Ketogenic diet

How it works: Bring on the bacon. This high-fat, very low carbohydrate diet typically means eating fewer than 50 grams of carbs a day — less than four slices of bread's worth.

What it promises: Getting most of your calories from fat forces your body to use different energy pathways. Instead of carbs for energy, the body burns fat, entering a state called ketosis.

The upsides: While the precise mechanisms are unclear, ketosis is thought to have brain-protecting benefits: As many as half of young people with epilepsy had fewer seizures after following the diet. And some early research suggests it may have benefits for blood sugar control among people with diabetes. An upcoming study will look at the ketogenic diet as a weight maintenance strategy.

The downsides: While the research is exciting, there's very little evidence to show that this type of eating is effective — or safe — over the long term for anything other than epilepsy. Plus, very low carbohydrate diets tend to have higher rates of side effects, including constipation, headaches, bad breath and more. Also, meeting the diet's requirements means cutting out many healthy foods, making it difficult to meet your micronutrient needs.

Mayo's verdict: While the ketogenic diet may be recommended for some people with uncontrolled epilepsy, the high fat content — and especially the high level of unhealthy saturated fat — combined with limits on nutrient-rich fruits, veggies and grains is a concern for long-term heart health.