Grilled and fried food may raise Alzheimer's risk, study finds
It's not just what we eat, but how we cook it, that influences our health as we grow older
Swapping that grilled beef burger for beef stew, or having poached salmon instead of fish and chips, could benefit more than just your waistline. New research suggests that avoiding foods that have been fried, grilled or smoked could help keep your risk of Alzheimer's disease in check.
According to the study, published in the Journal of Alzheimer's Disease earlier this month, the culprit lies in a group of compounds called advanced glycation end products (AGEs) that are formed when foods are cooked or processed using high temperatures, or aged for a long time (such as hard cheese).
Based on data from clinical studies involving 19 countries, estimates of AGEs in national diets were found to "correspond well" with Alzheimer's disease prevalence, say researchers from the University of Poitiers in France and San Francisco's Sunlight, Nutrition and Health Research Centre.
In typical national diets, meat made the highest contribution of AGEs, followed by vegetable oils, cheese and fish, according to the study report. Foods such as cereals/grains, eggs, fruit, legumes, milk, nuts, starchy roots and vegetables generally make low contributions to the total amount of AGEs in a diet, either because they are generally prepared at low temperatures or they comprise smaller portions of diets.
In their analysis, the researchers used AGE values of many types of food from a 2010 study by scientists at the Icahn School of Medicine at Mount Sinai in New York. The Icahn scientists had cooked 549 foods using different methods: boiling (100 degrees Celsius), grilling (225 degrees), deep-frying (180 degrees), oven-frying (230 degrees) and roasting (177 degrees).
It was found that the higher the cooking temperature, the higher the AGE content. A hundred grams of raw beef, for example, had 707 kilounits (kU) of AGEs, while 100g of stewed beef had 2,230kU.
Apart from heat, the level of water present during food preparation had an effect on dietary AGEs content. Exposure to higher temperatures and lower moisture levels coincided with higher AGEs levels for equal weight of food as compared to foods prepared at lower temperatures or with more moisture.
For example, scrambled eggs prepared in an open pan over medium-low heat had about half the AGEs of eggs prepared in the same way but over high heat. Poached or steamed chicken had less than a quarter of the AGEs of roasted or broiled chicken.
Exposing foods to an acidic environment prior to cooking has also been found to decrease AGEs. That means using lemon juice, vinegar, tomato juice or wine in marinades or cooking liquids could help.
Although it's not yet known just how much dietary AGE intake is harmful, in animal studies a reduction of AGEs by half of the usual intake has been linked with reduced levels of oxidative stress, less deterioration of insulin sensitivity and kidney function with age, and a longer lifespan.
AGEs, which occur naturally at low levels in the body as a result of metabolism and ageing, are a group of compounds that are combinations of sugars and proteins and other large molecules. AGEs increase the risk of various chronic diseases through several mechanisms, including increased inflammation and oxidative stress.
Studies by Icahn researchers in recent years have revealed how dietary AGEs affect our health, in particular increasing body weight and the risk of diabetes.
In their most recent study that appeared this month in the journal Plos One, tests in pre-diabetic mice showed that chronic exposure to dietary AGEs promoted age-accelerated degeneration of spinal discs.
According to Icahn doctors Jaime Uribarri and Weijing Cai, the new study by the researchers in France and San Francisco supports their previous findings in animals and humans of an important role for dietary AGEs in Alzheimer's disease.
"We found that mice kept on a diet high in AGEs similar to Western diets had deposits of amyloid-beta, a component of the plaques characteristic of Alzheimer's disease, while at the same time developed declines in cognitive and motor abilities," they say.
The mice fed a low AGE diet remained free of these conditions.
"The findings point to an easily achievable goal that could reduce the risk of dementia through the consumption of non-AGE-rich foods, raising the importance of not just what we eat, but also how we prepare what we eat."
Five things you should know about foods and AGEs
The Icahn School of Medicine at Mount Sinai in New York cooked food using various methods and measured their levels of dietary advanced glycation end products (AGEs). Here's what they found:
Highest levels of AGEs per serving: meat group Although gram-for-gram, fats tend to contain more AGEs, meats are likely to contribute more to overall dietary AGEs intake because meats are served in larger portions than are fats. Among meat category items prepared by similar methods, the highest AGEs levels are seen in beef and cheeses, followed by poultry, pork, fish and eggs. Lamb ranked relatively low in AGEs compared to other meats.
Lowest levels of AGEs: grains, legumes, breads, vegetables, fruits and milk Non-fat milk had significantly lower AGEs than whole milk. Milk-related products with a high moisture index such as yogurt, pudding and ice cream were also relatively low in AGEs. However, hot cocoa made from a dehydrated concentrate contained significantly higher amounts of AGEs.
Oils and other high-fat products among foods highest in AGEs These include high-fat spreads such as butter, cream cheese, margarine and mayonnaise. With heat kept constant, the type of cooking fat used led to different amounts of AGEs. For example, scrambled eggs prepared with a cooking spray, margarine or oil had about 50 to 75 per cent less AGEs than if cooked with butter.
Carbs tend to contain lower amounts of AGEs This may be due to the often higher water content or higher level of antioxidants and vitamins in these foods, which may diminish new AGEs formation, the researchers say. In this category, dry-heat processed foods such as crackers, chips and cookies contained the highest levels of AGEs per gram, although still far below those present in meats.
Even uncooked animal products can contain large amounts of AGEs Cheese is a key example, and researchers suggest this is likely due to pasteurisation and/or holding times at ambient room temperatures used during curing or ageing processes. Higher-fat and aged cheeses, such as full-fat American and parmesan, contain more AGEs than lower-fat cheeses such as reduced-fat mozzarella, 2 per cent milk cheddar and cottage cheese.