Photo by moria/FlickrCC
I wish I could answer all of the questions I receive as feedback or comments, but I cannot except occasionally. A recent rainy day in New York, however, seemed 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 four 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 accompanied by vitamins, minerals, and fiber that are well worth eating. Honey, table sugar, and high-fructose corn syrup (a misnomer) are about 50 percent 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.