Yep, I've heard that: "Rapid weight loss programs should be avoided because fat-soluble environmental contaminants and toxins stored in body fat are released into the milk when caloric intake is severely restricted."
Below is the complete information from Kellymom.com:
Yes! Nursing mothers can lose weight safely if they follow some basic guidelines.
Six guidelines for safe dieting during lactation
1. Wait until your baby is two months old before dieting
It's best not to do anything consciously to lose weight until after the second month. This gives your body enough time to successfully establish a healthy milk supply that is less likely to be adversely affected if your caloric intake is restricted. Breastfeeding your baby, on average, burns 200-500 calories per day (above what you needed to maintain your pre-pregnancy weight) -- so keep in mind that even without a weight loss program you are burning extra calories.
2. Breastfeed without restriction
Research tells us that both more frequent breastfeeding and breastfeeding longer than six months increases maternal weight loss.
3. Eat at least 1500-1800 calories per day
While nursing, you should not consume less than 1500-1800 calories per day, and most women should stay at the high end of this range. Some mothers will require much more than this, but studies show that going below this number may put supply at risk.
4. Keep weight loss at less than 1.5 pounds per week
Most moms can safely lose up to 1.5 pounds per week or 6 pounds per month after the second month and not affect milk supply or baby's well being. One study has suggested that short-term weight loss of 2.2 pounds (1 kg) per week is not a problem (in this study, moms dieted for 11 days).
5. Decrease the calories gradually
A sudden drop in calories can reduce milk supply. Some moms notice this during an illness, although dehydration and/or medication use could also be a factor in reduced milk supply when mom is sick. It has been hypothesized that a sudden calorie decrease can cause mom's body to go into "starvation mode" and cut nonessential resources such as milk production.
6. Avoid quick-fix solutions
Liquid diets, low-carb diets, fad diets, weight loss medication, etc. are not recommended while breastfeeding.
What might happen if a nursing mother loses weight too quickly?
In most instances of too-rapid weight loss, it is not the breastmilk (composition or supply) that would suffer, but mom's nutrition and/or health.
Excessive dieting can result in a reduced milk supply.
According to Breastfeeding and Human Lactation (3rd Edition, Riordan, pp 440), it is noted that fad or rapid weight loss programs should be avoided because fat-soluble environmental contaminants and toxins stored in body fat are released into the milk when caloric intake is severely restricted. I was unable to find a definition of "severely restricted" but I expect that it is significantly under 1500 calories per day (which they called a "modest intake"). I've included information on a study on this subject below. See also the info at this website regarding environmental contaminants and breastfeeding.
Three great tips for weight loss (whether you are nursing or not)
Make dietary changes -- decrease your fat intake to 20-25% or less of total calories; keep your protein intake up to prevent loss of muscle mass (Recommended Intake of protein for nursing mothers is 65 grams/day for the first 6 months and 62 grams/day between 6 and 12 months).
Spread your calorie intake out -- instead of 2-3 meals, eat 3 smaller meals with snacks between. Your body will be less likely to go into "starvation mode" if you are eating small amounts throughout the day.
Exercise moderately so that you burn more fat while keeping lean muscle mass. Resistance/weight training is a good way to build muscle mass. A person with more muscle mass burns more calories, even when resting.