McArdle disease, glycogen storage disease type V, is a rare metabolic disease. Affected individuals are unable to utilize sugar stored as glycogen in muscle. Investigators hypothesize that a modified ketogenic diet could be a potential treatment option, by providing ketones as alternative fuel substrates for working muscle.

This blinded, placebo-controlled, cross-over study will investigate the potential effects of an optimal modified ketogenic diet found in part A (75% fat, 15%protein, 10%carbohydrates) in patients with McArdle disease compared with a healthy balanced placebo diet (>100grams of carbohydrates per day).

A blinded randomized, placebo-controlled, cross-over study to investigate the effects of a modified ketogenic diet in patients with McArdle disease.

McArdle disease, glycogen storage disease type V, is a rare metabolic disease caused by mutations in the PYGM gene resulting in absence of the enzyme muscle phosphorylase. Affected individuals are unable to utilize sugar stored as glycogen in muscle, leading to exercise intolerance, exercise-induced muscle pain, contractures and rhabdomyolysis. Currently, there are no satisfactory treatment options for McArdle disease. Ketones are feasible fuel alternatives to muscle glycogen when muscle glycogenolysis is blocked as in McArdle disease. A key element of alleviating symptoms in McArdle disease is to provide alternative fuels for energy metabolism. Ketosis can potentially provide alternative fuel substrates by provision of endogenous ketone bodies (KBs) which are desirable fuels for skeletal muscle and brain. Ketosis can be reached by fasting and can be induced by adhering to a modified ketogenic diet, which entails a high-fat, low-carbohydrate diet, which simulates the metabolic effects of fasting.

The study design is a placebo-controlled, blind, cross over design. The study will be carried out at two sites: CNMC (Copenhagen), NHNN (London). Subjects will be randomized 1:1 to receive either a modified ketogenic diet (75% fat, 15%protein, 10% carbohydrates) or a placebo diet (>100grams of carbohydrates per day) first. Subjects will follow the diet for 4 weeks, followed by 2-4 weeks wash-out, followed by 4 weeks on the opposite diet. During the 10-12 weeks trial period, subjects will visit the trial site in London on five occasions. Effects of the diet will be evaluated by improvement in exercise capacity during submaximal exercise test on a cycle ergometer. Subjective improvements will be evaluated by questionnaires and a dietary diary.

If the diet improves exercise capacity, it will provide a safe and cheap treatment option that may lead to reduced risk of muscle injury and enhance quality of life in patients with McArdle disease.

Inclusion Criteria:

• Genetically confirmed GSDV
• Patient is willing and able to provide written informed consent prior to participation.
• Patient is ambulatory.
• Women in fertile age must be willing to practice the following medically acceptable methods of birth control

Exclusion Criteria:

• Patient has any prior or current medical conditions that, in the judgment of the Investigator, would prevent the patient from safely participating in and/or completing all study requirements.
• Pregnancy or breastfeeding
• Patient does not have the cognitive capacity to understand/comprehend and complete all study assessments
• Patients with porphyria or disorders of fat metabolism (primary carnitine deficiency, carnitine palmitoyltransferase I or II, β-oxidation defects etc.).

Disguise products (Oral supplements: Nutricia 'ketocal' (intervention) or Nutricia 'fortini' (placebo)