By Brenda Davis RD, co-author of the very new “Becoming Vegan: Comprehensive Edition, The Complete Reference to Plant Based Nutrition” (2014) and of the award winning “Becoming Vegan: Express Edition” (2013).
When foods are subjected to heat, especially at high temperatures, several by-products can form that are very damaging to human health. Among the most notorious are heterocyclic amines (HCAs), polycyclic aromatic hydrocarbons (PACs), advanced glycation end-products (AGEs), and acrylamide.
Heterocyclic Amines and Polycyclic Aromatic Hydrocarbons
Heterocyclic amines (HCAs) are chemicals that are created when meat, poultry, fish and eggs are subjected to high temperature cooking, such barbequing, frying, or grilling. The formation of these compounds increases with temperature and duration of cooking. Cooking methods that use direct heat (e.g. frying or grilling) produce more HCAs than do indirect-heat methods such as stewing, steaming or poaching.
Many vegetarians wonder if HCAs are produced in a similar manner when vegetables are cooked at high temperatures. The answer is no because the formation of these compounds involves the condensation of creatinine with amino acids (the building blocks of protein), and creatinine is found exclusively in muscle tissue. In January, 2005, HCAs were officially added the list of cancer causing agents put out by the National Institutes of Health (NIH).
Polycyclic aromatic hydrocarbons (PAHs) are a group of over a hundred different chemicals that are formed by the incomplete burning of organic substances such as oil, gas, coal, forests, garbage, tobacco and food or the pyrolysis of fat at temperatures in excesses of 392 degrees F. Food accounts for over 90 percent of our PAH exposure. The most concentrated sources of PAHs are grilled or charred meat, poultry and fish, grains, fats and oils, and sweets. Vegetables, fruits, beverages and dairy products also contribute to overall intake. However, PAHs in vegetables and fruits are due largely to environmental contaminations of the air and soil. Where contamination is minimal, produce will contain few PAHs.
A host of factors affects PAH formation, apart from environmental contamination. The primary factors include the temperature of cooking, distance from the heat source, duration of cooking, type of fuel used in heating, amount of fat in the food, and whether the fat drips onto the heat source and rises back onto the food. Raw grains tend to be low in PAHs, however levels increase with certain processing techniques, such as direct combustion gas heating and toasting. While sprouted grains are a very minor source of PAHs, ready-to-eat cereal generally contain much higher levels. Processed sweets can also contribute to PAH intake. Fats and oils are a significant source, as the foods from which they are extracted are often contaminated. In addition, direct combustion gases and solvents used in processing can further increase levels.
Heterocyclic amines and polycyclic aromatic hydrocarbons are known to be mutagenic (that is, they damage DNA). Evidence suggests that HCAs increase our risk for a variety of cancers, including colorectal, stomach, pancreatic and breast cancers. Cancers thought to be linked to PAH intake include lung, skin and genitourinary. Most of our exposure to these compounds comes through our food, and the vast majority through high temperature cooking. Vegetarian diets generally contain negligible amounts of HCAs, and moderate amounts of PAHs. Intakes of those eating raw or high-raw diets would be considerably lower.
Advanced Glycation End-Products (AGEs)
Advanced glycation end products (AGEs) are harmful end-products of the Maillard reaction or of fat oxidation, which are formed when food is heated to high temperatures. They can also be formed within the body when sugar molecules attach to protein, fat or DNA.
Begin Breakout Box
What is the Maillard Reaction?
The Maillard reaction is a form of non-enzymatic browning that occurs when sugars, such as glucose or fructose, combines with an amino acid. The chemical reaction, which is called glycation, results in intermediate products of the Maillard reaction. If heating continues, further chemical reactions may result in irreversible bonding of amino acids (protein cross-links), and the formation of advanced glycation end-products (AGEs). The cross-linking of protein may also result in the destruction of essential amino acids or a reduction in their availability.
The Maillard reaction begins to escalate when foods are heated to temperatures of 310 degrees F or higher. Products of the Maillard reaction are minimized when foods are boiled or steamed, as temperatures reach only 212 degrees F. The food industry purposefully uses the Maillard reaction to improve the flavor and color of foods. Good examples of this are the production of colas, coffee, caramel and many baked goods. The potential harmful effects of the end-stage products of the Maillard reaction were not well recognized until the 1970’s when products of non-enzymatically glycated hemoglobin where found in diabetic patients.
There is evidence that AGEs impair immune system function, accelerate aging, and contribute to the progression of Alzheimer’s disease, cardiovascular disease, diabetes, eye diseases, kidney disease, nerve diseases and stroke.
Research has shown that about 10 percent of the AGEs in food are absorbed into the system. Restricting foods rich in AGEs has been shown to significantly reduce circulating AGE levels in the body, and levels of C-reactive protein, a marker for inflammation (high levels of C-reactive protein indicate greater inflammation in the body).
Foods that are most concentrated in AGEs are broiled, grilled and fried meats. See Table 3.5 for a list of AGEs in common foods. The average daily AGE intake (based on food records) for people eating a standard Western diet has been estimated at approximately 16,000 kilounits (ku). Vegetarian diets tend to contain lesser amounts, unless they are very high in processed and fried foods. Raw food diets contain minimal AGEs.
AGE Content of Common Foods
|Food Group||Amount||AGEs (kU)|
|Frankfurter, broiled 5 minutes||3 oz (90 grams)||10,143|
|Chicken breast, breaded, fried 25 minutes||3 oz (90 grams)||8,965|
|Hamburger, fried, fast food||3 oz (90 grams)||4,876|
|American processed cheese||1 oz (30 grams)||2,603|
|Trout, roasted 25 minutes||3 oz (90 grams)||1,924|
|Cottage cheese, 1%||4 oz (120 grams)||1,744|
|Egg, fried with margarine||45 grams||1,237|
|Infant formula||1 cup (250 ml)||1,212|
|Chicken breast, boiled 1 hour||3 oz (90 grams)||1,011|
|Chicken breast, raw||3 oz (90 grams)||692|
|Human milk||1 cup (250 ml)||16|
|Whole fluid milk||1 cup (250 ml)||12|
|Fats and Oils|
|Butter||1 tablespoon (15 grams)||3,972|
|Margarine||1 tablespoon (15 grams)||2,628|
|Plant Foods (including processed foods)|
|Tofu, broiled||3 oz (90 grams)||3,696|
|Roasted nuts (average)||1 oz (30 grams)||2,000-3,000|
|Potatoes, fried, fast food||100 grams||1,522|
|Waffle, frozen||1 oz (30 grams)||861|
|Tofu, raw||3 oz (90 grams)||709|
|Rice Krispies||1 oz (30 grams)||600|
|Beans, kidney, canned||100 grams||191|
|Potatoes, boiled 25 minutes||100 grams||117|
|Beans, kidney, raw||100 grams||116|
|Whole wheat bread, toasted||1 oz (30 grams)||30|
|Whole wheat bread||1 oz (30 grams)||19|
|Oatmeal, instant, dry||1 oz (30 grams)||4|
Acrylamide is a chemical that is used to make polyacrylamide, which is employed in the manufacturing of certain glues, cosmetics, food packaging materials, soil conditioning agents, plastics and grouting agents. It is also used in water treatment. Although polyacrylamide is non-toxic, small quantities of acrylamide residues, which appear to be less benign, remain in finished products.
In April 2002, Swedish researchers discovered the presence of acrylamide in some starchy foods. They noted that acrylamide appears to form spontaneously when certain foods, particularly those that are high in carbohydrate and low in protein such as potatoes, are subjected to high cooking temperatures. The higher the cooking temperature and the longer the cooking time, the greater the acrylamide concentration becomes. Canadian scientists discovered that most acrylamide in food is formed when the amino acid asparagine reacts with naturally occurring sugars such as glucose. This generally occurs at the later stages of baking, roasting, or frying when the moisture content falls, and the surface temperature rises.
The most concentrated food sources of acyrlamide are processed foods such as potato chips and other baked or fried salty snacks and French fries, as potatoes are particularly high in asparagine. Other food sources include crackers, crispbreads, pretzels, breads (especially toasted), cold cereals, and other foods processed at high temperatures such as coffee and cocoa. Acrylamide is also formed during the home preparation of starchy foods. Any significant formation of acrylamide requires temperatures of 248 degrees F or higher. Consequently, it is found only in insignificant amounts in steamed or boiled foods.
There are a number of potential health concerns associated with acrylamide consumption. Occupational and accidental exposures to high doses have been shown to cause neurotoxicity in humans. Experimental studies suggest that it could also be toxic to genetic material, increasing risk of cancer and reproductive problems, although these findings have not been confirmed in humans. Acrylamide was evaluated by the International Agency for Research on Cancer in 1994 and classified as “probably carcinogenic to humans” on the basis of a positive cancer bioassay result.
Vegetarians are not necessarily at an advantage over non-vegetarians in terms of acrylamide intake. If vegetarians consume large amounts of baked or fried starchy foods, they may be at a disadvantage. On the other hand, vegetarians relying more heavily on raw foods would have significantly lower intakes.
Experts consistently agree that we should keep our intakes of heterocyclic amines, polycyclic aromatic hydrocarbons, advanced glycation end products, and acrylamide as low as possible. Increasing our intake of raw foods, and selecting lower temperature, wet cooking methods will help to minimize the production of these harmful compounds in our food.
This article is adapted from Becoming Vegan: Comprehensive Edition (Davis, B, Melina, V. Book Publishing Company, 2014). Further information, references and resources can be found in this book.