA (D) (% of total fatty acids) content of cooking oil as a function of heating temperature. Data were fitted using a mixed multilevel linear regression model adjusted for heating time and oil type with random intercepts for studies and spline knots at 200 and 240 °C. Slopes represent change in TFA per 10 °C change in heating temperature within a particular spline range (<200 °C, 200–240 °C, >240 °C). Data point colours represent different studies.

Table 1. Levels of different trans fatty acid (TFA) (% of total fatty acids) as a function of heating temperature. Changes in TFA levels resulting from heating oils above room temperature as well as the difference in TFA levels in oils heated to high (>200 °C) versus usual cooking temperatures (

\leq

200 °C) are depicted for each TFA studied.

		TFA Concentration (% of Total Fatty Acids)
		Unheated		<200 °C		200–240 °C		>240 °C
Fatty acid		n studies (n samples)	Median (IQR)/ Estimate (95% CI); p	n studies (n samples)	Median (IQR)/ Estimate (95% CI); p	n studies (n samples)	Median (IQR)/ Estimate (95% CI); p	n studies (n samples)	Median (IQR)/ Estimate (95% CI); p
16:1t	Median	0 (0)	-	4 (60)	0.02 (0.01; 0.03)	2 (30)	0.02 (0.01; 0.03)	1 (16)	0.02 (0.00; 0.02)
	Difference from usual cooking temperatures		-		Reference		0.00 (−0.00, 0.01); p = 0.62		−0.01 (−0.01, 0.00); p = 0.052
18:1t	Median	3 (14)	0.35 (0.06, 0.56)	13 (306)	0.24 (0.03, 1.49)	5 (79)	0.07 (0.01, 3.72)	3 (37)	1.08 (0.08, 3.20)
	Difference from unheated		Reference		0.71 (−1.11, 2.54); p = 0.45		0.61 (−1.26, 2.48); p = 0.52		2.06 (0.10, 4.02); p = 0.04
	Difference from usual cooking temperatures				Reference		−0.10 (−0.97, 0.77); p = 0.82		1.33 (0.07, 2.60); p = 0.039
18:2t	Median	1 (1)	0.13 (-)	10 (178)	0.31 (0.01, 0.50)	4 (69)	0.42 (0.03; 0.62)	1 (48)	0.48 (0.21; 0.98)
	Difference from unheated		Reference		0.33 (−0.51, 1.18); p = 0.45		0.38 (−0.47, 1.23); p = 0.38		0.76 (−0.10, 1.61); p = 0.084
	Difference from usual cooking temperatures				Reference		0.05 (−0.07, 0.18); p = 0.42		0.43 (0.28, 0.57); p < 0.001
18:3t	Median	2 (7)	0.01 (0.00, 0.20)	4 (56)	0.01 (0.00, 0.27)	3 (27)	0.01 (0.00, 0.30)	1 (12)	0.70 (0.53, 1.48)
	Difference from unheated		Reference		0.11 (−0.23, 0.45); p = 0.52		0.20 (−1.14, 0.53); p = 0.25		0.80 (0.44, 1.15); p < 0.001
	Difference from usual cooking temperatures				Reference		0.05 (−0.11, 0.21); p = 0.52		0.62 (0.37, 0.88); p < 0.001
Total TFA	Median	1 (1)	0.09 (-)	10 (117)	0.97 (0.62, 1.53)	5 (52)	1.42 (0.87; 3.70)	2 (17)	1.54 (0.97; 4.10)
	Difference from unheated		Reference		2.14 (−8.24, 12.52); p = 0.69		2.49 (−7.93, 12.90); p = 0.64		3.78 (−6.84, 14.40); p = 0.49
	Difference from regular cooking temperatures				Reference		0.34 (−1.42, 2.10); p = 0.70		1.64 (−1.22, 4.49); p = 0.26

Even at the temperature range >200 °C (where significant increase in TFA was observed), the magnitude of the increase appears to be relatively small. For instance, the expected increase in total TFA for an increase in temperature between 25 °C (room temperature) and 220 °C is 7.4% (after a median cooking time of 45 min of the included studies), whereas the level of TFA in partially hydrogenated vegetable oil, the major target of global TFA elimination, is typically around 25–40% [46]. Nonetheless, these findings affirm recommendations from several European countries that to minimise formation of harmful TFA, frying oil should not exceed common cooking temperatures (i.e., <200 °C) and support other public health recommendations that prolonged and/or repeated use of cooking oils should be avoided, which may be particularly relevant for some informal food sectors in low- and middle-income countries where such practices may be common [47,48,49,50]. However, there appears to be a lack of formal guidance on the repeated use of cooking oils at both an international and national level, with the Food Safety and Standards Authority of India being a notable exception [47,51]. There may be a need to support vendors in both the formal and informal food sectors to avoid the practice of reusing cooking oils, such as through targeted education programs or subsidised access to fresh oils and used oil waste removal.

Though some evidence to suggest that under various heating conditions each of the subtypes of TFA assessed here and total TFA increase, C18:3t was found to be the TFA that most readily increased (i.e., even below 200 °C, it showed significant increase). This may indicate that the precursor fatty acid (C18:3, alpha-linolenic acid) is more susceptible to the effects of heating than other mono- and polyunsaturated fatty acids (i.e., oleic acid and linoleic acid for C18:1 and C18:2, respectively). This is a novel finding which suggests that as such, the avoidance of cooking oils that contain high levels of C18:3, such as various seed oils, in cooking methods reaching high temperatures may be a useful additional way to avoid the generation and consumption of TFA. It should be noted that 240 °C is above the smoking point of most cooking oils, i.e., the temperature at which oil starts to vaporise, which is typically undesirable for appearance, taste, and utility. Heating oils above their smoking point would not only impact the fat quality and lead to the generation of TFA but could also increase the levels of carcinogenic compounds [52]. Thus, cooking at temperatures much lower than 240 °C is important for avoidance of generation of such compounds in addition to TFA [43].

While heating edible oils to commonly used cooking temperatures has little effect on TFA generation, heating to higher temperatures and for a longer period of time can increase TFA levels. These findings provide further evidence that prolonged heating of edible oils to very high temperatures may be harmful and should be avoided to reduce dietary intake of TFA.

This entry is adapted from the peer-reviewed paper 10.3390/nu14071489

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