Weight loss diets: low, moderate or high carbohydrate?

1. Very low carbohydrate (very high protein) diets

Very low carbohydrate diets, such as the Atkin's diet, are in fashion again.
Many health professionals have not supported the Atkin's diet because of its high fat content and restrictions on fruit and vegetables. It has not helped that the Atkin's diet has not been well studied. However, a recent study published in the New England Journal of Medicine (May 2003) showed that after 6 months on a very low carbohydrate (<25% energy) high protein (>35% energy) "Atkins" diet, 63 obese study participants lost more than double the weight (7kg) compared with those following a high carbohydrate diet low protein diet (60% energy as carbohydrates, 15% energy as protein) (3kg). Total blood and LDL cholesterol did not change on the low carbohydrate group, but the good HDL cholesterol increased and the undesirable triglycerides decreased. Both diets lowered blood pressure and made the body more sensitive to insulin.

Two other recent studies had similar results - low carbohydrate more effectively reduced weight than conventional low energy, low fat, without any adverse impact on cardiovascular and diabetic risk factors. However, compliance figures in these studies was poor; in the first study nearly half dropped out after a year. Furthermore, at 12 months low and high carbohydrate diets had regained about half the lost weight and there was no longer any statistical difference between the two groups.

It is still uncertain about the long term safety of a diet very low in carbohydrates (with small amounts of plant food) but high in saturated fat (which allows unlimited intake of animal fat). Some experts believe it's a recipe for a heart attack. What is needed are follow up studies over years, rather than months to determine safety of very low carbohydrate diets.

2. Moderate carbohydrate (moderate protein) VERSUS high carbohydrate (low protein) diets

Moderate carbohydrate/protein diets were recently shown by the CSIRO in Adelaide in 2002 to be a safe and effective option to conventional high carbohydrate/low protein weight loss diets in 100 overweight women.

The moderate carbohydrate diet contained moderate amounts of carbohydrate (46%energy) and protein (34% protein) and low amount of fat (20% energy).
Sample diet: 35g wholegrain breakfast cereal, 250ml low fat milk, 2 slices wholegrain bread, 2 fruits, 100g chicken/fish/meat for lunch, 200g beef/lamb for dinner, 2.5 cups vegetables, 200g diet yoghurt, 3 teaspoons canola oil; 2 glasses wine a week (optional)

The high carbohydrate diet was high in carbohydrate (63% energy), low in fat (20%) and protein (17%).
Sample diet: 35g wholegrain breakfast cereal, 250ml low fat milk, 3 slices wholegrain bread, 3 fruits, 80g chicken/fish/meat for dinner, 3 low fat biscuits, 120g cooked pasta/rice, 2.5 cups vegetables, 200g diet yoghurt, 3 teaspoons canola oil, 2 glasses wine a week (optional)

Over 12 weeks the women on the moderate carbohydrate/high protein diet lost 8kg weight (especially from the abdomen) versus 6kg on the high carbohydrate diet i.e lost 25% more weight. Both diets lowered blood lipids and insulin/glucose levels but women with high blood triglycerides lowered these to a greater extent (28% lower) on the moderate carbohydrate diet compared with the high carbohydrate diet (10% lower). The drop out was three times greater on the high carbohydrate diet. The researchers conclude moderate carbohydrate/ protein diets are a safe and effective weight loss program for those people showing symptoms of metabolic syndrome (increased blood fats and glucose) and that there does not appear to be any adverse effect on kidney function and bone.

Long-term weight loss?

The US National Weight Control Registry concluded that fewer than 1% of people who had successfully lost weight had followed a low carbohydrate diet. This suggests that this diet is not realistic for the achievement of long term weight loss. However, there have been reports of significant weight loss on fatty low carbohydrate diets. The mechanism may have something to do with the effect fat has on gastric emptying. Fat can delay the emptying of food from the stomach into the intestine which in turn can make one feel "full". This may result in less food being eaten (which may translate into a negative energy balance). Even though fat is very energy dense, the satiating effect of fat resulting in less food being consumed wins out over the energy density of fat. This hypothesis needs further exploration and research.

Weight gain comes from too many calories
The basic principle of any low carbohydrate diet is that weight gain is caused by carbohydrates, which is misleading. Weight gain is caused by consuming too many kilojoules (or calories) whether they are from carbohydrate, protein or fat.

The advocates of many of these diets advise people to consume kilojoules mainly from protein and fat sources. This means you need to limit cereals, some vegetables and fruit, while eating more meat, dairy foods and fat. Typical foods eaten on a low-carbohydrate diet include beef, chicken, bacon, fish, eggs, non-starchy vegetables, as well as fats such as oils butter and mayonnaise . Forbidden foods include fruit, bread, grains, starchy vegetables and dairy products other than cheese, cream or butter.

Low carbohydrate diets cause weight loss because they restrict kilojoules or energy. This basically starves the body of the nutrients it needs and causes major metabolic disturbances to the body. There is nothing special about the proportions of protein to carbohydrate - there are just fewer kilojoules consumed in these diets, which causes the weight loss.

A CSIRO study (Professor Peter Clifton, study leader) provided calorie-reduced diet plans to people with insulin resistance and people who are not insulin resistant. The diet plans strictly controlled the amount of protein, carbohydrate and fat in the meals of subjects. About 30% of energy intake came from protein, double the amount in a normal diet. This meant eating 200 grams of meat for the evening meal. The researchers found that it was the calorie reduction that counted, not the protein. It did not appear to matter if the diets were high protein, high carbohydrate or even high fat, as long as there was calorie reduction. Furthermore, the insulin resistant subjects lost just as much weight as subjects who were non-insulin resistant.

A diet high in fruits and vegetables, wholegrains, legumes and low-fat dairy products, as well as being moderate in fat and calories, will result in the greatest chance of weight loss and maintenance. Such a diet also assists in fullness and can reduce the risk of chronic diseases.

Short term side effects of very low carbohydrate diets
Within a short period of time, the effects of a very low carbohydrate diet include:
Nausea
Dizziness
Constipation
Lethargy
Dehydration
Bad breath
Loss of appetite.

However, a recent systematic review (JAMA 2003 Apr 9;289(14):1837-50) of over 100 trials of low carbohydrate diets concluded that, at least in the short term, such diets "had no significant adverse effect on serum lipid, fasting serum glucose and fasting serum insulin levels or blood pressure"..

Potential effects on health if a very low carbohydrate diet is consumed long-term
The potential effects on health if consumed long-term are unknown. However,
very low carbohydrate diets are at risk of being nutritionally inadequate due to their low plant food content. They tend to be low in fibre, thiamin, folate, vitamins A, E and B6, calcium, magnesium, iron, potassium and antioxidant phytochemicals. Such a diet could increase a person's risk of developing cancer. Very low carbohydrate diets also tend to be high in saturated fat, which can raise blood cholesterol and contribute to heart disease. Diets that are top-heavy in protein and fats are associated with abdominal obesity and obesity-related disorders including heart disease, diabetes and cancer. Evidence exists that the heart may not be able to function to its full ability when ketone bodies are its main source of fuel. Ketones are formed when the diet contains little carbohydrate. There is also increased excretion of calcium, which may increase the risk of developing osteoporosis.

Carbohydrates, glycaemic index (GI), glycaemic load (GL) and insulin
Carbohydrates are the only fuel source for many vital organs, including the brain, central nervous system and kidneys. The digestive system breaks down carbohydrates into glucose and the pancreas secretes a hormone called insulin to help the glucose migrate from the blood into the cells. Some authors of low carbohydrate diets propose that increased insulin levels cause weight gain, to justify a diet low in carbohydrates. However, proteins and fats also prompt the pancreas to release insulin. Nevertheless, there is emerging evidence that if a diet is high in refined high glycemic index (GI) carbohydrates then this may overstimulate insulin production, but diets high in unrefined high fibre lower GI carbohydrates have a more attenuated effect on insulin. Higher insulin levels have been associated with greater appetite and higher energy intakes as well as inhibition of fat oxidation. Studies on rats have shown that on diets containing the same amount of calories differing only in GI, weight was gained on the high GI diet while maintained on the low GI.

There is also emerging evidence for the potential role of Glycaemic Load (GL) in the cause and 'cure' of obesity and studies have shown that it can predict the risk of type 2 diabetes, heart attack, blood fats and some cancers. GL is based on the Glycaemic Index of the food/meal and the total amount of carbohydrate in the serve/meal consumed. This concept is important because some low GI foods can contain a large amount of total carbohydrate due to the large serving size that can be consumed - for example orange juice. If a food/meal has a low GI and a moderate amount of total carbohydrates it will have a low GL. The GL may have an important role to play in how an individual actually responds to a high carbohydrate weight loss diet - with insulin resistance actually acting to work against weight loss. An individual's insulin sensitivity is an important guide when considering an appropriate diet. For example, a 'moderate' intake of carbohydrates comprising low GI foods may have its greatest benefits in people with insulin resistance.

Carbohydrates and weight gain
You are more likely to gain weight from protein and fats than from carbohydrates because dietary fat has more than double the amount of kilojoules per gram (37) than carbohydrate (16). Protein has slightly higher amounts of energy at 17 kilojoules per gram, making protein and fat more energy-dense than carbohydrate. It's also important to remember that:
A quarter of the kilojoules eaten as carbohydrates is used up just converting glucose to body fat.
Only 3 per cent of kilojoules eaten as fat are used up in converting to body fat.

Vegetarians and people who consume predominantly plant-based diets are generally slimmer and have much lower rates of obesity, heart disease and cancer than people who eat meat-based diets. This supports current thinking that diets high in unrefined carbohydrates help to prevent overweight and obesity.
So how and why do low carbohydrates work to cause weight loss in the short term? Is the weight loss the result of changes in insulin response? Some believe that, in evolutionary terms, humans are well suited to a diet of relatively high protein and low carbohydrate (particularly processed/refined carbohydrate), and that this may be crucial to determining our insulin resistance (Eur J Clin Nutr 2002 Mar;56 Suppl 1:S30-5). Others are less sure of this argument (Curr Diab Rep 2002;Oct;2(5):457-64). The questions remain open, and we are bound to hear more on this.

Weight loss is mainly water in the short term
The body stores excess glucose as glycogen and converts glycogen back into glucose to use as fuel if there is insufficient carbohydrate in the diet. Around three grams of water are needed to release one gram of glycogen, so the rapid initial weight loss on a low carbohydrate diet is mostly water, not body fat. When a normal diet is resumed, some muscle tissue is rebuilt, water is restored and weight quickly returns, more than otherwise would be the case, mostly as fat. This can contribute to the problem yo-yo effect of dieting.

Low carbohydrate diets limit nutrition
A diet that restricts cereals, vegetables and fruit reduces your intake of essential vitamins and minerals. Over time, deficiencies can cause a variety of ailments. Diets low in carbohydrates are also low in fibre, which can lead to constipation in the short term and more serious intestinal disorders like diverticulitis in the long term. Diets chronically low in dietary fibre have also been associated with certain cancers.

The risks of a diet high in animal fats
A diet high in animal fat, especially saturated fat, has been associated with abdominal obesity. Carrying too much body fat around the middle is a risk factor in many diseases, including coronary heart disease and diabetes. Some research suggests that saturated fats (found in large amounts in animal fats) are more likely to contribute to abdominal fat than polyunsaturated or monounsaturated fats, even though they have the same kilojoule content. High blood cholesterol levels, usually caused by consuming a diet high in saturated fat, have been associated with coronary heart disease.

A very high protein diet can be dangerous
High protein foods, such as meat, are usually high in saturated fats. Foods high in saturated fats are usually high in cholesterol too. The long term health risks of a diet high in protein include:
High intake of animal products are usually recommended which can be high in saturated fats and cholesterol, associated with a range of conditions including heart disease.
Increased risk of developing gout and gall bladder colic
Kidney problems in those with already impaired kidney function or people with diabetes
The Nurses Healthy Study in the US on 121,700 women has shown (Knight et al., Annuals Internal Med 2003; 138: 460-67) that high protein intakes (especially from animal foods and fish) between 86g to 150g per day can cause further decline in kidney function in women who already have impaired function. In contrast, high protein intake did not affect women with normal kidney function. Researchers estimate that about 30% of all adults may have mild undetected kidney problems which could be worsened by high intakes of animal foods.
Liver problems
Loss of bone mineral content - a recent study, however, has shown that losses in bone minerals are correlated with the loss of body fat mass not the macronutrient content of the diet

A very low carbohydrate diet can lead to ketosis
If the body doesn't receive sufficient carbohydrate (about 40-50g per day or <25% energy as carbohydrate), it breaks down muscle and other tissue to produce glucose. This causes a build-up of waste products called 'ketones'. This state, known as 'ketosis', is an indication of body protein and/or muscle breakdown and is commonly seen in people who are starving, suffering from anorexia nervosa or have untreated insulin-dependent diabetes. Ketones make the blood acidic. Ketosis can be fatal in severe conditions, particularly for pregnant women, their unborn babies, and for people with diabetes.

Select carbohydrates, proteins and fats carefully if you follow a low-moderate carbohydrate diet
If you do choose to follow a lowish carbohydrate diet, do not avoid carbohydrates completely - at least a 'moderate' amount of unrefined low GI carbohydrates is desirable. You must have some carbohydrate in your diet to metabolise fat. Choose unrefined low GI sources of carbohydrates including wholegrains and fruit, rather than the more refined and energy-dense forms such as cakes, sweets and soft drinks. Remember that some refined carbohydrates can have a low GI (such as fruit juice, some types of white rice, some pasta) - these are best consumed in small amounts. Watch your portion sizes of carbohydrate rich foods to avoid a high GL and spread these foods out across the day. Include unrefined carbohydrates with a lower GI in your diet every day - include a couple of pieces of fresh fruit daily along with at least 3 serves of whole grain cereals (e.g grainy bread, natural muesli, brown rice). Have a variety of vegetables daily and try not to rely on potatoes as your main vegetable every day.

Select a variety of protein-rich foods that are also low in fat, especially saturated fat, for example:
Lean cuts of red meat
Fish (including fatty fish)
Lean chicken and pork.
You could also select protein rich foods that are plant-based, for example:
Nuts
Legumes such as beans and pulses
Soy products including tofu.

Select a variety of fat containing foods. If you must follow a low carbohydrate diet consume a variety of fats from plant sources (e.g olives, olive oil, canola oil, peanuts, peanut oil, soy, soy oil) rather than from animal sources (e.g butter, meat fat).

Ultimately, to avoid weight gain, energy intake should not exceed energy output over a period of time. Avoiding large portion sizes will help keep energy intake in check. For long term weight management, the benefits of regular physical activity can not be emphasised enough. Check out HECs dietary assessment tools.

Things to remember
Very low carbohydrate diet combined with very high protein intakes are not recommended. They can be unhealthy if too much animal fat is consumed and if plant foods are overly restricted . The long term safety of these diets is unknown
In the short term, very low carbohydrate diets can result in greater weight loss than the high carbohydrate diets,but in the long term, weight loss differences appear to be minimal.
Moderate carbohydrate/protein diets may be more effective than the high carbohydrate diet for weight loss (especially from the abdomen) in some people - especially in people showing symptoms of metabolic syndrome/elevated triglycerides. These diets will also have a lower GL and thus may be more suitable for people with insulin resistance.
Include unrefined high fibre carbohydrates with a lower GI in your diet every day

Related articles: Fact sheet on Protein

Article co-authored by Better Health Channel
(Australian -Victorian Government website)

Last Updated: April 2004