You only need to eat a gluten-free diet if you have Celiac disease or gluten sensitivity (gluten sensitivity is a littler more difficult to determine). You only need to avoid dairy products if you have an allergy to dairy products or if you have lactose intolerance. In these cases, you should speak with your health care provider about your condition. A dietitian or nutritionist can help you set up a diet that takes these conditions into consideration and still eat a balanced and nutritious diet.
If you don't have any of these conditions, then you don't need to avoid two whole food groups. If you're looking at a diet that tells you to do that, you're just looking at another fad diet, and you're probably wasting your time.
Fad diets (like the Paleolithic diet or alkaline diet) usually require you to avoid specific food groups like grains or dairy products. Or they may claim that specific foods that contain certain components like gluten or phytic acid are bad (they may refer to them as anti-nutrients). These diets aren't based on credible research studies for weight loss or other health claims, but they may try to sound scientific. Taking a few minutes to review their sources will reveal a lack of evidence beyond opinion and conjecture.
Most fad diets come and go quickly - probably because they're difficult to follow long-term, and they don't help you lose or maintain your weight.
The best way to watch your weight and provide your body with all the nutrition you need is to follow a healthfully balanced diet. Choose a variety of foods from each food group to ensure a sufficient intake of macronutrients, fiber, vitamins and minerals:
- Fruits and vegetables: two or three cups of fruits and vegetables each day.
- Grains and cereals: at least half your grains should be whole grains.
- Protein sources: choose low-fat protein sources like lean meat, poultry, seafood, fish or legumes.
- Dairy foods: you need two or three servings of dairy or other calcium-rich foods every day.
- Fats and oils: get a little bit of healthful fat like omega-3 and monounsaturated fats.
It's also important to watch your portion sizes and keep your calorie intake to a level that lets you reach and maintain a healthy weight.
Academy of Nutrition and Dietetics. "Total Diet Approach to Healthy Eating." Accessed February 9, 2013. http://www.eatright.org/About/Content.aspx?id=8356.