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.
Heating to temperatures most commonly used in cooking (≤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.
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% [13]. 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 [14][15][16][17]. 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 [14][18]. 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 [19]. Thus, cooking at temperatures much lower than 240 °C is important for avoidance of generation of such compounds in addition to TFA [20].
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