October 24, 2024, 8:07 am | Read time: 5 minutes
Fats are important for health, but not all fatty acids are good for the body. A large-scale study by the University of Cambridge investigated the effect of saturated fatty acids on cardiovascular and total mortality risk over four decades. FITBOOK nutrition expert Sophie Brünke presents the results.
Saturated fats are not bad per se but should not be consumed in excess. They are mainly found in animal products, such as high-fat cheese, sausage, and meat products. The German Nutrition Society recommends that these fatty acids should not make up more than seven to ten percent of your daily energy intake – in favor of unsaturated fats. This would prevent numerous diseases, such as obesity, lipometabolic disorders, and coronary heart disease.1 But how are saturated fatty acids linked to the risk of death? A Norwegian research team has investigated this.
Overview
Study ran for four decades
The results of a Norwegian cohort study were recently presented in the British Journal of Nutrition. The scientists investigated the relationship between total saturated fat intake and specific saturated fatty acid (SFA) intake and cardiovascular and total mortality risk.2
A total of 78,725 subjects from three Norwegian counties participated, with an average age of 41.1 years. There were three screening appointments between 1974 and 1988, each with a participation rate of over 80 percent. Subsequently, the test subjects were followed up until their death, which could last up to 45 years. The information on the time and cause of death was taken from the national death registry.
How was the data collected?
Each screening included physical examinations and a questionnaire that collected information on lifestyle habits (e.g., smoking), medical history (e.g., stroke), and marital status. People with high-risk factors for cardiovascular disease were referred to their GP for a follow-up examination.
Diet was recorded at each screening with a semi-quantitative Food Frequency Questionnaire (FFQ), which takes into account foods consumed daily to infrequently, but not their quantity. For example, the test subjects were able to state how often they ate bread, spreads (including butter and margarine) and toppings, fats for cooking and baking, milk, cheese, meat, fish, potatoes, and eggs. For a more accurate assessment, some of the participants also completed 24-hour dietary recall logs at the screenings.
Saturated fats were associated with a higher risk of death
Overall, the study results showed a positive correlation between SFA intake and cardiovascular and all-cause mortality. In other words, the consumption of saturated fats increases the risk of death.
The average intake of SFA at the start of the study was between 10.7 and 19.6 percent energy. A higher intake of SFA during the follow-up period was associated with higher total cholesterol and lower HDL cholesterol levels – risk factors for cardiovascular disease.
Among all participants, 28,555 deaths occurred during an average follow-up period of 33.5 years, of which 9318 were due to cardiovascular disease. The risk of death increased with the consumption of SFA. Those subjects with the highest consumption of saturated fats had a 25 percent higher overall risk of death. A five percent increase in SFA intake was associated with an 18 percent higher overall mortality risk. With regard to death from cardiovascular disease, a five percent increase in SFA intake increased the risk of death by 16 percent.
Of the individual SFAs studied, myristic and palmitic acids were positively associated with all-cause mortality. Both are frequently found in food. The former is found in butter, for example, and the latter in lard.
Replacing saturated fats with other fats or carbohydrates reduced the risk of death
The scientists wanted to find out mathematically whether replacing some of the saturated fats with monounsaturated fatty acids or carbohydrates could reduce the risk of death – with the same total energy intake. In fact, every five percent increase in energy intake of carbohydrates (at the expense of SFA) resulted in a 15 percent reduction in overall mortality risk. In the case of unsaturated fats, this reduction amounted to 24 percent.
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Classification of the study
The study is characterized by a large sample size, repeated measurements, and a follow-up period spanning four decades. In addition, an unselected group of relatively young and healthy subjects (at baseline) was examined, which, according to the study authors, reduced the risk of reverse causality and survival bias. Another strength of the study was the consideration of comorbidities (e.g., stroke). The repeated measures were particularly important as dietary habits evolved over the years in Norway. An additional advantage was that the authors obtained detailed information on the composition of the kinds of margarine, including trans fatty acids, from the industry.
However, the study also had some weaknesses. For one thing, the subjects self-reported their diet, which favors potential inaccuracies. Secondly, the questionnaire focused mainly on fat intake, so other nutrients may have been underestimated. A comparison of the FFQs (questionnaires) with the 24-hour logs revealed that the FFQ underestimated total calorie intake by 17 to 20 percent, especially energy from fruit and vegetables. However, macronutrient densities (and therefore fat) were consistent. When analyzing specific SFAs, rarely occurring SFAs could not be taken into account. Nutrient composition calculations were based on tables that were current between 1984 and 1991. In addition, subjects were not asked if they were taking lipid-lowering medications during follow-up. Due to changes in diet and risk profiles, it remains unclear whether the study results can be fully extrapolated to today’s population, according to the authors.