by Marion Nestle
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.

  • Anthro

    What a nice surprise! Glad you had the time as I read these questions and have my interest piqued. It all seems to come back to basic, simple common sense. Eat a variety of (unprocessed) foods in moderation and limit sweets to very special occasions.

    People want there to be some magic to nutrition, but like weight loss, the truth is simple, but elusive for many.

  • Very good answers! thank you.

  • Good to know about Agave. I always wondered how much it was processed as compared to regular sugar.

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  • Thanks Marion,
    great answers in simple enough language that anyone can understand.
    Is it just me or do you think that, in relation to the fourth question, the media hasn’t paid much attention to the results of the meta-analysis. It seems any time there is a new study linking some aspect of nutrition to obesity, disease, etc., the media pounces all over the headline (usually leaving out the real details, but that’s another subject). And here, when an analysis seems to refute the sacred cow of a CVD/sat fat link, the mainstream press isn’t giving it much attention at all.


  • Carla

    I am no health expert of any stripe, just a housewife who has read about nutrition for over 30 years now. I used to attempt to follow the latest advice. I don’t anymore. A lifetime of watching expert recommendations proclaim, positively, that a certain nutrient is henceforth THE THING you must have or alternatively, forbidden — and then to watch these experts pedal backward furiously a few years later — this has made me wary of implementing current nutritional thought into my diet. I am going to eat. Real food. Mainly but not exclusively vegetarian. Organic if possible. Forget the rest.

  • “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.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.”

    So how are we simple lay persons supposed to know which experts to believe?

    The 50 Best Health Blogs

  • Organic Grocer

    Thanks Marion! And thanks for writing the the great book Pet Food Politics. That really explained the situation of contaminated soy to me!

  • Cathy Richards

    Thanks Marion. Good to see your explanation of the meta analysis re: sat fat and heart disease. I think there are a variety of reasons the fat issue has been so confusing. The main one is that trans fats and sat fats were lumped into one category in most of the studies in the 50’s and 60’s. This created the fat hypothesis, and that hypothesis was promoted heavily by the processed oils and foods industries. It also made scientists question any research results that didn’t support the fat hypothesis — I have heard that not everything was submitted for publication but collects dust in medical libraries (eg. some Framingham data analyses).

    I first heard Walter Willet questioning the fat hypothesis at a Toronto conference in 1986 — it opened up my “dietitian eyes” and made it difficult for me to work in cardiac settings. Of course, Willet now has got the support of the majority of researchers, but low fat and low sat fat continues to be parroted by physicians and dietitians and food marketers that struggle to abandon old paradigms.

    I’m enjoying some heart healthy nuts as I type. Will follow them up with some 85% cocoa solids chocolate.

  • Marion

    @Jim Purdy: I addressed the “who can we trust” question in a column in the San Francisco Chronicle that you can access through this site at This question comes up so often that I think I will add it to my Q and A page. Thanks for asking.

  • Bobby

    my partner makes my life as the cook of the house very complicated by insisting that only one kind of potato can be eaten at supper, along with a wide array of other seemingly inane rules based on how fast the food gets digested. My question is that since we are not eating those meal ingredients all by them selves, then the montignac rules are just a pile of bull crap and eating a balanced diet made up of many ingredients is healthier than forcing certain foods out of the diet at certain times of the day, moon, etc. I call it voodoo, and tell her to just eat a bit less and then you will lose weight but serving a smaller portion than me causes more conflict… I love cooking, but I hate being the cook.

  • What a great post! The controversy and new discoveries make the nutrition field truly fascinating, indeed.

  • Cathy Richards

    Bobby — oh man I empathize with you! Point out to her that the glycemic index of a potato is better if it is cold, or if it has acid with it (eg. vinegrette or mayo potato salad). If such simple modifications change the G.I., then what will eating your potato with an entire meal — complete with protein and fat — do to the GI?

    When we need to focus on such small nuances, it becomes clear that our bigger picture is blurred. Whatever that bigger picture is — food, stress, body image, diversion tactics, activity, relationship dynamics, mental health, etc.

    I’ll remind myself of that next time I’m deciding between a cereal with 3 vs 5 g of sugar.

  • Cindy

    Marion, There’s been a lot written about the benefits of grass-fed beef, regarding high levels of omega 3’s, vs omega 6’s. Does this hold true for all animals fed grass, rather than grain? (And farmed fish fed corn?)

  • Jess

    “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.”

    Micro-analysis doesn’t really help anything – just look at the history:

    Animal fat eaters have thrived for 2 million years of evolution. We will continue to do so, so long as we get rid of the grain-fed, factory-farmed industry and get back to supporting the life cycle through permacultural animal husbandry. Animal fats are the key component of the human diet – we need them in order to thrive both physiologically and psychologically, so we have to stop science messing with our fuel at the molecular level!

    Sugar eaters (i.e. processed sugars and grains) have been feeding their habit for a much shorter length of time, and just look at the explosion of diabetes and obesity and cancer… The fact that so many people CONTINUE to be aware that SUGAR is what makes you fat & causes heart disease etc horrifies me – just look at the way we ‘treat’ diabetes and don’t tell cancer sufferers that glucose is what fuels tumor cell development!

    Eat real food. Organic meat & veg. Leave the grains to the herbivores & don’t eat anything that required detoxification and processing to make it even slightly edible.

  • I hope that in my lifetime there is a major movement away from nutrition science as the major discipline of nutrition. Food is about much more than measure and chemical structure. The current focus is neither productive nor practical. If anything the growth of nutrition as a discipline in the public eye, with its often conflicting findings, has generated distrust and disdain.

  • Hylton

    The recent saturated fat analysis was supported by the National Dairy Council.

    That doesn’t make it automatically erroneous but it is something to bear in mind, especially with the track record you have recognized in the recent past:

  • Hylton-

    John Tierney wrote a piece in today’s times on this issue.

    A mass refusal of industry money would only serve as a major hindrance to researchers and greatly limit the amount of new research that is produced. I agree with Tierney’s sentiment; quality research is quality research, regardless of who pays for it.

  • Hylton

    Thanks for the follow up comment Daniel.

    I read that NYTimes article about two minutes after I have posted on this blog during my morning Internet rounds. I just knew a savvy person would link to it by time I got home.

    still stand by my initial comment because in this instance, there is evidence of patterns from the national Dairy Council and many corporately funded nutritional research projects. I understand the point John Tierney is making in that article, but he’s speaking in generalities.

    In her book “What to Eat,” Marion Nestle lays out solid reasons to hold a certain degree of skepticism when reviewing research sponsored by the dairy industry.

    Again, it doesn’t nullify the study, but we should weight this knowledge with everything else.

  • ET Addison


    By that measure, we should also be skeptical of any research sponsored by the American Heart Association, or the NIH, or any other government body, in that they have a vested interest in preserving and protecting whatever policies and recommendations they have been making for years now.

    They would NEVER, EVER sponsor research that would potentially negate all the stuff they’ve been espousing. . . fat will kill you, vegetables are perfect food, eat whole grains.

    Nutrition research is not really science, like physics is science. There is way more religion and preaching involved.

    Marion noted in a recent post that Nutrition is an infernally difficult field. (A ‘science’ not quite.) Research is ambiguous and hard to interpret (or spin).

    Yet, curiously, it’s the only ‘science’ that continually makes bold pronouncements, recommendations, and prescriptions based on iffy and shaky evidence.

    “we don’t really know, but do what we say, anyway.”


  • C. Price


    One thing to note about Wrangham’s thesis regarding the different caloric potential in cooked vs. raw foods is that he was including the calories burned in his analysis; in his view, digestion of raw meats consumes an substantially greater amount of calories than digestion of cooked meats, meaning that you will net sigificantly fewer calories when eating raw meat. Your response to this aspect of the query seems not to account for this, although I’d be interested in your take.

    Thanks for your important work.

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