by Marion Nestle

Currently browsing posts about: Fats-and-oils

Mar 22 2010

Saturated fat vs. heart disease: current state of the science

Despite recent publications finding no correlation between intake of saturated fat and coronary heart disease (CHD) – see, for example, the recent meta-analysis in the American Journal of Clinical Nutrition – the debates over the role of saturated fat continue.

In that same issue of the Journal, another study says that reducing saturated fat only works if you replace it with something better.  If you replace saturated fat with carbohydrates, the effects on heart disease will be worse.

The fat story is not simple (in What to Eat, I explain the biochemistry of food fats in the chapter on fats and oils and in an appendix).  The main reason for the complexity is that different kinds of fats do not occur separately in foods.

Without exception, food fats are mixtures of  three kinds of fatty acids: saturated (no double bonds and solid at room temperature), monounsaturated (one double bond), and polyunsaturated (two or more double bonds and liquid at room temperature).  Food fats just differ in proportions of the three kinds.

Meat, dairy, and egg fats generally are more saturated.  Plant fats and oils are generally more unsaturated.

How to make sense of the saturated fat story? An expert panel from WHO and FAO just produced a new review of the evidence.  The panel evaluated CHD morbidity and mortality data from epidemiological studies and controlled clinical trials.  It found:

  • Convincing evidence that replacing saturated fat with polyunsaturated decreases the risk of CHD.
  • Probable evidence that replacing saturated fat with largely refined carbohydrates (starch and sugar) has no benefit and even may increase the risk of CHD.
  • Insufficient evidence relating to the effect on the risk of CHD of replacing saturated fat with monounsaturated fats or whole grain carbohydrates, but a trend suggesting that these might decrease CHD risk.
  • Possible positive relationship between saturated fat and increased risk of diabetes.
  • Insufficient evidence for establishing any relationship of saturated fat with cancer.

The panel’s recommendations:  (1) Replace saturated fat with polyunsaturated fats (omega-3 and omega-6) in the diet, and (2) Limit saturated fat to 10% of daily calories or less.

Translation: Eat less animal fat and replace it with vegetable fats.

Historical note: These are precisely the same recommendations that have been standard in the U.S. for at least fifty years.  This was good advice in the late 1950s.  It is still good advice.

UPDATE, March 22,2011:  Another major review has just come to precisely the same conclusions, this one from an international expert panel.  It also suggests areas for future research.  See American Journal of Clinical Nutrition 2011;93:684-88.

Jan 25 2010

A quick Q and A: sugars and fats

I wish I could answer all of the questions that come into Feedback or Comments, but I cannot except occasionally.  It’s a rainy day in New York and today seems to be one of those occasions.

Q: Does the caloric value of a food change when it’s cooked?  In his latest book, “Catching Fire: How Cooking Made us Human,” Harvard Primatologist Richard Wrangham argues that cooking foods changes the available nutrient content and actually raises the available calories.

A:  The rules of physical chemistry tell us that matter cannot be destroyed or created so the number of calories available in a food does not change with temperature.  What can change is our ability to use (digest, absorb) the calories that are there as well as our desire to eat the foods.  Cooking makes the calories in potato starch more available, for example, but has hardly any effect on the calories in meat.  Both, in my opinion at least, taste better cooked.    But cooked or not, the calorie differences will be small and unlikely to account significantly for weight change.

The nutrient situation is also complicated.  Cooking destroys some nutrients (vitamin C is a good example) but makes others more available (beta-carotene).  This is another reason why nutritionists are always advising variety in food intake.  Variety applies to cooked and raw, as well.

Q.  Can you please explain what benefits, if any, there are in using a “natural” sweetener, e.g. agave, over regular sugar?  Are there any differences in terms of glucose/fructose makeup?

A.  Agave is more expensive so you probably won’t use as much of it.  Beyond that, it is higher in fructose than table sugar or honey.  This is because agave contains inulin, a polymer of fructose, which must be hydrolyzed (broken down by heat or enzymes) to fructose to make the sweetener.  It’s a processed sweetener requiring one hydrolysis step, requiring more processing than honey and less than high fructose corn syrup.  It has the same number of calories as any other sugar, about 4 per gram or 16 per teaspoon.

Q.  Also, you’ve written on a prior blog that fructose is “preferentially” metabolized into fat by the body.  Can you explain in more detail what that means?

A.  More and more evidence suggests that high amounts of fructose in the diet are not good for health.  Fructose occurs naturally in fruit and nobody worries about that because fruits don’t contain all that much and the sugar is accompanies by vitamins, minerals, and fiber that are well worth eating.  Honey, table sugar, and high fructose corn syrup (a misnomer) are about 50% each glucose and fructose.  Glucose and fructose are metabolized differently and some investigators believe that excessive amounts of fructose stress metabolism in ways that encourage fat deposition.  Eating a lot of sugars of any kind is not a great idea, which is why there are so many concerns about soft drinks these days.

Q.  I would appreciate some comments about the “Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease.

A.  The study concludes:  

A meta-analysis of prospective epidemiologic studies showed that there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD [coronary heart disease] or CVD [cardiovascular disease]. More data are needed to elucidate whether CVD risks are likely to be influenced by the specific nutrients used to replace saturated fat.

This is a review of previous epidemiological studies (not clinical trials).  These fail to find a correlation between consumption of saturated fat and heart disease.  This could be because there is no correlation or there is one but they can’t find it.  My interpretation: even if there is one, it is likely to be small.

I am increasingly convinced that studies of single nutrients – sugar, fructose, saturated fat, or even omega-3s – will give complicated results when removed from their dietary context.  People who eat foods containing a lot of sugars or animal fats eat and behave differently than people who do not, but not so differently that health differences will show up in the kinds of studies scientists are currently able to do.

Keep in mind: nutrition science is exceedingly difficult to do because there are so many factors in foods that affect health and so many behavioral, economic, and social factors that affect what people eat.

All of this is why I find nutrition so interesting but I can understand why others might find it frustrating.

Jan 19 2008

Nothing is simple: sustainable palm oil?

If you eliminate oils with trans fats, you have to replace them with fats with equivalent levels of saturation, and palm oils are highly saturated and work well as substitutes. One consequence of the increased demand for palm oils is destruction of tropical rainforests. “To improve the industry’s image and avert a consumer backlash,” food companies are pushing palm oil producers to go green and promise to produce palm oils sustainably. Will this work? It will be interesting to see.

In the meantime, the New York Times has plenty to say about how using palm oil for fuel drives up the cost of food.

Jan 11 2008

What’s the deal on saturated fat?

A reader, “rj,” sends a link to an article in Men’s Health (“What if bad fat isn’t so bad”), and asks about: “The supposed inconclusive evidence for sat fat being the culprit in atherosclerosis. Personally, I couldn’t find any credentials of the author but nevertheless would be much interested in your thoughts on the matter.”

My thoughts: As I keep saying, nutrition science is complicated and this article, by an excellent science journalist, is the latest in a series by excellent science journalists (see, for example, the recent books by Gary Taubes and Michael Pollan) to point out the inconsistencies in data on saturated fat and heart disease risk. Let me make several quick points: (1) All fats–no exceptions–are mixtures of saturated, unsaturated, and polyunsaturated fatty acids (2) Saturated fats occur in greater proportions in animal fats–meat and dairy foods, (3) Some epidemiologic evidence–but not all–suggests that people who eat a lot of meat and dairy foods have a higher risk of heart disease than people who eat a lot of fruit and vegetables (this is correlation, not causation), (4) The same clinical studies that show how trans fats do bad things to blood cholesterol levels also show that saturated fat does too, although not as much (But: people take in a lot more saturated fat than trans fat), and (5) Saturated fat is a single nutrient and the studies reviewed and discussed by the journalists take saturated fat out of its dietary context.

Out-of-context studies of single nutrients are exceedingly difficult to interpret. At the moment, today’s dietary (not single nutrient) advice is the same as it has been for the last fifty years. As I put it in What to Eat, “Eat less, move more, eat plenty of fruits and vegetables, and don’t eat too much junk food.” Michael Pollan gives exactly the same advice: “Eat food. Not too much. Mostly plants.” Do this, and you really don’t need to give a thought to single nutrients.

I discuss the politics of diet and disease recommendations in my book, Food Politics (now out in a new, expanded edition), and this particular question in “Ask Marion” on Eating Liberally.

Does this help at all? Thanks for asking.

Dec 31 2007

Trans-fat substitutes: How?

Here’s a quick question, just in: “I finally got the chance to finish What to Eat, and I noticed that you didn’t talk about non-hydrogenated margarine in your margarine section. I’m not wondering if it’s better for you because I’m sure it’s still soybean oil with a bunch of stabilizers, but I’m just wondering how it’s made.”

Response: I did actually, but in two other chapters, the next one and the one on fats and oils so the explanation is hard to find. Sorry about that. Here’s the deal: companies use variations of two methods: (1) substitute a highly saturated fat like palm kernal or coconut oils, or (2) mix a totally saturated fat (which will not have any trans) with an unhydrogenated fat (also trans-free) until you get the degree of thickness required. Both methods increase the amount of saturated fatty acids. Saturated fats raise the risk of heart disease, but not as much as trans. So the substitutes are likely to be marginally better than oils with trans.

Nov 7 2007

Trans Fat Dilemmas

I have long talked about trans fat as a calorie distracter. People think “trans fat-free” means “calorie-free” when it most definitely does not. Whatever replaces trans fats will have just as many calories–130 per tablespoon, meaning that each tablespoon is 5% of a day’s average calorie intake. That’s why I either laugh or cry when I see “zero grams trans fat”
on the labels of junk foods. Trans fats raise the risk of heart disease a bit more than do the saturated fats that occur naturally in foods. But trans fats are unnatural and unnecessary and it’s good to get rid of them. Yesterday’s Wall Street Journal explains how food companies are struggling to find replacements that do not increase the amount of saturated fat in processed foods. This, as it turns out, is not so easy to do. I discuss all this in the fats-and-oils chapter of What to Eat, so I’m happy to see the WSJ take it on.