According to the American Institute of Cancer Research, 40% of all cancers can be prevented by focusing on three things: maintaining a healthy weight, eating a well-balanced healthy diet and engaging in regular physical activity.
Some of the most effective dietary strategies to prevent cancer include the consumption of whole grains, garlic, berries, tomatoes, green tea, spinach, curcumin, and coffee. Behavioral changes include maintaining a healthy weight, regular exercise, smoking cessation, reducing alcohol intake, and managing stress.
What is Cancer?
In recent decades, it seems like nothing strikes fear in the heart so much as the sound of cancer. According to Medline Plus, more than 100 different types of cancer have been identified so far. Cancer is defined as the abnormal proliferation of cells. These abnormal cells can form a lump or tumor that can be either be not cancerous- benign or cancerous-malignant. Unfortunately, every cancer is slightly different, as it attacks the body differently in almost every case, and can mutate to protect itself, making consistent or universal treatment nearly impossible. 
Different varieties of cancer have significantly different mortality rates; for example, the one-year survival rate of pancreatic cancer is 25 percent, whereas the five-year survival rate for thyroid cancer is 97 percent. This means that many types of cancer can be “treated” and “sent into remission”, and there are many different treatments that have been tried. Chemotherapy, radiation therapy, immunotherapy, and surgery are the most commonly used strategies in formal medical situations, but there are many side effects and no guarantee of remission. Having cancer is a tragedy that strikes millions, so prevention is highly recommended. There is no telling when or in what form cancer will strike, so behavioral and dietary strategies for prevention are recommended. 
Tips to Help Prevent Cancer
Let us look at the ways we can potentially reduce the risk of cancer by focusing on diet and lifestyle factors.
One of the most widely recommended anti-cancer dietary staples is whole grains, namely due to the high content of fiber and antioxidants. One of the more common forms of cancer is colorectal cancer, and whole grains have been directly linked to preventing the development of this cancer. There is a low glycemic load in whole grains and wheat, rather than foods like white bread and potatoes, which can increase your blood sugar when ingested. Although intervention trials do not directly show the benefits of low glycemic food consumption and cancer prevention, we know low glycemic foods prevent insulin resistance and decrease inflammation. 
Garlic has many different compounds studied for their anti-cancer activity. One of them is allicin. Allicin has been found to inhibit enzymes involved in activating carcinogens, helping with DNA repair, slowing the growth of cancer cells, and inducing apoptosis (automatic cell death). Allicin is released in garlic when crushed or chopped. 
Eating fruits and vegetables is a fundamental way to prevent cancer, but berries, in particular, have a high concentration of antioxidant nutrients and compounds that can reduce the risk of cancer. These antioxidants include anthocyanins, catechins, and flavonols. Some of the best berries to prevent cancer are blueberries, which are often referred to as a “superfood” – and with good reason!  
Carotenoids are a group of phytochemicals that may inhibit cancer cell growth, work as antioxidants and improve immune function. Lycopene is a type of carotenoid found in tomatoes widely known for its antioxidant activity. Although additional consistent research is needed, tomatoes may prevent DNA damage in healthy individuals and possibly decrease the risk of prostate cancer.  
Many different phytochemicals have been studied in tea. Research has shown that tea exhibits antioxidant activity but the exact amount of antioxidants to prevent cancer is unknown. Further studies are needed to link tea consumption with cancer prevention. 
Green leafy vegetables have always been praised for their health benefits, namely due to its content of lutein, zeaxanthin, and beta-carotene, and in terms of cancer, these antioxidant compounds may limit the growth of cancerous cells, and promote the overall health of the body. 
Curcumin & Turmeric
Curcumin is the most active compound found in the spice, turmeric, widely used in Asian cuisines. Dr. Timothy J. Moynihan from the Mayo Clinic states that curcumin has been found to exhibit antioxidant and anti-inflammatory properties which may be useful in cancer prevention and treatment. At this time, the evidence from clinical trials is limited regarding curcumin’s direct role in fighting against cancer. Research is on-going and still in the early stages. Consuming turmeric in food is generally safe but turmeric supplements should not be taken without consulting your doctor.   
In the past, there was conflict regarding the role of coffee and it’s potential harm for increasing cancer risk. Research has now confirmed that along with its variety of phytochemicals, coffee may decrease the risk of endometrial and liver cancer like due to its antioxidant and anti-inflammatory properties. The USDA’s Dietary Guidelines for Americans state that 3-5 cups of coffee daily can be incorporated into a healthy eating plan. 
The AICR states that aside from smoking, maintaining a healthy weight is the most important preventative measure to protect against the risk for at least 12 known cancers including breast, pancreas, liver and kidney cancer. Maintaining a healthy weight throughout life can be accomplished by simple dietary modifications: avoid sugary beverages like sodas and energy drinks, limit the amount foods you eat that are high in fat and sugar, and exercise at least 30 minutes every day.  
Regular physical activity can decrease the risk of cancer by keeping hormone levels at a healthy range, providing a boost to the immune system and preventing weight gain which is a risk factor for many types of cancer. Current recommendations for physical activity is at least 150 minutes of moderate activity per week or 75 minutes of vigorous activity per week (AICR).  
According to the American Cancer Society, about half of all Americans who smoke will die because of this toxic habit. Cigarettes are packed with dangerous toxins and carcinogens that can greatly increase your chances of developing cancer – and not just lung cancer. More than a dozen other cancers have been linked to smoking, including bladder, kidney, and liver cancer, among others. Avoiding cigarettes is one of the first things to do if you want to prevent the onset of cancer. It is best to talk to your doctor about nicotine replacement products and strategies to quit smoking.   
Reduce Alcohol Intake
Excessive alcohol consumption can weaken your immune system, compromise your liver, and generally promote toxicity within the body. Although some people say that one drink a day can improve overall heart health, alcohol consumption does increase the risk of throat, liver, esophagus, and mouth cancer. To prevent cancer, many experts recommend to not drink alcohol at all. If you do drink alcohol, limit your drinks to no more than two drinks a day for men and no more than one drink a day for women. 
Ginger has a number of active ingredients that make it useful in keeping the body healthy, according to a report in the Food and Chemical Toxicology journal. These ingredients include gingerol and zingerone, which have anti-inflammatory and antioxidant properties. They can neutralize free radicals before they can cause your body harm, thereby protecting against a wide range of cancers. 
Oregano may have the ability to neutralize carcinogens in the body, due to the presence of carvacrol, the active ingredient of this herb. Research has linked oregano to a slower spread of cancerous cells, and a reduction in the size of tumors. 
According to a study published in The Lancet, they can weaken your immune system, exhaust your mind, increase oxidative stress, and compromise your metabolic activity. Relax your body by getting enough sleep every night and by regularly engaging in relaxing activities such as meditation and yoga.