What women should know before trying intermittent fasting

Intermittent fasting is an eating pattern that focuses on when you eat, rather than what you eat. There are different types of intermittent fasting, but most approaches alternate between periods of eating and not eating — or fasting.

Some methods suggest fasting for a set number of hours, while others extend fasting to an entire day, several times a week.

Intermittent fasting affects men and women differently. Women may need to take a more gradual approach in order to reduce possible negative side effects on reproductive health, bone health, and overall well-being.

Women with diabetes, those taking certain medications, or those who are pregnant or breastfeeding should talk to their doctor before starting an intermittent fasting plan.

Research suggests there may be health benefits to intermittent fasting, including possible weight loss, improved insulin sensitivity, and heart health.

At ZOE, we know that everyone’s responses to foods are different and that these can have an effect on many different areas of your health, including your weight. 

We run the largest nutrition and gut microbiome study in the world, and we’ve found strong links between specific gut microbes and body composition.

Read on to learn more about intermittent fasting for women and how personalized nutrition can help you achieve your health goals.

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What is intermittent fasting?

Intermittent fasting focuses on when you eat, rather than the actual foods you eat. When you follow intermittent fasting, you alternate between periods of eating and fasting.

There are several different approaches to intermittent fasting. Some involve an extended fasting window each day, while others have entire fasting days a couple of times per week.

No matter which approach you choose, the goal of intermittent fasting is typically to extend the amount of time that your body goes without food.

When your body is in a fasting state, fat cells release stored sugar that your body then uses for energy. One of the ideas behind intermittent fasting is that as you use this stored energy, your weight may go down.

The most common approaches to intermittent fasting are the twice-a-week method, alternate-day fasting, and time-restricted eating.

Twice-a-week method

The twice-a-week method, also known as 5:2, involves eating a normal, healthy diet for 5 days out of the week. The other 2 days are fasting days, when you eat no more than 500–800 calories. You can choose which days you fast, as long as there is at least 1 non-fasting day between them.

Alternate-day fasting

As the name suggests, alternate-day fasting involves fasting every other day. On fasting days, you limit what you eat to around 500–800 calories. On non-fasting days, you eat a normal, healthy diet.

Time-restricted plans

Time-restricted plans separate eating and fasting into two windows, or cycles, within 1 day. The idea is to extend the fasting window that already happens when you’re sleeping. 

Since the majority of the fasting hours are during sleep, many people find this method easier to follow. 

Two common time-restricted eating methods are the 16/8 method and the 14/10 method.

• 16/8 method: Fast for 16 hours of the day and limit your eating to the other 8 hours. For example, eating between 9 a.m. and 5 p.m. or noon and 8 p.m.

• 14/10 method: Fast for 14 hours of the day and only eat during the other 10 hours. For example, eating between 10 a.m. and 8 p.m. 

• There is also an 18/6 and a very restrictive 20/4 version of this approach.

People may follow these patterns of eating and fasting anywhere from a couple of times a week to every day.

Why is intermittent fasting different for women and men?

Intermittent fasting affects men and women differently. There are possible risks for women to consider around reproductive health, bone health, and overall well-being.

Although you can eat normally during the non-fasting periods, certain forms of intermittent fasting may reduce your calorie intake. Calories provide your body with energy, and when energy is in short supply, your body will focus its attention on survival over other functions.

Research with women as participants is limited, but some studies that involve rodents suggest intermittent fasting may cause changes in estrogen levels and negatively affect reproductive functions, such as menstrual regularity, fertility, pregnancy, and lactation. 

The hormone levels of male rodents are also affected, but the disruptions don’t seem to change their reproductive functions as much as they do for female rodents. 

One possible reason for this is that some aspects of reproduction in women — including pregnancy and breastfeeding — require more energy than those in men. 

So, if energy intake is too low, these functions can be negatively affected. However, it’s not clear at the moment if human females are affected in the same way that rodent females are. 

Potential risks

A growing body of research suggests that significantly limiting calorie intake may also reduce bone strength and bone density. 

However, according to a recent review of studies, intermittent fasting does not appear to impact the density of the minerals in your bones in the same way that calorie-restricted diets do.

Most of the studies that show this are small, though, so more research is needed.

Other possible side effects of intermittent fasting include:

• headaches

• tiredness

• constipation

• overeating during non-fasting periods

To manage these health risks, women may need to take a different approach to intermittent fasting than men. Making small, gradual changes over several months, rather than drastic ones in a short period of time, could help to reduce unwanted side effects.

If you’re considering trying intermittent fasting, speak with your doctor to discuss any potential risks and whether it is right for you.

Nutrition is personal

At ZOE, our research has shown that everyone responds differently to food. Even twins have unique food responses, which tells us that our metabolic health isn't tied to genes and is changeable.

The ZOE at-home test can tell you which of the 15 “good” and 15 “bad” gut bugs that we’ve identified live in your gut. These are linked to higher or lower risk of increased belly fat, heart disease, and type 2 diabetes.

Along with your gut microbiome, the test analyzes your blood sugar and blood fat responses to food, and uses the latest science to determine the best foods for your unique metabolism.

With the ZOE program, you can eat for your unique biology to improve your gut health and overall health. You can take a free quiz to learn more.

Benefits of intermittent fasting for women

While the long-term effects of intermittent fasting are still unclear, scientists have found evidence of short-term health benefits including weight loss, improvements in heart health, and reduced risk of diabetes. 

Weight loss

Several studies suggest that intermittent fasting could be an effective way for women to lose weight. 

In one study, women over the age of 60 saw an average body fat loss of 2 kilograms — about 4.5 pounds — after 6 weeks of following a daily 16/8 intermittent fasting plan. 

In other research, people with overweight followed alternate-day fasting for between 8 and 12 weeks. Eating a low-calorie diet every second day, they lost an average of 5–6% of their body weight and reduced their waistlines by 5–7 centimeters.

So, why might you lose weight with intermittent fasting? One possibility is that you may simply be eating less overall. 

If you regularly reduce your calorie intake, or stop eating completely — whether for a number of hours each day or on certain days each week — there’s a good chance you’ll end up in an energy deficit. In other words, you’re using up more energy than you’re consuming.

Another possibility is that it’s the metabolic switch that happens during periods of fasting taking effect. 

Your body usually uses glucose from the food you eat for energy, but as you fast, it starts to break down your body fat for fuel. This can result in short-term weight loss, although further research is needed to determine how effective — or healthy — it is in the long term.

Heart health and inflammation

Heart disease is the leading cause of death in the world. 

Intermittent fasting may promote heart health by:

• improving cholesterol levels

• reducing blood pressure

• improving blood fat levels

• reducing chronic inflammation

Chronic inflammation is a long-term, continuing response by your immune system. It’s been linked to a number of serious diseases including heart disease.

It’s important to note that while many of the benefits around heart health were seen when people were following their intermittent fasting plans, they weren’t permanent. Once participants returned to normal eating patterns, the improvements quickly disappeared. 

Reduced diabetes risk

Intermittent fasting has also been linked to a reduced risk of type 2 diabetes.

Insulin sensitivity is a term that experts use to describe how sensitive the body is to the effects of insulin, and it varies from one person to another. Low insulin sensitivity, which is also called insulin resistance, can lead to a variety of health issues, including type 2 diabetes.

In one study, alternate-day fasting resulted in weight loss and improvements in insulin sensitivity. Another study showed that time-restricted intermittent fasting increased insulin sensitivity, even without weight loss. Research is ongoing in this area.

Help with cancer treatments

Scientists are currently studying the effects that intermittent fasting might have alongside cancer treatments on different types of cancer. 

Studies are in their early stages, but some researchers believe that fasting may improve the effects of cancer treatments, such as chemotherapy, and reduce their negative side effects. 

Protection from neurodegenerative illnesses

Animal studies suggest that alternate-day fasting may protect against neurodegenerative conditions such as stroke, Alzheimer’s disease, and Parkinson’s disease, by switching between carbohydrate and fat as the fuel your body uses for energy. 

However, further research is needed on whether these results might be applied to people.

Women who should avoid intermittent fasting

While there are many possible benefits of intermittent fasting, some women should avoid it unless cleared by their doctor. Reasons to talk to your medical provider before starting intermittent fasting include:

• Pregnancy or breastfeeding: Intermittent fasting is generally not recommended for women who are pregnant, as it may impact the growth of the baby and cause other negative side effects.

• Diabetes: Fasting may increase the risk of low blood sugar (hypoglycemia).

• Medications: Some drugs need to be taken with food, so extended fasting could interfere with scheduled doses.

If you have an existing medical condition or a history of disordered eating, talk to your doctor before starting any diet or eating plan.

Summary

Intermittent fasting is an eating pattern that alternates between periods of eating and fasting. There are different methods of intermittent fasting, including twice-a-week, alternate-day, and time-restricted.

Women and men are affected differently by intermittent fasting. There are possible risks for women to consider around their reproductive health, bone health, and overall well-being.

Evidence suggests there are also possible health benefits to intermittent fasting, including weight loss, diabetes prevention, and improvements in heart health. 

Everyone’s biology is unique. Understanding how you respond to different foods can help you eat in a way that’s best for you and your metabolic health. 

The ZOE at-home test analyzes your blood sugar and blood fat responses after eating, in combination with your gut microbiome. Based on your individual results, your ZOE program helps you identify the best foods for your body and your health. 

Take our quiz to learn more about how ZOE can help you reach your health goals by putting your insights into action.