Before you take any supplements for disease prevention, it's important to know whether the potential benefits outweigh the risks. To make that conclusion, you need to look at the results of well-designed studies. A recent randomized trial in men suggested multivitamins have possible benefits for cancer prevention. For many of the other popular supplements, including vitamin D and omega-3 fatty acids, results from randomized controlled trials should be available within the next five years, according to Dr. Manson.
Healthy eating is a way of eating that improves your health and helps prevent disease. It means choosing different types of healthy food from all of the food groups (fruits, vegetables, grains, dairy, and proteins), most of the time, in the correct amounts for you. Healthy eating also means not eating a lot of foods with added sugar, sodium (salt), and saturated and trans fats.
Vitamins and minerals are most easily digested with food. Fat-soluble vitamins should be taken with food that contains fat. Vitamins tend to work synergistically, meaning that they work together in order to be effective. For instance, vitamin E requires some of the B-complex vitamins and the minerals selenium and zinc for most effective absorption. Some minerals may not be absorbed or may inhibit each other when taken in improper ratios. Generally, a high quality, broad-spectrum vitamin and mineral supplement will be formulated to prevent unfavorable interactions.
Men who choose to drink and can do so responsibly may benefit from one to two drinks a day, counting 5 ounces of wine, 12 ounces of beer, or 1.5 ounces of spirits as one drink. But women face an extra risk: Even low doses of alcohol can raise their risk of breast cancer. So women who choose to drink might be wise to limit themselves to half as much as men.
Supplementation with EPA and/or DHA does not appear to affect the risk of death, cancer or heart disease. Furthermore, studies of fish oil supplements have failed to support claims of preventing heart attacks or strokes. In 2017, the American Heart Association issued a science advisory stating that it could not recommend use of omega-3 fish oil supplements for primary prevention of cardiovascular disease or stroke, although it reaffirmed supplementation for people who have a history of coronary heart disease.
There is more than one way to eat healthfully and everyone has their own eating style. Make healthier choices that reflect your preferences, culture, traditions, and budget. Choose fruits, vegetables, grains, dairy, and protein foods to get the most nutrition and meet your personal calorie needs. Aim for a variety of foods and beverages from each food group and limit saturated fat, sodium, and added sugars.
6. Keep healthy foods in larger packages and containers, and unhealthy foods in smaller ones. Big boxes and containers tend to catch your eye more, take up space in your kitchen and pantry, and otherwise get in your way. As a result, you’re more likely to notice them and eat them. Meanwhile, smaller items can hide in your kitchen for months. (Just take a look at what you have lying around right now. It’s probably small cans and containers.)
Nutrition is particularly important when you are pregnant. Weight gain during pregnancy is normal—and it's not just because of the growing fetus; your body is storing fat for lactation. The National Academy of Sciences/Institute of Medicine (NAS/IOM) has determined that a gain of 25 to 35 pounds is desirable. However, underweight women should gain about 28 to 40 pounds, and overweight women should gain at least 15 pounds. The IOM has not given a recommendation for an upper limit for obese women, but some experts cap it as low as 13 pounds. If you fit into this category, discuss how much weight you should gain with your health care professional. Remember that pregnancy isn't the time to diet. Caloric restriction during pregnancy has been associated with reduced birth weight, which can be dangerous to the baby.
Reduced nutritional density in many foods, combined with the use of refined "foods" like sugar, white flour and refined oils, places a greater priority on eating the most nutritious foods. Farm produce grown organically generally has higher levels of essential nutrients such as trace minerals because the soil contains higher levels of trace minerals and the produce grows slower and thus has more time to absorb nutrients from the soil. Examples of nutrient dense foods are sardines, wild salmon, shellfish, eggs, liver, kale, collards and spinach, sea plants (seaweed), garlic, blueberries, and dark chocolate. 
The transition from an existence as hunter and gatherers to urban agriculture around 10,000 years ago began an epoch when foods were mass-produced but had lower nutritional density, compared with the previous food eaten by our ancestors. The nutritional density in many foods has fallen significantly since human societies transformed from hunter-gatherers into resident farmers. This is especially true in the last 60-70 years after agriculture was changed from small, versatile ecologically driven family farms to large, chemical-based, industrial agriculture.  The reduction of nutritional content in modern crops, compared with older varieties, is well documented.  It is a consequence of soil erosion, loss of essential minerals from continual heavy use, combined with breeding of new varieties, which has increased the size and growth rate of plants by increasing the content of sugar and water and decreasing their mineral content compared to ancient species. At the same time, the relative content of other macronutrients (fat, protein/amino acids) and antioxidants may have been reduced.
Scientific evidence shows that some dietary supplements are beneficial for overall health and for managing some health conditions. For example, calcium and vitamin D are important for keeping bones strong and reducing bone loss; folic acid decreases the risk of certain birth defects; and omega-3 fatty acids from fish oils might help some people with heart disease. Other supplements need more study to determine their value. The U.S. Food and Drug Administration (FDA) does not determine whether dietary supplements are effective before they are marketed.
Processing of food reduces its nutritional content, and the finished products are often based on fractions of the original foods. One example is milling grain to make white flour,  which has a lower nutritional density than whole grain flour. The reduction in nutritional value has accelerated since whole foods are now divided into pieces, for example, boneless chicken breast. When meat is injected with saline to increase the volume, the relative level of essential nutrients is reduced. In the United States, many supermarkets in low-income rural and inner city areas have a limited selection of nutrient-dense foods, compared with high-income areas. 
Dietary supplements are complex products. The FDA has established good manufacturing practices (GMPs) for dietary supplements to help ensure their identity, purity, strength, and composition. These GMPs are designed to prevent the inclusion of the wrong ingredient, the addition of too much or too little of an ingredient, the possibility of contamination, and the improper packaging and labeling of a product. The FDA periodically inspects facilities that manufacture dietary supplements.