Are fasting diets effective?

I’m a student in a Master of Science in Applied Nutrition program. That means I’m not qualified to give health advice and shouldn’t take health advice from me (yet!). But I’m willing to look at the science if you have a question. Here’s one question I was asked recently.

What do you think about fasting diets?

“How about Thursday? I’m fasting today,” is something I’ve been hearing lately. I’m not talking about fasting for religious reasons or because bad luck scheduling led to a fasting blood test in the late afternoon (always schedule those for first thing in the morning!).

Alternate-day fasting is said to speed weight loss and decrease risk of diabetes and heart disease. As it sounds, participants fast one day then eat whatever they want the “feast day.” Some modify the plan to consume 20-25 percent of their daily energy expenditure calories on the fasting day. Others restrict calories two days per week and eat normally five days a week. This is known as intermittent fasting.

Fasting has been shown to be effective in losing weight quickly, but it comes with a problem of elevated hunger, which has been shown to be difficult to maintain long-term.3

One study found that metabolically healthy obese adults did not show significant weight loss when following an alternate-day fasting diet versus following a controlled calorie restriction diet. No significant differences in blood pressure, heart rate, triglycerides, or diabetes markers were noted. 1

However, the HDL “good” cholesterol levels in the alternative fasting group were improved after the six-month diet period. But, when the group was tested again following a six-month maintenance period, no significant differences were seen. LDL “bad” cholesterol levels were similar to the control group’s after the six-month diet period, but the alternative fasting group had significantly higher LDL levels after 12 months.1

The study dropout rate was also highest for the alternate-day fasting group. The alternate-day fasting group also ate more than prescribed on fast days and less than prescribed on feast days. People in the control group generally met their calorie goals. This indicates that alternate-day fasting is not sustainable long-term and is no more effective than traditional calorie cutting for weight loss and improving health.1

Another study showed that when combined with an exercise program, alternate-day fasters had significant changes in weight loss and lipid indicators of coronary artery disease risk when compared to those who only fasted, only exercised, or neither. Also interesting, only two participants dropped out of the diet-exercise combination group whereas nine dropped out of the diet-only group.2

One study suggested further research on what macronutrients to consume on fasting days. The hypothesis is that eating protein on restriction days might help reduce feelings of hunger and help with diet retention.3

So should you try fasting? I wouldn’t suggest it, but I wouldn’t try to talk you out of it either. The science points to no real harm in trying, so if you want to try it and see if it works for you, go nuts.


  1. Wise J. Alternate day fasting is no better for weight loss than conventional diets, study finds. BMJ : British Medical Journal (Online). 2017;357. doi:
  2. Bhutani S, Klempel MC, Kroeger CM, Trepanowski JF, Varady KA. Alternate day fasting and endurance exercise combine to reduce body weight and favorably alter plasma lipids in obese humans. Obesity. 2013;21(7):1370-1379. doi:
  3. Johnstone A. Fasting for weight loss: An effective strategy or latest dieting trend? Int J Obes. 2015;39(5):727-733. doi:


Published by Candace

I’m a journalist, nutritionist, doting auntie, one-time bobsledder, and wannabe health nut (who loves chocolate and pizza too much to fully commit). I don't want you to think my life is perfect. It's not.

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