Healthy Eating: A Guide to the New Nutrition - Some foods are good for you, some are bad. But which are which? The answers, according to the latest nutritional science, are not the same as previously thought. While some age-old advice like “eat your vegetables” still holds true, many early assumptions have turned out to be wrong. Scientists have learned much more about why some foods help prevent disease and why others promote it.
It used to be so simple. Fats were the villains, and carbohydrates were the heroes. More protein was good. But that simplified thinking has changed. Fats, carbohydrates, and proteins, known as macronutrients, are the basic categories of nutrients that humans need. They provide the body with energy, and they enable the body to carry out many normal biological functions. A healthy diet consists of a mix of foods from each of these three macronutrient categories, although not in equal amounts.
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All about fat
Fat now makes up about one-third of the average American's diet. At one time, experts encouraged people to eat less fat. These days, experts are more concerned about the type of dietary fat, especially for heart health, thanks in part to decades of studies at the Harvard Medical School and the Harvard School of Public Health. This changed concept was supported in 2006 by the Women’s Health Initiative, a study of about 49,000 women, which showed that a low-fat diet did not significantly reduce the women’s risk of heart disease, breast cancer, or colon cancer.
The body needs fat. It’s a major energy source and also helps you absorb certain vitamins and nutrients. Only some fats are bad for you: saturated fats (found mainly in meat, butter, whole milk, and cheese) and trans fatty acids, or trans fats (which come mostly from the partially hydrogenated oils used in restaurant fryers, many margarines, and packaged snacks and baked goods, and in lesser amounts from dairy products and meats). These bad fats boost your chances of developing heart disease by increasing blood levels of two of its main risk factors, LDL cholesterol and triglycerides. Trans fats also lower levels of the protective HDL cholesterol.
Unsaturated fats — which come from fish and such plant sources as vegetable oils, nuts, and whole grains — are good for you. There are two types of unsaturated fats: polyunsaturated fats and monounsaturated fats. These good fats can help lower LDL, prevent abnormal heart rhythms, and prevent heart disease.
A low-fat diet is no guarantee of good health. In fact, a diet with only 20% of calories from fat can be virtually a junk-food diet if you make up for the lost fat calories with sugary foods such as soft drinks, nonfat cookies, and high-starch carbohydrates such as white bread and potatoes. An overabundance of these foods increases the risk for heart disease and diabetes. What’s the difference between a good fat and a bad fat? All fats have a similar chemical structure: a chain of carbon atoms bonded to hydrogen atoms. What differs is the length and shape of their carbon chains and the number of hydrogen atoms connected to the carbon atoms. Seemingly slight differences in structure translate into crucial differences in the body.
What about carbohydrates?
Carbohydrates encompass a broad range of foods, including table sugar, fruits and vegetables, and grains such as rice and wheat. Most of your carbohydrates should come from whole-grain foods, vegetables, and fruits. If most of the carbohydrates you eat are bad carbohydrates (white bread, white potatoes, white rice, and other refined starches or sugars), you could end up gaining weight and putting yourself at risk for disease.
The list of bad carbohydrates may come as a surprise. Why are potatoes bad for you? They’re vegetables, after all. Why are they in the same category as sweets? To answer these questions, you have to consider the glycemic index of a food.
The glycemic index reflects the spike in blood sugar caused by eating a certain amount of a particular food compared with that of white bread or pure sugar. In general, healthy carbohydrate foods have a lower glycemic index. The term glycemic load is often a more useful term because it describes the amount of carbohydrate in a serving of food. The glycemic load is easy to grasp as simply the food’s glycemic index times the amount of carbohydrate in a serving. For example, a big slice of white bread will spike the blood sugar more than a little slice of the same bread. For menu-planning purposes, the glycemic load is more meaningful than the glycemic index because it is based on real-life portions of particular foods eaten throughout a day.
Foods with a high glycemic index are digested more quickly than foods with a low glycemic index. Rapidly digested foods can be dangerous because they flood your bloodstream with sugar all at once. Sudden, high spikes of blood sugar trigger a gush of insulin to clear the sugar from your blood. The problem is that this quick surge of insulin can leave your blood sugar too low after just a few hours. When your blood sugar is too low, you feel hungry; if it’s low soon after a meal, you’re apt to overeat and possibly gain weight.
Another problem with a steady diet of high-glycemic meals is that over many years, your body’s system of responding to insulin could become impaired. This is called insulin resistance. When your cells are less responsive to insulin, the resulting overload of sugar in your bloodstream forces the pancreas to step up its production of insulin in an effort to move the sugar from the blood into the cells. This itself will further reduce the response of muscles to insulin to take up sugar from the blood. As this situation worsens, blood sugar rises chronically to an abnormal level defined as type 2 diabetes, the type that often develops in older adults. If the pancreas is forced into overdrive for a sustained period, it may wear down and eventually lose some of its ability to produce insulin, leading to insulin deficiency and worsening of type 2 diabetes. Insulin resistance can also cause other problems, including heart disease and perhaps some cancers.
Update on protein
How much protein do you need? That question has been the subject of debate for decades. You’re better off, if you choose proteins from foods like fish, skinless chicken, beans, soy, and nuts rather than foods laden with saturated fats, like red meat and cheese. Reach for the beans, not the burgers.
Protein is everywhere in your body. It’s in your muscle, bone, skin, and nearly every other body part or tissue. It has many functions, including building the enzymes that trigger many of the important chemical reactions in the body. Providing the raw material for the body’s proteins are about 20 amino acids. Following genetic instructions, the body strings together these amino acids into chains to make the specific proteins the body needs. The body stores amino acids in muscle protein, and mobilizes them continuously by breaking down muscle. This happens every day as our metabolism moves from fed to fasting. In the morning, the body is using its muscle for amino acids.
Ideally the body needs a daily supply of amino acids to make new proteins. This supply comes from the protein in food. Dietary proteins are as essential as calcium and vitamin D for bone health and osteoporosis prevention, according to a recent review in the Journal of the American College of Nutrition. A lack of protein in the diet can slow growth, reduce muscle mass, lower immunity, weaken the heart and respiratory system, and even cause death. These problems are more likely to arise in people in poorer countries; most people in industrialized countries eat plenty of protein.
So how much protein should you eat? In establishing the upper limit of 35%, the Institute of Medicine cautioned against exceeding that amount because some studies show that high-protein diets may promote some diseases, such as osteoporosis. When you eat a lot of protein, your body draws some calcium out of your bones to help neutralize the acids that digest protein. Going on a high-protein diet for a few weeks probably won’t weaken your bones, but doing so for longer could begin to take a toll on your bones.