For decades, nutrition experts told us that eating a low-fat diet was the key to preventing heart disease, obesity, and even cancer. The food industry responded by creating all kinds of packaged foods, from cookies and cakes to ice cream and chips, that were lower in fat but loaded with refined carbohydrates.

Now the pendulum has swung in the opposite direction, and the supermarket shelves are full of products that are low in refined sugar and high-glycemic carbs, but often very high in fat, such as low-carb chocolate and low-carb ice cream.

“Too much fat (especially of the wrong kind) is just as unhealthy as too many refined carbohydrates,” Dr. Northrup cautions. “The right answer, as always, is balance.”

Not all fat is bad, according to Dr. Northrup. We need the right amount of the essential fatty acids found in seeds, nuts, fatty fish, and some oils. These are the omega-6 and omega-3 EFAs. No minimum daily requirement for these has been established, but most research (not all) suggests that the ratio between omega-6 and omega-3 fats should be about 3:1.

Because the American diet is already quite high in omega-6 fats (from seed oils such as canola, oil, peanut oil, and so on.), Dr. Northrup says it’s far more important to get enough omega-3s from sources such as flaxseed, hemp seed, and wild salmon. Their effect on the brain, the immune system, and virtually every cell in the body has been well established.

Saturated fat, the kind found in animal foods such as beef and eggs, can be a healthy addition to one’s diet as long as the animals that supply it are organically raised with the right feed. For example, free-range hens produce eggs with higher omega-3 content. But in individuals who are genetically predisposed to high homocysteine levels, saturated fat is a risk factor for heart disease and stroke.

Animal fat is also high in arachadonic acid, which can be a setup for the production of inflammatory eicosanoids in susceptible individuals. Those who have a history of arthritis should avoid animal fat for a couple of weeks to see if joint pain lessens. If pain returns when saturated fat is reintroduced, they should eliminate most of it permanently.

And watch out for partially hydrogenated fats (also called trans fats). “These are just plain dangerous,” Dr. Northrup says. “This type of fat isn’t found in nature. It’s created by blowing hydrogen into polyunsaturated fats at very high temperatures and pressures. This renders them solid at room temperature—as in margarine—and gives them a shelf life that seems to approach infinity.”

The problem is that these fake fats are metabolized into the membranes of virtually every cell in the body, including the fat-rich brain and nerve cells. Over time, they accumulate in the system and disrupt cell membrane function, contributing to the development of cancer, heart disease, and mood problems.

Read labels and avoid buying foods that contain these fake fats,” she advices. “You’ll be shocked to discover how many packaged foods do. But it will spur you to find healthier alternatives.”