A Spanish study published in the December issue of American Journal of
Clinical Nutrition found in deep-frying mixed oil and sunflower oil raised
blood pressure compared with olive oil. Reusing oil degraded in previous
heatings also raised blood pressure, even when olive oil was used.

Below comments from news articles about the study:

Reused cooking oil raises blood pressure
<URL:http://www.healthandage.com/Home/105!gid1=5328>

   "The number of times you re-use a cooking oil could have
   a significant impact on your health, say researchers. We
   already know that the kind of oil you use for cooking is
   important - with olive oil often being cited as having
   health benefits. Now researchers in Spain have
   discovered a new link between re-using a cooking oil and
   high blood pressure.

   People who use a lot of olive oil - which can be
   expensive - sometimes use an oil 'bath' or deep fryer to
   fry certain goods. But when the oil is heated, it
   changes its chemical composition and creates substances
   called polymers and polar compounds which seem to raise
   blood pressure. In fact, re-heating olive oil more than
   twice probably cancels out all its health benefits. If
   you want to fry foods in oil and can't afford fresh
   olive oil each time, it may be better to mix it with a
   cheaper vegetable oil but do not re-heat it.

Reusing cooking oil ups blood pressure
<URL:http://www.heartcenteronline.com/myheartdr/home/research-detail.cfm?reutersid=4046>

   Experts say that different types of cooking oils are
   better for you than others, and a new study suggests
   that the number of times you reuse cooking oils can also
   affect your health.

   Spanish researchers found that people whose kitchens
   contained any type of oil that had been reused many
   times over were more likely to have high blood pressure
   than people whose cooking oils were changed more
   frequently.

   People who ate foods cooked with sunflower oil also
   showed a higher risk of high blood pressure, while
   consuming more olive oil appeared to protect people from
   high blood pressure.

   Based on the findings, study author Dr. Federico
   Soriguer of the Hospital Civil Plaza in Malaga
   recommends that people cook with olive oil whenever
   possible, and discard any oils after using them up to
   two or three times.

   Although olive oil is generally considered to be a
   healthy addition to meals, the findings suggest that
   repeatedly reusing the oil may invalidate its favorable
   effects on health, Soriguer told Reuters Health.

   Many of the study participants consumed the so- called
   Mediterranean diet, which features liberal use of olive
   oil and has been linked with numerous health benefits.

   Soriguer explained that followers of the Mediterranean
   diet often use an "oil bath," or deep fryer, to fry
   foods. However, when the same pot of oil is repeatedly
   reheated, the oils begin to degrade, releasing
   substances known as polymers and polar compounds that
   can become absorbed by food.

   To measure the effects of repeated use of cooking oils,
   Soriguer and his team measured levels of polymers and
   polar compounds from oil samples taken directly from the
   kitchens of 538 adults.

   The more polar compounds and polymers present in oil
   samples taken from a participant's house -- a sign the
   oil had been reused repeatedly -- the more likely it was
   that participant had high blood pressure.

   However, people whose blood samples contained a
   relatively high concentration of fats predominantly
   found in olive oil were less likely than others to have
   high blood pressure, Soriguer and his team report in the
   American Journal of Clinical Nutrition.

   Olive oil that had been repeatedly reused tended to show
   a lower concentration of polymers and polar compounds
   than other types of oil, suggesting that olive oil
   degrades more slowly than other types, Soriguer
   explained. ..."

The study is:

Soriguer F, Rojo-Martinez G, Dobarganes MC, Garcia Almeida JM, Esteva I,
Beltran M, Ruiz De Adana MS, Tinahones F, Gomez-Zumaquero JM, Garcia-Fuentes
E, Gonzalez-Romero S.
Hypertension is related to the degradation of dietary frying oils.
Am J Clin Nutr. 2003 Dec;78(6):1092-7.
PMID: 14668269 [PubMed - indexed for MEDLINE]
<URL:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1
4668269&dopt=Abstract>
( http://tinyurl.com/28xu8 )

Abstract:

   "BACKGROUND: The family kitchen resembles an
   uncontrolled laboratory experiment, and some
   discrepancies in the relation between the risk of
   hypertension and dietary fat may be partly due to the
   manipulation to which the fats were subjected.
   OBJECTIVE: We investigated whether deterioration in the
   quality of the cooking oils in the family household
   contributes to the risk of high blood pressure. DESIGN:
   The study was cross-sectional. Anthropometric
   measurements were obtained for 1226 persons aged 18-65 y
   who were selected randomly from the municipal census of
   Pizarra, Spain. An oral-glucose-tolerance test was given
   to 1020 of these persons. Samples of the cooking oil
   being used were taken from the kitchens of a random
   subset of 538 persons. The concentrations of polar
   compounds and polymers were used as markers of the
   deterioration of the oils. The strength of association
   between variables was measured by calculating the odds
   ratio from logistic models. RESULTS: Hypertension was
   strongly associated with obesity and was influenced by
   sex, diabetes, and age. The presence of excess polar
   compounds in the cooking oil and the use of sunflower
   oil were related to the risk of hypertension, whereas
   the concentration of monounsaturated fatty acids in the
   serum phospholipids was negatively related to this risk.
   These associations remained after inclusion in the
   models of age, sex, obesity, and the presence of
   carbohydrate metabolism disorder. CONCLUSIONS: The risk
   of hypertension is positively and independently
   associated with the intake of cooking oil polar
   compounds and inversely related to blood concentrations
   of monounsaturated fatty acids."

--
Matti Narkia