Low-Fat Diet

A diet is considered to be "low-fat" when the total calories from fat are < 10-30%.

Here is a trick to teach your patients to determine if a product is 30% fat or less

Look for "calories from fat" on label, and multiply by 3
Compare to "total calories"
If "calories from fat" x 3 is less than "total calories," then product fits into a less than 30% fat diet plan
Diets approaching 10% fat are usually vegetarian

Evidence base for low-fat diets

Meta-analysis of studies > 1 year duration
Low-fat diets produce a mean weight loss = 3.55 kg (95% CI, 2.55 - 4.54 kg)
This corresponds to a 7.9 lb weight loss (5.6 - 10 lbs.) over a year, i.e., modest weight loss
Blood pressure, lipids, and fasting glucose also improved
In four studies: low-fat diets may prevent Type 2 diabetes and decrease the use of antihypertensive meds for up to three years

Reference