Why low-carb is best for PCOS
Updated: Jul 3
Polycystic Ovary Syndrome (PCOS) affects 10% of women in the UK and is the leading cause of female infertility worldwide. At its core, PCOS is a disease of hyperinsulinemia or excess insulin. High insulin disrupts normal hormonal balance, preventing ovulation and leading to the formation of ovarian cysts. The good news that we can lower insulin! A low-carb diet lowers blood glucose which reduces the secretion of insulin – Simples! Here we describe more about the condition and outline why we think a low-carb diet should be offered as the standard for every woman with PCOS in the UK.
What is PCOS?
PCOS is a common endocrine/metabolic disorder which affects 1 in 10 women in the UK and usually arises in adolescence or early adulthood. The NHS lists the main features of PCOS as:
· Irregular periods or no periods at all
· Difficulty getting pregnant due to irregular ovulation or failure to ovulate
· Excessive hair growth (hirsutism) on the face, chest or back
· Weight gain
· Thinning hair or hair loss from the head
· Oily skin or acne
Diagnosis is usually made using the Rotterdam Criteria, defined by the presence of two of the following criteria:
Irregular, infrequent or absent menstrual periods - the ovaries don't regularly produce eggs (ovulation)
Excess androgen – high levels of ‘male’ hormones, which may cause physical signs such as excess facial hair
Women with PCOS have a greater risk than others of developing other metabolic health problems, such as hypertension, pre-diabetes, type 2 diabetes and cardiovascular disease in the future than other women
Infertility and PCOS
Around 85% of women with PCOS have menstrual irregularities which often cause infertility. Many women are unaware they have PCOS until they try to conceive. PCOS is the most common cause of infertility in women and affects between 4-20% of women of reproductive age worldwide.
Overweight, obesity and PCOS
PCOS is more commonly found in women who are obese than women of normal weight. The prevalence of PCOS in women with a BMI of less than 25 is 4.3% compared to 14% in women with a BMI of over 30. The risk of women with PCOS becoming obese is four times higher than for others.
What causes PCOS?
PCOS is a metabolic condition - one in which excess insulin disturbs the normal, complex, hormonal patterns involved in the menstrual cycle.
One of the features of PCOS is that most women with this condition are resistant to the impact of insulin – that is, they are insulin resistant, and produce higher levels of insulin to regulate blood sugar than normal. Excess insulin in the body then impacts on the delicate balance of hormones which regulate the menstrual cycle.
The normal menstrual cycle relies on a complex arrangement in which hormones govern the development of follicles, until one follicle becomes dominant and develops into an egg which is released, a process known as ovulation, as shown in the diagram below:
Oestrogen plays a key role in the menstrual cycle, however, the higher levels of insulin found in PCOS inhibit the normal production of oestrogen, thereby disturbing the normal monthly cycle.
In his book, ‘Why we get Sick’, Ben Bickman explains how insulin impacts on the hormones involved in ovulation. Normally an enzyme called aromatase converts male hormones, such as testosterone, into oestrogen. However, when insulin is high, aromatase is inhibited, so that the conversion of is reduced. Oestrogen usually rises dramatically mid-cycle, signalling to the brain to produce luteinising hormone (LH) and leading to ovulation and the degradation of other developing eggs. Without the necessary spike in oestrogen, ovulation cannot occur, the ovaries retain the eggs, and cysts develop on the ovaries.
The reduced conversion of androgens to oestrogen also leads to elevated levels of androgens, which may result in facial and body hair, male pattern baldness and acne.
Dietary changes for PCOS
By itself, weight loss can make a very positive impact on the symptoms of PCOS and is considered a first line treatment for PCOS in those who are overweight or obese. PCOS is more commonly found in women who are obese as insulin resistance stimulates the storage of fat leading to weight gain. Excess weight can further exacerbate insulin resistance and symptoms of PCOS – a vicious circle!
Weight loss of just 5% improves the symptoms of PCOS and reduces fasting insulin. Weight loss is therefore usually recommended to women with this condition, and even modest weight reduction can be effective at improving symptoms.
Why Low Carbohydrate Diets are best for PCOS
The good news is that changes to diet and lifestyle improve symptoms and help reduce the impact of this condition. A recent systematic review and meta-analysis of 20 RCTs (randomised control studies) on dietary changes for PCOS concluded that dietary modification improved fertility outcomes, such as menstrual regularity, ovulation and pregnancy, and decreased testosterone and hirsutism. Of all the diets assessed, the study found that low carbohydrate diets were the most effective for improving reproductive outcomes. Low-carb diets reduce insulin and improve insulin sensitivity, so it makes sense that they would be beneficial for women with PCOS, and a growing body of research evidence shows this to be the case.
Insulin is a hormone which controls blood glucose levels in the body; when blood glucose levels rise after a meal, insulin is released to drive the glucose out of the blood and into the cells of the body where it is used for energy.
Women with PCOS are less sensitive to the impact of insulin and release more insulin to normalise blood glucose levels after a meal, and it is this excess insulin which is problematic in this condition as it impacts adversely on the hormones that regulate the menstrual cycle. Reducing intake of carbohydrate foods, such as sugars, bread, pasta, potatoes and rice, that increase or spike blood glucose levels is therefore helpful in PCOS as when blood glucose levels stay lower, the body releases less insulin. Carbohydrate foods increase blood glucose levels to a much greater degree than proteins or fats, as shown in the diagram below, so it makes sense to reduce their intake in order to reduce the release of insulin.
What is the evidence for low-carb for PCOS?
There is a strong body of evidence that low carb diets are effective in the management of other metabolic diseases, such type 2 diabetes, pre-diabetes and obesity. Research also shows low-carb diets are beneficial for reducing the symptoms of PCOS and improving fertility. A 2017 systematic review of studies assessed the impact of low-carb diets on PCOS found that 5 of 7 studies reported significant improvements in fasting insulin and testosterone, with 4 of 7 studies reporting improvement in menstrual cycles and ovulation. Four studies reported on pregnancy outcomes, with 3 of the 4 studies showing improved pregnancy rates. A 2019 meta-analysis of 8 RCTs (randomised controlled trials) found similar findings to the 2017 study, and concluded that:
‘proper control of carbohydrate intake provides beneficial effects on some aspects of PCOS and may represent one of the most important interventions improving the clinical symptoms of affected patients’
How far to lower carbohydrates?
A low carbohydrate diet is one which carbohydrate is limited to less than 130g/per day, with a very low carbohydrate diet being one which limits carbohydrates to between 20-50g/per day.
Currently there is not enough research available to determine an optimal level of carbohydrates for women with PCOS, but it seems that any lowering of carbohydrates is likely to be valuable. A 2013 study showed that even a modest reduction in carbohydrates, to 41% of the dietary intake resulted in numerous beneficial outcomes in the metabolic profiles of women with PCOS, such as improved fasting glucose and insulin, reduced testosterone and improved insulin sensitivity.
However, other research suggests that reducing carbohydrate intake further may have greater benefits, and a number of studies have shown benefits from a greater reduction of carbohydrates. One small study used a very low carb diet (a ketogenic diet) over 12 weeks in 14 overweight women, with significant reductions in body weight, BMI, fat body mass, glucose and insulin and improvements in reproductive hormone levels and function. Carbohydrates in the diet were limited to 30g per day.
Low-carbohydrate diets and pregnancy
A low carbohydrate diet may increase regularity of menstrual cycles and ovulation, which can improve fertility. Anecdotal evidence suggests that some women become pregnant after adopting a low-carb diet. A clinical case report of four women who adopted a high protein, very low carbohydrate diet for 6 months, reported that all four women lost weight and regained regular menstrual periods, with two of the four becoming pregnant during this time.
What is a low-carb diet?
Following a low-carb diet is easy and involves eating delicious foods! A low carb-diet can be summarised as follows:
Avoid starchy carbs, like bread, pasta, potatoes, and rice, but eat plenty of non-starchy fruit and vegetables
Eat plenty of protein, such as fish, meat, eggs, tofu, eggs, cheese, nuts
Use natural fats, such as olive oil, coconut oil, butter and avoid vegetable oils
Cook from scratch and avoid ultra-processed and sweetened foods
Other nutrition and lifestyle changes to reduce symptoms of PCOS?
Other measures which may improve the symptoms include exercise, time-restricted eating and dietary supplements, all of which have a positive impact as they help lower insulin.
Exercise has long been recognised as beneficial in PCOS and is considered an important lifestyle intervention in guidelines for this condition. One systematic review concluded that vigorous aerobic exercise and resistance training provides the most benefit to women with PCOS, as these have the most benefit on improving insulin sensitivity and reducing androgen levels.
Time restricted eating
Eating within a shorter period each day can improve insulin sensitivity, so it would seem likely that this option may be beneficial to women with PCOS. Not a great deal of research has been undertaken on assessing time restricted eating in PCOS, however a recent 6-week trial studied the impact of restricting eating (between 8am and 4pm in a 24-hour period) in a group of 18 women with PCOS. The study found significant improvement in a range of metabolic and endocrine measures, and an improvement in menstrual cycle regularity in 73% of women.
Myo-inositol is a ‘vitamin-like’ substance, often considered to be a member of the B-complex group of vitamins, also appears to be helpful in managing PCOS. It is made by the body and found naturally in many foods such as organ meats, fruit, grains, nuts and beans. However, low levels of myo-inositol, have been observed in individuals with impaired insulin sensitivity and PCOS. In one randomised controlled trial, 4mg of myo-inositol taken daily for 6 months, appeared to have similar impact to the blood-glucose lowering medication, Metformin, (1500mg daily) in reducing blood glucose and restoring the menstrual cycle.
Chromium is an essential mineral, found naturally in foods such as meat, legumes and nuts which appears to have an important role in the regulation of insulin. A systematic review and meta-analysis published in 2017 suggests that supplementation with chromium picolinate may have beneficial effects on decreasing BMI, fasting insulin and free testosterone in PCOS patients.
At its core, PCOS is a disease of excess insulin, and strategies to lower circulating insulin, such as using a low-carb diet, are logically the best measures to take when seeking to manage the symptoms of this condition. Every woman with PCOS should be given this information. They should also be given the support they may need to help them adopt a low-carb lifestyle and to maximise the benefit of reducing the problematic levels of insulin in PCOS. This approach relieves symptoms of PCOS and gives hope to women with this condition.
In addition to the links in the post above, we recommend the following books:
'The PCOS Plan' by Nadia Brito Pateguana, ND and Jason Fung MD
'Why we get sick' by Benjamin Bickman