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Biochemistry in the news: trans fatty acids

Food and Drug Administrion News Release, July 9, 2003 " Under the new FDA regulations, by Jan. 1, 2006, consumers will be able to find trans fat listed on food nutrition labels directly under the line for saturated fat. The new information is the first significant change on the Nutrition Facts panel since it was established in 1993."

 

What is trans fat, how does it get into our food, and what are its effects on human health?

Traditionally, animal fats like butter and lard (saturated fats) were used as spreads and for baking. Vegetable oils (unsaturated fats) were cheaper, but as liquids they were not a suitable alternative. Unsaturated fats have a structure with kinks; these kinks result in a liquid state at room temperature. In the early 20th century, a chemical process called hydrogenation was developed that converts vegetable oils into saturated, more solid fats (margarine and vegetable shortening). When it was discovered that eating saturated fats increases the risk for coronary heart disease, the food industry turned to partial hydrogenation. This process lowerd the content of saturated fat in vegetable shortening and margarine, but also dramatically increased the amount of a certain kind of fat - trans fat - in our diets. While suppliers praised processed vegetable oils as healthy unsaturated and cholesterol-free substitutes for animal fats, there is now strong evidence that introducing trans-fatty acids into our diets does more harm than good.

Structure and properties of fatty acids

Fat molecules are made of three fatty acid molecules that are bound to glycerol. Fatty acids consist of a carboxylic acid group (shown in red in the figure) attached to a linear chain of carbons, a so-called alkyl chain. Different sources of fat (vegetable fat from soybean, corn, olives etc, animal fat from pork, beef, fowl or fish) have different compositions of fatty acids. The figure gives three examples of fatty acids that all contain 18 carbons (shown in grey in the figure), but differ in the number and placement of hydrogen atoms (white spheres in the figure). Stearic acid is a so-called saturated fatty acid, in which every carbon atom in the alkyl chain is bound to four other atoms. These alkyl chains are called saturated because the carbon atoms bind to the maximum possible number of hydrogen atoms.

In contrast, oleic acid is an unsaturated fatty acid; the carbon atoms shown in blue are bound to three atoms only (to two carbon atoms and one hydrogen atom each). To "make up" for the missing binding partners, the two carbon atoms form a double-bond, which has consequences for the shape of the entire molecule. (In the chemical formulas on the left, all hydrogen atoms are omitted except for the ones on the double bond.) Double bonds come in two flavors, cis and trans. In trans-compounds, two hydrogen atoms are on opposite sides of the double bond (the alkyl chain runs zig-zag in this case), whereas in cis-compounds, two hydrogen atoms are on the same sides of the double bond (the alkyl chain runs zig-zig in that case). Cis and trans compounds are different molecules, with different physical and chemical properties.

For saturated alkyl chains, we do not have to distinguish between cis and trans because the molecules are able to rotate around the single bonds, so every bond can be transoid at one time and cisoid at another. In the figure, the molecules are shown as all-transoid (i.e. zig-zag-zig-zag...) because this is the most stable conformation. At very low temperature, fatty acids will predominantly have the shown elongated shape, but at higher temperatures, they will be dynamic, kinking in all kinds of ways.

As the figure shows, the shape of trans fatty acids is in between that of cis fatty acids, which are kinked, and that of unsaturated fatty acids, which are straight. The straighter the molecule, the easier it is to pack densely in a solid. Thus, oleic acid (cis-unsaturated) is liquid at room temperature (it solidifies below 13C), whereas its trans isomer elaidic acid is a solid at room temperature (it melts at 44C). The saturated stearic acid is a solid that melts at even higher temperature, 72C.

 

Sources of trans-fatty acid in our food

Animal and plants do not produce trans fatty acids. Milk and beef do contain some trans fatty acid, produced by bacteria that help cows digest foods in their digestive systems. However, most of the trans-fats in our current diet originates from processed vegetable fats.

Traditionally, lard (pork fat) was used for baking, and butter to spread on bread. Both butter and lard contain saturated fats and cholesterol. Vegetable fats are cheaper but they don't keep as well and are often liquid rather than solid (soybean, corn, sunflower oil) because they contain unsaturated fatty acids. Hydrogenation is a process invented in the 1930's in which the double bonds of unsaturated fats are opened and hydrogen atoms are added. If all double bonds are removed, the product is saturated fat; if some of the double bonds remain, the product is called partially hydrogenated vegetable oil.

Two commonly used products are Crisco (contains partially hydrogenated soybean and cottonseed oils) and margarine (the Land o Lakes brand for instance contains liquid soybean oil and partially hydrogenated soybean oil, with 22% of the fat composed of trans fatty acids) . With the rise in life-expectancy and sedentary lifestyle, and subsequent rise in cardio-vascular disease, partially hydrogenated fats were marketed as healthy alternative to animal fats because the former don't contain any cholesterol.

In the process of partial hydrogenation, some of the fatty acids rearrange in a way that the natural cis-double bonds are replaced by trans-double bonds. This cis-trans isomerization also occurs in other processes in which fats are heated for extended time, for instance in the deoderization of rapeseed or fish oils.

 

Effects of trans-fats on human health

The fats and oil industry is a big business : "In the USA markets for oils and fats constitute about USD 5 billion (Euromonitor) or about 25% of the world market.  The vegetable oil sales account for USD 1,3 billion and in volume terms constitute 2 668 000 tons  (Euromonitor)." Source (For comparison, the 2000 GNP was 10,500 billion dollars).

When issuing statements about the risk of trans-fats, federal agencies like the FDA and the NIH strike a balance between potential improvement in public health and economic impact. The quote below reflects this balance, and is formulated quite tentatively.

"Trans Fat: Experts could not provide a reference value for trans fat nor any other information that FDA believes is sufficient to establish a Daily Value or %DV. Scientific reports link trans fat (and saturated fat) with raising blood LDL ("bad") cholesterol levels, both of which increase your risk of coronary heart disease, a leading cause of death in the US.

Important: Health experts recommend that you keep your intake of saturated fat, trans fat and cholesterol as low as possible as part of a nutritionally balanced diet." Source: FDA

Compare this with the statement of health benefits of olive oil:

"Limited and not conclusive scientific evidence suggests that eating about 2 tablespoons (23 grams) of olive oil daily may reduce the risk of coronary heart disease due to the monounsaturated fat in olive oil.  To achieve this possible benefit, olive oil is to replace a similar amount of saturated fat and not increase the total number of calories you eat in a day. One serving of this product contains [x] grams of olive oil." Source FDA

The Harvard School of Public Health is more direct in its warning against intake of trans fats:

"Trans fats are even worse for cholesterol levels than saturated fats because they raise bad LDL and lower good HDL. While you should limit your intake of saturated fats, it is important to eliminate trans fats from partially hydrogenated oils from your diet." Source Harvard SPH

 

Fat consumption in the US

Although it has been known for quite some time that sedentary people eat too much fat, the per capita amount of added fat consumed in the US has increased over the past 100 years (see table below). The striking changes in the types of fats consumed reflect changes in technology and pricing, but also changing ideas about the nutrional value of different types of fat.

 

Added fats and oils: Per capita consumption
Year
U.S. total population, July 1 1
Butter
Margarine
Lard 2
Edible beef tallow 2
Shortening
Salad and cooking oils 3 4
Other edible fats and oils 5
Total product weight 6
Total fat content 6 7
Animal
Vegetable
Total
  Millions
----------------------------------------------------------------------------------- Pounds -------------------------------------------
1910
92.407
18.4 1.6
8.2
NA
8.0
NA
1.5
37.7
NA
NA
33.7
44.8
45.1
82.1
1930
123.188
17.6 2.6
12.7
NA
9.8
NA
6.2
48.8
NA
NA
1960
180.671
7.7 9.3
7.4
NA
12.6
NA
11.5
48.5
NA
NA
2000
282.388
4.5 7.5
1.9
4.0
31.3
33.7
1.5
84.5
12.6
69.5
NA = not available.1 Prior to 1930, except for the war years, 1917-19, resident population only, from 1930 and thereafter, resident population plus Armed Forces overseas. 2 Direct use. Excludes use in margarine, shortening, and nonfood products. Edible beef tallow is included in lard before 1979. 3 Included in other edible fats and oils before 1965. 4 Includes edible rapeseed (canola) oil beginning in 1985. 5 Specialty fats used mainly in confectionery products and non-dairy creamers. 6 Computed from unrounded data. 7 Fat content of butter and margarine is calculated at 80 percent of product weight.
Source: USDA/Economic Research Service estimate using data from Census Bureau. Data last updated Dec. 21, 2004.

 

What is my take-home message?

I think the changing technology, changing patterns of consumption and emerging data on health effects for fats and oils illustrates the challenges we face in providing healthy and satisfying food for a growing world population. The food industry develops new products rapidly, and the data on the potential health benefits or risks lags behind (by a decade or more because the effects are long-term and because studies have to be conducted on large groups of people to uncover the effects of a single risk factor among many affecting our health). Moreover, humans have not yet adapted to new kinds of foods (and won't in another 10,000 years), so any new product carries a certain amount of risk. On the other hand, humans haven't adapted either to high population density, environmental pollution and sedentary life style. In addition, life expectancy has increased steadily, with major causes of death changing from starvation, infectious disease, and murder to coronary heart disease, cancer and Alzheimer's in parts of the world. Thus, we face new food-related risks not only because foods have changed, but because many other circumstances have, too.

Weighing all the risks and benefits, I think that it is best to eat unprocessed, home-prepared foods as much as possible, if you can afford to and have the time to prepare them. Adjusting dietary intake by choosing among foods that have been consumed by humans for centuries rather than switching to novel foods seems the best course of action to me. It is also the tastiest way to eat!

Sources

ALFONSO VALENZUELA and NORA MORGADO, 1999

Sandra Laursen and Heather Mernitz

Harvard School of Public Health, Review

...and links

Margarine brand names

scientificpsychic .. on food labels

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Karsten Theis, 3-29-2005